Endotherms, ectotherms, and mitochondrial genome-size variation

Drivers of interlineage variability in mitogenomic evolutionary rates in Platyhelminthes

Studies of forces driving interlineage variability in the evolutionary rates (both sequence and architecture) of mitochondrial genomes often produce contradictory results. Flatworms (Platyhelminthes) exhibit the fastest-evolving mitogenomic sequences among all bilaterian phyla. To test the effects of multiple factors previously associated with different aspects of mitogenomic evolution, we used mitogenomes of 223 flatworm species, phylogenetic multilevel regression models, and causal inference. Thermic host environment (endothermic vs. ectothermic) had nonsignificant impacts on both sequence evolution and mitogenomic size. Mitogenomic gene order rearrangements (GORR) were mostly positively correlated with mitogenomic size (R2 ≈ 20-30%). Longevity was not (negatively) correlated with sequence evolution in flatworms. The predominantly free-living "turbellaria" exhibited much shorter branches and faster-evolving mitogenomic architecture than parasitic Neodermata. As a result, "parasitism" had a strong explanatory power on the branch length variability (>90%), and there was a negative correlation between GORR and branch length. However, the stem branch of Neodermata comprised 63.6% of the total average branch length. This evolutionary period was also marked by a high rate of gene order rearrangements in the ancestral Neodermata. We discuss how this period of rapid evolution deep in the evolutionary history may have decoupled sequence evolution rates from longevity and GORR, and overestimated the explanatory power of "parasitism". This study shows that impacts of variables often vary across lineages, and stresses the importance accounting for the episodic nature of evolutionary patterns in studies of mitogenomic evolution.

The mitochondrial genome of Bottapotamon fukienense (Brachiura: Potamidae) is fragmented into two chromosomes

BACKGROUND

China is the hotspot of global freshwater crab diversity, but their wild populations are facing severe pressures associated with anthropogenic factors, necessitating the need to map their taxonomic and genetic diversity and design conservation policies.

RESULTS

Herein, we sequenced the mitochondrial genome of a Chinese freshwater crab species Bottapotamon fukienense, and found that it is fragmented into two chromosomes. We confirmed that fragmentation was not limited to a single specimen or population. Chromosome 1 comprised 15,111 base pairs (bp) and there were 26 genes and one pseudogene (pseudo-nad1) encoded on it. Chromosome 2 comprised 8,173 bp and there were 12 genes and two pseudogenes (pseudo-trnL2 and pseudo-rrnL) encoded on it. Combined, they comprise the largest mitogenome (23,284 bp) among the Potamidae. Bottapotamon was the only genus in the Potamidae dataset exhibiting rearrangements of protein-coding genes. Bottapotamon fukienense exhibited average rates of sequence evolution in the dataset and did not differ in selection pressures from the remaining Potamidae.

CONCLUSIONS

This is the first experimentally confirmed fragmentation of a mitogenome in crustaceans. While the mitogenome of B. fukienense exhibited multiple signs of elevated mitogenomic architecture evolution rates, including the exceptionally large size, duplicated genes, pseudogenisation, rearrangements of protein-coding genes, and fragmentation, there is no evidence that this is matched by elevated sequence evolutionary rates or changes in selection pressures.

Comparative mitochondrial genomics in Nematoda reveal astonishing variation in compositional biases and substitution rates indicative of multi-level selection

BACKGROUND

Nematodes are the most abundant and diverse metazoans on Earth, and are known to significantly affect ecosystem functioning. A better understanding of their biology and ecology, including potential adaptations to diverse habitats and lifestyles, is key to understanding their response to global change scenarios. Mitochondrial genomes offer high species level characterization, low cost of sequencing, and an ease of data handling that can provide insights into nematode evolutionary pressures.

RESULTS

Generally, nematode mitochondrial genomes exhibited similar structural characteristics (e.g., gene size and GC content), but displayed remarkable variability around these general patterns. Compositional strand biases showed strong codon position specific G skews and relationships with nematode life traits (especially parasitic feeding habits) equal to or greater than with predicted phylogeny. On average, nematode mitochondrial genomes showed low non-synonymous substitution rates, but also high clade specific deviations from these means. Despite the presence of significant mutational saturation, non-synonymous (dN) and synonymous (dS) substitution rates could still be significantly explained by feeding habit and/or habitat. Low ratios of dN:dS rates, particularly associated with the parasitic lifestyles, suggested the presence of strong purifying selection.

CONCLUSIONS

Nematode mitochondrial genomes demonstrated a capacity to accumulate diversity in composition, structure, and content while still maintaining functional genes. Moreover, they demonstrated a capacity for rapid evolutionary change pointing to a potential interaction between multi-level selection pressures and rapid evolution. In conclusion, this study helps establish a background for our understanding of the potential evolutionary pressures shaping nematode mitochondrial genomes, while outlining likely routes of future inquiry.

Relaxed purifying selection pressure drives accelerated and dynamic gene rearrangements in thrips (Insecta: Thysanoptera) mitochondrial genomes

Insect mitochondrial genomes (mitogenome) generally present a typical gene order, which is considered as the ancestral arrangement. All sequenced mitogenomes in the Thysanoptera display high levels of gene rearrangement. Due to limited number of thrips mitogenomes sequenced, how gene rearrangement may be shaped by evolution remain unclear. Here, we analyzed 33 thrips mitogenomes, including 14 newly sequenced. These mitogenomes were diverse in organization, nucleotides substitution and gene arrangements. We found 28 highly rearranged gene orders with the breakpoints of gene rearrangements from 25 to 33. Reconstruction of the ancestors mitochondrial gene arrangements states indicated that Tubulifera have more complex pathways than Terebrantia in the gene order evolution. Molecular calibration estimated that divergence of two suborders occurred in the middle Triassic while the radiation of thrips was associated with the arose and flourish of angiosperm. Our evolutionary hypothesis testing suggests that relaxation of selection pressure enabled the early phase of Thysanoptera evolution, followed by a stronger selective pressure fixed diversification. Our analyses found gene inversion increases the nonsynonymous substitution rates and provide an evolutionary hypothesis driving the diverse gene orders.

Disentangling Positive Selection from Relaxed Selection in Animal Mitochondrial Genomes

AbstractDisentangling different types of selection is a common goal in molecular evolution. Elevated dN/dS ratios (the ratio of nonsynonymous to synonymous substitution rates) in focal lineages are often interpreted as signs of positive selection. Paradoxically, relaxed purifying selection can also result in elevated dN/dS ratios, but tests to distinguish these two causes are seldomly implemented. Here, we reevaluated seven case studies describing elevated dN/dS ratios in animal mitochondrial DNA (mtDNA) and their accompanying hypotheses regarding selection. They included flightless lineages versus flighted lineages in birds, bats, and insects and physiological adaptations in snakes, two groups of electric fishes, and primates. We found that elevated dN/dS ratios were often not caused by the predicted mechanism, and we sometimes found strong support for the opposite mechanism. We discuss reasons why energetic hypotheses may be confounded by other selective forces acting on mtDNA and caution against overinterpreting singular molecular signals, including elevated dN/dS ratios.

Rapid evolution, rearrangements and whole mitogenome duplication in the Australian stingless bees Tetragonula (Hymenoptera: Apidae): A steppingstone towards understanding mitochondrial function and evolution

The extreme conservation of mitochondrial genomes in metazoans poses a significant challenge to understanding mitogenome evolution. However, the presence of variation in gene order or genome structure, found in a small number of taxa, can provide unique insights into this evolution. Previous work on two stingless bees in the genus Tetragonula (T. carbonaria and T. hockingsi) revealed highly divergent CO1 regions between them and when compared to the bees from the same tribe (Meliponini), indicating rapid evolution. Using mtDNA isolation and Illumina sequencing, we elucidated the mitogenomes of both species. In both species, there has been a duplication of the whole mitogenome to give a total genome size of 30,666 bp in T. carbonaria; and 30,662 bp in T. hockingsi. These duplicated genomes present a circular structure with two identical and mirrored copies of all 13 protein coding genes and 22 tRNAs, with the exception of a few tRNAs that are present as single copies. In addition, the mitogenomes are characterized by rearrangements of two block of genes. We believe that rapid evolution is present in the whole Indo-Malay/Australasian group of Meliponini but is extraordinarily elevated in T. carbonaria and T. hockingsi, probably due to founder effect, low effective population size and the mitogenome duplication. All these features - rapid evolution, rearrangements, and duplication - deviate significantly from the vast majority of the mitogenomes described so far, making the mitogenomes of Tetragonula unique opportunities to address fundamental questions of mitogenome function and evolution.

Amblyopinae Mitogenomes Provide Novel Insights into the Paraphyletic Origin of Their Adaptation to Mudflat Habitats

The water-to-land transition is one of the most important events in evolutionary history of vertebrates. However, the genetic basis underlying many of the adaptations during this transition remains unclear. Mud-dwelling gobies in the subfamily Amblyopinae are one of the teleosts lineages that show terrestriality and provide a useful system for clarifying the genetic changes underlying adaptations to terrestrial life. Here, we sequenced the mitogenome of six species in the subfamily Amblyopinae. Our results revealed a paraphyletic origin of Amblyopinae with respect to Oxudercinae, which are the most terrestrial fishes and lead an amphibious life in mudflats. This partly explains the terrestriality of Amblyopinae. We also detected unique tandemly repeated sequences in the mitochondrial control region in Amblyopinae, as well as in Oxudercinae, which mitigate oxidative DNA damage stemming from terrestrial environmental stress. Several genes, such as ND2, ND4, ND6 and COIII, have experienced positive selection, suggesting their important roles in enhancing the efficiency of ATP production to cope with the increased energy requirements for life in terrestrial environments. These results strongly suggest that the adaptive evolution of mitochondrial genes has played a key role in terrestrial adaptions in Amblyopinae, as well as in Oxudercinae, and provide new insights into the molecular mechanisms underlying the water-to-land transition in vertebrates.

New Insights into the Origin and Evolution of Mysmenid Spiders (Araneae, Mysmenidae) Based on the First Four Complete Mitochondrial Genomes

The mitochondrial genome (mitogenome) is recognized as an effective molecular marker for studying molecular evolution and phylogeny. The family Mysmenidae is a group of widely distributed and covert-living spiders, of which the mitogenomic information is largely unclear. In this study, we obtained the first four complete mitogenomes of mysmenid spiders (one aboveground species: Trogloneta yuensis, and three cave-dwelling species: T. yunnanense, Yamaneta kehen and Y. paquini). Comparative analyses revealed that their lengths ranged from 13,771 bp (T. yuensis) to 14,223 bp (Y. kehen), containing a standard set of 37 genes and an A + T-rich region with the same gene orientation as other spider species. The mitogenomic size of T. yunnanense was more similar to that of Yamaneta mitogenomes than that of T. yuensis, which might indicate the convergent evolution of cave spiders. High variability was detected between the genera Trogloneta and Yamaneta. The A + T content, the amino acid frequency of protein-coding genes (PCGs) and the secondary structures of tRNAs showed large differences. Yamaneta kehen and Y. paquini contained almost identical truncated tRNAs, and their intergenic spacers and overlaps exhibited high uniformity. The two Yamaneta species also possessed a higher similarity of start/stop codons for PCGs than the two Trogloneta species. In selective pressure analysis, compared to Yamaneta, Trogloneta had much higher Ka/Ks values, which implies that selection pressure may be affected by habitat changes. In our study, the phylogenetic analysis based on the combination of 13 PCGs and two rRNAs showed that Mysmenidae is a sister clade to the family Tetragnathidae. Our data and findings will contribute to the better understanding of the origin and evolution of mysmenid spiders.

Complete mitogenomes of Turkish tree squirrels, Sciurus anomalus and S. vulgaris, (Sciuridae: Rodentia: Mammalia) and their phylogenetic status within the tribe Sciurini

The genus Sciurus, a member of the family Sciuridae, is widely distributed in the Holarctic region. To better understand mitogenomic characteristics and to reveal internal phylogenetic relationships of the genus, 20 complete mitogenomes of Turkish tree squirrels were successfully sequenced for the first time, including 19 for S. anomalus (from 16,505 bp to 16,510 bp) and one for S. vulgaris (16,511 bp). The mitogenomes of two species were AT-biased. All tRNAs for two species displayed a typical clover-leaf structure, except for tRNA^Ser(AGY). The tRNA Serine1 (S1)-GCT structure lacked the dihydrouridine (DHU) loop and stem. Based on mitogenomic dataset for phylogeny of Sciurinae, phylogenetic analyses (Bayesian Inference and Maximum Likelihood) did not support monophyly of Sciurus and proposed that S. anomalus, the most basal taxa in the Sciurini tribe, had at least five mitogenome lineages, which were also supported by network analysis. The dissimilarities among the five linegaes of S. anomalus ranged from 0.0042 (0.42%) to 0.0062 (0.62%) using K2P sequence pairwise distances. In addition to this mitogenomic analysis result, phylogenetic analyses using the CYTB + D-loop dataset proposed the existence of at least nine lineages for S. anomalus, which was different than those of the previous studies. The current study proposed that the use of mitogenomic data for reconstructing the phylogeny of Turkey' Sciurus holds an important value for revealing evolutionary relationships.

Inverted base composition skews and discontinuous mitochondrial genome architecture evolution in the Enoplea (Nematoda)

Background

Within the class Enoplea, the earliest-branching lineages in the phylum Nematoda, the relatively highly conserved ancestral mitochondrial architecture of Trichinellida is in stark contrast to the rapidly evolving architecture of Dorylaimida and Mermithida. To better understand the evolution of mitogenomic architecture in this lineage, we sequenced the mitogenome of a fish parasite Pseudocapillaria tomentosa (Trichinellida: Capillariidae) and compared it to all available enoplean mitogenomes.

Results

P. tomentosa exhibited highly reduced noncoding regions (the largest was 98 bp), and a unique base composition among the Enoplea. We attributed the latter to the inverted GC skew (0.08) in comparison to the ancestral skew in Trichinellidae (-0.43 to -0.37). Capillariidae, Trichuridae and Longidoridae (Dorylaimida) generally exhibited low negative or low positive skews (-0.1 to 0.1), whereas Mermithidae exhibited fully inverted low skews (0 to 0.05). This is indicative of inversions in the strand replication order or otherwise disrupted replication mechanism in the lineages with reduced/inverted skews. Among the Trichinellida, Trichinellidae and Trichuridae have almost perfectly conserved architecture, whereas Capillariidae exhibit multiple rearrangements of tRNA genes. In contrast, Mermithidae (Mermithida) and Longidoridae (Dorylaimida) exhibit almost no similarity to the ancestral architecture.

Conclusions

Longidoridae exhibited more rearranged mitogenomic architecture than the hypervariable Mermithidae. Similar to the Chromadorea, the evolution of mitochondrial architecture in enoplean nematodes exhibits a strong discontinuity: lineages possessing a mostly conserved architecture over tens of millions of years are interspersed with lineages exhibiting architectural hypervariability. As Longidoridae also have some of the smallest metazoan mitochondrial genomes, they contradict the prediction that compact mitogenomes should be structurally stable. Lineages exhibiting inverted skews appear to represent the intermediate phase between the Trichinellidae (ancestral) and fully derived skews in Chromadorean mitogenomes (GC skews = 0.18 to 0.64). Multiple lines of evidence (CAT-GTR analysis in our study, a majority of previous mitogenomic results, and skew disruption scenarios) support the Dorylaimia split into two sister-clades: Dorylaimida + Mermithida and Trichinellida. However, skew inversions produce strong base composition biases, which can hamper phylogenetic and other evolutionary studies, so enoplean mitogenomes have to be used with utmost care in evolutionary studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08607-4.

The Mitochondrial Genome of a Freshwater Pelagic Amphipod Macrohectopus branickii Is among the Longest in Metazoa

There are more than 350 species of amphipods (Crustacea) in Lake Baikal, which have emerged predominantly through the course of endemic radiation. This group represents a remarkable model for studying various aspects of evolution, one of which is the evolution of mitochondrial (mt) genome architectures. We sequenced and assembled the mt genome of a pelagic Baikalian amphipod species Macrohectopus branickii. The mt genome is revealed to have an extraordinary length (42,256 bp), deviating significantly from the genomes of other amphipod species and the majority of animals. The mt genome of M. branickii has a unique gene order within amphipods, duplications of the four tRNA genes and Cox2, and a long non-coding region, that makes up about two thirds of the genome’s size. The extension of the mt genome was most likely caused by multiple duplications and inversions of regions harboring ribosomal RNA genes. In this study, we analyzed the patterns of mt genome length changes in amphipods and other animal phyla. Through a statistical analysis, we demonstrated that the variability in the mt genome length may be a characteristic of certain phyla and is primarily conferred by expansions of non-coding regions.

Characterization and Phylogenetic Analysis of the Complete Mitochondrial Genome of Saturnia japonica

The complete mitochondrial genome (mitogenome) of Saturnia japonica (Lepidoptera: Saturniidae) was sequenced and annotated. It is a circular molecule of 15, 376 bp, composed of 13 protein-coding genes (PCGs), 22 transfer RNA genes (tRNAs), two ribosomal RNA genes (rRNA), and an adenine (A) + thymine (T)-rich region. All protein-coding genes (PCGs) are initiated by the ATN codon except for cytochrome c oxidase subunit 1 (cox1) gene that is seemingly initiated by the CGA codon. Except for cox2 and nad4, which were terminated by incomplete stop codon T or TA, the rest were terminated by canonical stop codon TAA. The A + T-rich region is high conservative, including 'ATAGA' motif followed by a 19 bp poly-T stretch, a microsatellite-like element (AT)₉ and also a poly-A element, with a total length of 332 bp. The Asn codon was the most frequently used codon, followed by Ile, Leu2, Lys, Met, Phe, and Tyr, while Cys was the least frequently used codon. Phylogenetic relationships analysis based on the 13 PCGs by using maximum likelihood (ML) and neighbor Joining (NJ) revealed that S. japonica belongs to the Saturniidae family. In this study, the annotation and characteristics of the mitogenome of S. japonica were resolved for the first time, which laid a foundation for species classification and the molecular evolution of Lepidoptera: Saturniidae.

Comparative Mitogenomic Analysis of Two Longhorn Beetles (Coleoptera: Cerambycidae: Lamiinae) with Preliminary Investigation into Phylogenetic Relationships of Tribes of Lamiinae

The subfamily Lamiinae is the most taxonomically diverse subfamily of Cerambycidae, but relationships between tribes of Lamiinae are still unresolved. In order to study the characteristics of the mitogenomes of Lamiinae and the tribal-level phylogenetic relationships, we sequenced the mitogenomes of two species representing two tribes, Agapanthia amurensis (Agapanthiini) and Moechotypa diphysis (Ceroplesini), with a total length of 15,512 bp and 15,493 bp, respectively. The gene arrangements of these two new mitogenomes were consistent with the inferred ancestral insect mitogenomes. Each species contained 37 typical mitochondrial genes and a control region (A + T-rich region), including 13 protein-coding genes (PCGs), 22 transfer RNA genes (tRNAs), and two ribosomal RNA genes (rRNAs). All PCGs initiated with the standard start codon ATN, and terminated with the complete stop codons of TAA and TAG, or incomplete stop codon T. All tRNAs could be folded into a clover-leaf secondary structure except for trnS1, in which the dihydrouridine (DHU) arm was reduced. Moreover, we studied the phylogenetic relationships between some tribes of Lamiinae based in mitochondrial PCGs in nucleotides; our results show that the relationships were as follows: (Onciderini + ((Apomecynini + Acanthocinini) + ((Ceroplesini + Agapanthiini) + ((Mesosini + Pteropliini) + ((Dorcaschematini + (Saperdini 1 + (Phytoeciini + Saperdini 2))) + (Batocerini + Lamiini)))))).

Mitogenomes of Accipitriformes and Cathartiformes Were Subjected to Ancestral and Recent Duplications Followed by Gradual Degeneration

The rearrangement of 37 genes with one control region, firstly identified in Gallus gallus mitogenome, is believed to be ancestral for all Aves. However, mitogenomic sequences obtained in recent years revealed that many avian mitogenomes contain duplicated regions that were omitted in previous genomic versions. Their evolution and mechanism of duplication are still poorly understood. The order of Accipitriformes is especially interesting in this context because its representatives contain a duplicated control region in various stages of degeneration. Therefore, we applied an appropriate PCR strategy to look for duplications within the mitogenomes of the early diverged species Sagittarius serpentarius and Cathartiformes, which is a sister order to Accipitriformes. The analyses revealed the same duplicated gene order in all examined taxa and the common ancestor of these groups. The duplicated regions were subjected to gradual degeneration and homogenization during concerted evolution. The latter process occurred recently in the species of Cathartiformes as well as in the early diverged lineages of Accipitriformes, that is, Sagittarius serpentarius and Pandion haliaetus. However, in other lineages, that is, Pernis ptilorhynchus, as well as representatives of Aegypiinae, Aquilinae, and five related subfamilies of Accipitriformes (Accipitrinae, Circinae, Buteoninae, Haliaeetinae, and Milvinae), the duplications were evolving independently for at least 14-47 Myr. Different portions of control regions in Cathartiformes showed conflicting phylogenetic signals indicating that some sections of these regions were homogenized at a frequency higher than the rate of speciation, whereas others have still evolved separately.

Slow crabs - fast genomes: Locomotory capacity predicts skew magnitude in crustacean mitogenomes

Base composition skews (G-C/G+C) of mitochondrial genomes are believed to be primarily driven by mutational pressure, which is positively correlated with metabolic rate. In marine animals, metabolic rate is also positively correlated with locomotory capacity. Given the central role of mitochondria in energy metabolism, we hypothesised that selection for locomotory capacity should be positively correlated with the strength of purifying selection (dN/dS), and thus be negatively correlated with the skew magnitude. Therefore, these two models assume diametrically opposite associations between the metabolic rate and skew magnitude: positive correlation in the prevailing paradigm, and negative in our working hypothesis. We examined correlations between the skew magnitude, metabolic rate, locomotory capacity, and several other variables previously associated with mitochondrial evolution on 287 crustacean mitogenomes. Weakly locomotory taxa had higher skew magnitude and ω (dN/dS) values, but not the gene order rearrangement rate. Skew and ω magnitudes were correlated. Multilevel regression analyses indicated that three competing variables, body size, gene order rearrangement rate, and effective population size, had negligible impacts on the skew magnitude. In most crustacean lineages selection for locomotory capacity appears to be the primary factor determining the skew magnitude. Contrary to the prevailing paradigm, this implies that adaptive selection outweighs nonadaptive selection (mutation pressure) in crustaceans. However, we found indications that effective population size (nonadaptive factor) may outweigh the impact of locomotory capacity in sessile crustaceans (Thecostraca). In conclusion, skew magnitude is a product of the interplay between adaptive and nonadaptive factors, the balance of which varies among lineages.

Detection of high heteroplasmy in complete loggerhead and hawksbill sea turtles mitochondrial genomes using RNAseq

Sea turtle populations around the world face rapid decline due to the effect of anthropogenic and environmental factors. Among the affected populations are those of hawksbill turtles (Eretmochelys imbricata) and loggerhead turtles (Caretta caretta), which is why a greater effort is currently being made in their monitoring and tracing. The intragenic degree of heteroplasmic mutations, commonly associated with diseases of variable symptoms, has not been analyzed in these species. In this study, heteroplasmy in the complete mitogenome (mtDNA) of three loggerhead turtles and one hawksbill turtle was identified from data obtained by RNAseq. Individuals Cc3, Ei1, Cc1 and Cc2 presented 0.3, 1.7, 1.8 and 7.1% of heteroplasmic mutations in all their mtDNA, respectively. The protein-coding genes that presented the highest percentage of heteroplasmy were ND4 and ND5 in individual Cc2 with 16 and 38.6%, respectively. Of the tRNA genes, only tRNA^Tyr was heteroplasmic in the four individuals with 5.63% (Cc1), 25.35% (Ei1 and Cc2) and 49.3% (Cc3). In this study, we identified the critical sites of heteroplasmy in each individual and the genetic variability of their mitogenomes. The data obtained represents the baseline for future projects that evaluate the population status of these species.

Complete mitochondrial genome sequence of Lepus yarkandensis Günther, 1875 (Lagomorpha, Leporidae): characterization and phylogenetic analysis

Lepusyarkandensis is a national second-class protected animal endemic to China and distributed only in the hot and arid Tarim Basin in Xinjiang. We sequenced and described the complete mitogenome of L.yarkandensis to analyze its characteristics and phylogeny. The species’ DNA is a 17,047 bp circular molecule that includes 13 protein-coding genes (PCGs), two rRNA genes, 22 tRNA genes, and one control region. The overall base composition was as follows: A, 31.50%; T, 29.40%; G, 13.30% and C, 25.80%, with a high A+T bias of 60.9%. In the PCGs, ND6 had deviation ranges for AT skew (–0.303) and GC skew (0.636). The Ka/Ks values of ND1 (1.067) and ND6 (1.352) genes were >1, indicating positive selection, which might play an important role in the adaptation of L.yarkandensis to arid and hot environments. The conserved sequence block, the central conserved domain, and the extended termination-associated sequences of the control region and their features were identified and described. The phylogenetic tree based on the complete mitogenome showed that L.yarkandensis was closely related to the sympatric Lepustibetanuspamirensis. These novel datasets of L.yarkandensis can supply basic data for phylogenetic studies of Lepus spp., apart from providing essential and important resource for further genetic research and the protection of this species.

Comparative mitogenome analyses uncover mitogenome features and phylogenetic implications of the subfamily Cobitinae

BACKGROUND

Loaches of Cobitinae, widely distributed in Eurasian continent, have high economic, ornamental and scientific value. However, the phylogeny of Cobitinae fishes within genera or family level remains complex and controversial. Up to now, about 60 Cobitinae mitogenomes had been deposited in GenBank, but their integrated characteristics were not elaborated.

RESULTS

In this study, we sequenced and analyzed the complete mitogenomes of a female Cobits macrostigma. Then we conducted a comparative mitogenome analysis and revealed the conserved and unique characteristics of 58 Cobitinae mitogenomes, including C. macrostigma. Cobitinae mitogenomes display highly conserved tRNA secondary structure, overlaps and non-coding intergenic spacers. In addition, distinct base compositions were observed among different genus and significantly negative linear correlation between AT% and AT-skew were found among Cobitinae, genus Cobitis and Pangio mitogenomes, respectively. A specific 3 bp insertion (GCA) in the atp8-atp6 overlap was identified as a unique feature of loaches, compared to other Cypriniformes fish. Additionally, all protein coding genes underwent a strong purifying selection. Phylogenetic analysis strongly supported the paraphyly of Cobitis and polyphyly of Misgurnus. The strict molecular clock predicted that Cobitinae might have split into northern and southern lineages in the late Eocene (42.11 Ma), furthermore, mtDNA introgression might occur (14.40 Ma) between ancestral species of Cobitis and ancestral species of Misgurnus.

CONCLUSIONS

The current study represents the first comparative mitogenomic and phylogenetic analyses within Cobitinae and provides new insights into the mitogenome features and evolution of fishes belonging to the cobitinae family.

Characterization and Phylogenetic Analysis of the Complete Mitochondrial Genome of Laelia suffusa (Lepidoptera: Erebidae, Lymantriinae)

In this study, the complete mitochondrial genome of a white tussock moth, Laelia suffusa (Walker, 1855) (Lepidoptera: Erebidae, Lymantriinae), was sequenced and annotated. The genome sequence was 15,502 bp in length and comprised 13 PCGs, 2 rRNAs, 22 tRNAs, and a single noncoding control region (CR). The nucleotide composition of the genome was highly A + T biased, accounting for 79.04% of the whole genome and with a slightly positive AT skewness (0.015). Comparing the gene order with the basal species of Lepidoptera, a typical trnM rearrangement was detected in the mitogenome of L. suffusa. Besides, the trnM rearrangement was found at the head of trnI and trnQ, rather than at the back. The 13 PCGs used ATN as their start codons, except for the cox1 which used CGA. Out of the 22 tRNAs, only 1 tRNA (trnS1) failed to fold in a typical cloverleaf secondary structure. The conserved motif 'ATAGA + poly-T' was detected at the start of the control region which was similar to other Lepidoptera species. In total, 10 overlapping regions and 19 intergenic spacers were identified, ranging from 1 to 41 and 2 to 73 bp, respectively. Phylogenetic analysis showed that Lymantriinae was a monophyletic group with a high support value and L. suffusa was closely related to tribe Orgyiini (Erebidae, Lymantriinae). Moreover, the phylogenetic relationship of Noctuoidea (Lepidoptera) species was reconstructed using two datasets (13 PCGs and 37 genes) and these supported the topology of (Notodontidae + (Erebidae + (Nolidae + (Euteliidae + Noctuidae)))).

Mitogenomics reveals phylogenetic relationships of Arcoida (Mollusca, Bivalvia) and multiple independent expansions and contractions in mitochondrial genome size

Arcoida, comprising about 570 species of blood cockles, is an ecologically and economically important lineage of bivalve molluscs. Current classification of arcoids is largely based on morphology, which shows widespread homoplasy. Despite two recent studies employing multi-locus analyses with broad sampling of Arcoida, evolutionary relationships among major lineages remain controversial. Interestingly, mitochondrial genomes of several ark shell species are 2-3 times larger than those found in most bilaterians, and are among the largest bilaterian mitochondrial genomes reported to date. These results highlight the need of detailed phylogenetic study to explore evolutionary relationships within Arcoida so that the evolution of mitochondrial genome size can be understood. To this end, we sequenced 17 mitochondrial genomes and compared them with publicly available data, including those from other lineages of Arcoida with emphasis on the subclade Arcoidea species. Our phylogenetic analyses indicate that Noetiidae, Cucullaeidae and Glycymerididae are nested within a polyphyletic Arcidae. Moreover, we find multiple independent expansions and potential contractions of mitochondrial genome size, suggesting that the large mitochondrial genome is not a shared ancestral feature in Arcoida. We also examined tandem repeats and inverted repeats in non-coding regions and investigated the presence of such repeats with relation to genome size variation. Our results suggest that tandem repeats might facilitate intraspecific mitochondrial genome size variation, and that inverted repeats, which could be derived from transposons, might be responsible for mitochondrial genome expansions and contractions. We show that mitochondrial genome size in Arcoida is more dynamic than previously understood and provide insights into evolution of mitochondrial genome size variation in metazoans.

Characterization of the complete mitochondrial genome of Orthaga olivacea Warre (Lepidoptera Pyralidae) and comparison with other Lepidopteran insects

Orthaga olivacea Warre (Lepidoptera: Pyralidae) is an important agricultural pest of camphor trees (Cinnamomum camphora). To further supplement the known genome-level features of related species, the complete mitochondrial genome of Orthaga olivacea is amplified, sequenced, annotated, analyzed, and compared with 58 other species of Lepidopteran. The complete sequence is 15,174 bp, containing 13 protein-coding genes (PCGs), 22 transfer RNA (tRNA) genes, 2 ribosomal RNA (rRNA) genes, and a putative control region. Base composition is biased toward adenine and thymine (79.02% A+T) and A+T skew are slightly negative. Twelve of the 13 PCGs use typical ATN start codons. The exception is cytochrome oxidase 1 (cox1) that utilizes a CGA initiation codon. Nine PCGs have standard termination codon (TAA); others have incomplete stop codons, a single T or TA nucleotide. All the tRNA genes have the typical clover-leaf secondary structure, except for trnS(AGN), in which dihydrouridine (DHU) arm fails to form a stable stem-loop structure. The A+T-rich region (293 bp) contains a typical Lepidopter motifs 'ATAGA' followed by a 17 bp poly-T stretch, and a microsatellite-like (AT)13 repeat. Codon usage analysis revealed that Asn, Ile, Leu2, Lys, Tyr and Phe were the most frequently used amino acids, while Cys was the least utilized. Phylogenetic analysis suggested that among sequenced lepidopteran mitochondrial genomes, Orthaga olivacea Warre was most closely related to Hypsopygia regina, and confirmed that Orthaga olivacea Warre belongs to the Pyralidae family.

Data on heteroplasmic mutations in mitochondrial genomes of loggerhead and hawksbill sea turtles: First approach

The populations of loggerhead (Caretta caretta) and hawksbill (Eretmochelys imbricata) sea turtles are suffering an exponential decline due to anthropic and environmental actions that threaten their survival. In these turtle populations, the degree of heteroplasmic mutations commonly related with pathologies, has not been studied. In this data report, the specifications of each heteroplasmic site (region, mutation, length) and the percentage of heteroplasmy of each gene for four mitochondrial genomes of turtles (loggerhead: Cc1, Cc2, Cc3 and hawksbill: Ei1) are presented. The highest value of heteroplasmy in tRNA was of 83.33% for the Cc2 turtle (tRNA^Ser gene), in protein coding genes was 38.62% for Cc2 (ND5), and in rRNA genes of 0.74% for Ei1 turtle (rRNA-16S). The variability data obtained will be useful for further conservation projects, evolution studies and population health of these species. This is the first study of heteroplasmy in complete mitogenomes of loggerhead and hawksbill turtles.

Description of soybean aphid (Aphis glycines Matsumura) mitochondrial genome and comparative mitogenomics of Aphididae (Hemiptera: Sternorrhyncha)

The complete mitochondrial genome of the soybean aphid (Aphis glycines Matsumura), a major agricultural pest in the world, is described for the first time, which consists of 13 protein-coding genes, 22 tRNA genes, 2 rRNA genes, as well as a large repeat region between tRNA-Glu and tRNA-Phe, and an AT-rich control region. The 17,954 bp mtgenome is the largest reported from the family Aphididae, and its gene order follows the ancestral insect mtgenome except for the repeat region, which contains a 195 bp unit repeated 11.9 times, representing the highest reported repeats among the known aphid mtgenomes to date. A new molecular phylogeny of Aphidae is reconstructed based on all available aphid mtgenomes, and it is shown that the mtgenome data can robustly resolve relationships at the subfamily level, but do not have sufficient phylogenetic information to resolve deep relationships. A phylogeny-based comparative analysis of mtgenomes has been performed to investigate the evolution of the repeat region between tRNA-Glu and tRNA-Phe. So far, among aphids, 13 species are known to have this repeat region of variable lengths, and a phylogenetic analysis of the repeat region shows that a large proportion of the sequences are conserved across the phylogeny, suggesting that the repeat region evolved in the most recent common ancestor of Aphidinae and Eriosomatinae, and that it has gone through numerous episodes of lineage-specific losses and expansions. Combined together, this study provides novel insights into how the repeat regions have evolved within aphids.

Mitogenomics of Central American weakly-electric fishes

Electric fishes are a diverse group of freshwater organisms with the ability to generate electric organ discharges (EODs) that are used for communication and electrolocation. This group (ca. 200 species) has originated in South America, and six species colonized the Central American Isthmus. Here, we assembled the complete mitochondrial genomes (mitogenomes) for three Central American electric fishes (i.e. Sternopygus dariensis, Brachyhypopomus occidentalis, and Apteronotus rostratus), and, based on these data, explored their phylogenetic position among Gymnotiformes. The three mitogenomes show the same gene order, as reported for other fishes, with a size ranging from 16,631 to 17,093 bp. We uncovered a novel 60 bp intergenic spacer (IGS) located between the COII and tRNA^Lys genes, which appears to be unique to the Apteronotidae. Furthermore, phylogenetic relationships supported the traditional monophyly of Gymnotiformes, with the three species positioned within their respective family. In addition, the genus Apteronotus belongs to the early diverging lineage of the order. Finally, we found high sequence divergence (13%) between our B. occidentalis specimen and a sequence previously reported in GenBank, suggesting that the prior mitogenome of B. occidentalis represents a different South American species. Indeed, phylogenetic analyses using Cytochrome b gene across the genus placed the previously reported individual within B. bennetti. Our study provides novel mitogenome resources that will advance our understanding of the diversity and phylogenetic history of Neotropical fishes.

Evolutionary progression of mitochondrial gene rearrangements and phylogenetic relationships in Strigidae (Strigiformes)

The bird mitogenome is generally considered to have a conservative genome size, consistent gene content, and similar gene order. As more mitogenomes are sequenced, mitochondrial (mt) gene rearrangements have been frequently identified among diverse birds. Within two genera (Bubo and Strix) of typical owls (Strigidae, Strigiformes), the rearrangement of the mt gene has been a subject of debate. In the current study, we first sequenced the whole mitogenomes of S. uralensis and B. scandiaca and resequenced the entire mitogenome of B. bubo. By combining our data with previously sequenced mitogenomes in Strigidae, we examined the mt gene rearrangements in the family and attempted to reconstruct the evolutionary progression of these rearrangements. The mitogenomes were then used to review the phylogenies of Strigidae. Most mitogenomes exhibited the ancestral gene order (A) in Strigidae. The ancestral gene order in the previously published mitogenome of B. bubo was found to be incorrect. We determined the mt gene order (the duplicate tRNA^Thr-CR, B) and discovered two additional mt gene orders (the duplicate tRNA^Glu-L-CR and CR, C and D) in the Bubo and Strix genera. Gene order B was likely derived from A by a tandem duplication of the region spanning from tRNA^Thr to CR. The other two modified gene orders, C and D, were likely derived from B by further degenerations or deletions of one copy of specific duplicated genes. We also preliminarily reconstructed the evolutionary progression of mt gene rearrangements and discussed maintenance of the duplicated CR in the genera. Additionally, the phylogenetic trees based on the mitogenomes supported the division of Strigidae into three subfamilies: Ninoxinae + (Surniinae + Striginae). Within the Striginae clade, the four genera formed a phylogenetic relationship: Otus + (Asio + (Bubo + Strix)). This suggests that Otus firstly diverges in their evolutionary history, and Bubo and Strix show a close relationship. B. bubo, B. blakistoni and B. scandiaca form a clade should be considered members of the same genus. The well-supported topology obtained in our Bayesian inference (BI) and maximum likelihood (ML) analyses of Strigid mitogenomes suggests that these genomes are informative for constructing phylogenetic relationships.

Comparative analyses of the complete mitochondrial genomes of Dosinia clams and their phylogenetic position within Veneridae

Mitochondrial genomes have proved to be a powerful tool in resolving phylogenetic relationship. In order to understand the mitogenome characteristics and phylogenetic position of the genus Dosinia, we sequenced the complete mitochondrial genomes of Dosinia altior and Dosinia troscheli (Bivalvia: Veneridae), compared them with that of Dosinia japonica and established a phylogenetic tree for Veneridae. The mitogenomes of D. altior (17,536 bp) and D. troscheli (17,229 bp) are the two smallest in Veneridae, which include 13 protein-coding genes, 2 ribosomal RNA genes, 22 tRNA genes, and non-coding regions. The mitogenomes of the Dosinia species are similar in size, gene content, AT content, AT- and GC- skews, and gene arrangement. The phylogenetic relationships of family Veneridae were established based on 12 concatenated protein-coding genes using maximum likelihood and Bayesian analyses, which supported that Dosininae and Meretricinae have a closer relationship, with Tapetinae being the sister taxon. The information obtained in this study will contribute to further understanding of the molecular features of bivalve mitogenomes and the evolutionary history of the genus Dosinia.

Characterization of the complete mitochondrial genome of Biston marginata (Lepidoptera: Geometridae) and phylogenetic analysis among lepidopteran insects

The complete mitochondrial genome (mitogenome) of Biston marginata (Lepidoptera: Geometridae) was determined and annotated. The circular genome is 15,470bp long and it contains the entire set of 37 genes usually present in lepidopteran mitogenomes. The nucleotide composition of the genome is highly A+T biased, accounting for 81.20%, with a slightly positive AT skewness (0.028), indicating the occurrence of more As than Ts, as found in other Geometridae species. Except for cox1 gene starts with non-canonical initial codon CGA, all protein-coding genes start with ATN codon. Three of the 13 PCGs (protein coding gene) had an incomplete termination codon, T or TA, while the others terminated with TAA. All tRNA genes are predicted to fold into typical clover-leaf secondary structure, except for the trnS1 (AGN), in which the dihydrouridine (DHU) arm could not form a stable stem-loop structure. The A+T-rich region of 343bp is comprised of non-repetitive sequences, but have several distinctive features, including the motif "ATAGA" followed by a 19bp poly-T stretch, a microsatellite-like (TA)₇ element next to the ATTTA motif. The phylogenetic analyses support the view that the B. marginata is closely related to the Biston pantrinaria, and confirm that Biston marginata belongs to the family Geometridae.

Adaptive Patterns of Mitogenome Evolution Are Associated with the Loss of Shell Scutes in Turtles

The mitochondrial genome encodes several protein components of the oxidative phosphorylation (OXPHOS) pathway and is critical for aerobic respiration. These proteins have evolved adaptively in many taxa, but linking molecular-level patterns with higher-level attributes (e.g., morphology, physiology) remains a challenge. Turtles are a promising system for exploring mitochondrial genome evolution as different species face distinct respiratory challenges and employ multiple strategies for ensuring efficient respiration. One prominent adaptation to a highly aquatic lifestyle in turtles is the secondary loss of keratenized shell scutes (i.e., soft-shells), which is associated with enhanced swimming ability and, in some species, cutaneous respiration. We used codon models to examine patterns of selection on mitochondrial protein-coding genes along the three turtle lineages that independently evolved soft-shells. We found strong evidence for positive selection along the branches leading to the pig-nosed turtle (Carettochelys insculpta) and the softshells clade (Trionychidae), but only weak evidence for the leatherback (Dermochelys coriacea) branch. Positively selected sites were found to be particularly prevalent in OXPHOS Complex I proteins, especially subunit ND2, along both positively selected lineages, consistent with convergent adaptive evolution. Structural analysis showed that many of the identified sites are within key regions or near residues involved in proton transport, indicating that positive selection may have precipitated substantial changes in mitochondrial function. Overall, our study provides evidence that physiological challenges associated with adaptation to a highly aquatic lifestyle have shaped the evolution of the turtle mitochondrial genome in a lineage-specific manner.

The Evolution of Dark Matter in the Mitogenome of Seed Beetles

Animal mitogenomes are generally thought of as being economic and optimized for rapid replication and transcription. We use long-read sequencing technology to assemble the remarkable mitogenomes of four species of seed beetles. These are the largest circular mitogenomes ever assembled in insects, ranging from 24,496 to 26,613 bp in total length, and are exceptional in that some 40% consists of non-coding DNA. The size expansion is due to two very long intergenic spacers (LIGSs), rich in tandem repeats. The two LIGSs are present in all species but vary greatly in length (114-10,408 bp), show very low sequence similarity, divergent tandem repeat motifs, a very high AT content and concerted length evolution. The LIGSs have been retained for at least some 45 my but must have undergone repeated reductions and expansions, despite strong purifying selection on protein coding mtDNA genes. The LIGSs are located in two intergenic sites where a few recent studies of insects have also reported shorter LIGSs (>200 bp). These sites may represent spaces that tolerate neutral repeat array expansions or, alternatively, the LIGSs may function to allow a more economic translational machinery. Mitochondrial respiration in adult seed beetles is based almost exclusively on fatty acids, which reduces the need for building complex I of the oxidative phosphorylation pathway (NADH dehydrogenase). One possibility is thus that the LIGSs may allow depressed transcription of NAD genes. RNA sequencing showed that LIGSs are partly transcribed and transcriptional profiling suggested that all seven mtDNA NAD genes indeed show low levels of transcription and co-regulation of transcription across sexes and tissues.

Characterization of the complete mitochondrial genome of Acanthoscelides obtectus (Coleoptera: Chrysomelidae: Bruchinae) with phylogenetic analysis

Acanthoscelides obtectus is a common species of the subfamily Bruchinae and a worldwide-distributed seed-feeding beetle. The complete mitochondrial genome of A. obtectus is 16,130 bp in length with an A + T content of 76.4%. It contains a positive AT skew and a negative GC skew. The mitogenome of A. obtectus contains 13 protein-coding genes (PCGs), 22 tRNA genes, two rRNA genes and a non-coding region (D-loop). All PCGs start with an ATN codon, and seven (ND3, ATP6, COIII, ND3, ND4L, ND6, and Cytb) of them terminate with TAA, while the remaining five (COI, COII, ND1, ND4, and ND5) terminate with a single T, ATP8 terminates with TGA. Except tRNA ^Ser , the secondary structures of 21 tRNAs that can be folded into a typical clover-leaf structure were identified. The secondary structures of lrRNA and srRNA were also predicted in this study. There are six domains with 48 helices in lrRNA and three domains with 32 helices in srRNA. The control region of A. obtectus is 1354 bp in size with the highest A + T content (83.5%) in a mitochondrial gene. Thirteen PCGs in 19 species have been used to infer their phylogenetic relationships. Our results show that A. obtectus belongs to the family Chrysomelidae (subfamily-Bruchinae). This is the first study on phylogenetic analyses involving the mitochondrial genes of A. obtectus and could provide basic data for future studies of mitochondrial genome diversities and the evolution of related insect lineages.

NUCLEOTYPIC EFFECT IN HOMEOTHERMS: BODY-MASS INDEPENDENT RESTING METABOLIC RATE OF PASSERINE BIRDS IS RELATED TO GENOME SIZE

The resting metabolic rate of passerines is shown to be negatively correlated with genome size when body mass is held constant (r = -0.75, P < 0.01). This finding extends previous conclusion for mammals to this bird order. The result holds when higher taxonomic levels are used instead of the species (for genera, r = -0.76, P < 0.03; for families, r = -0.991, P < 0.01) as well as when the independent contrasts derived from the resolved phylogeny are used instead of the taxa (r = -0.73, P < 0.02), with the evolutionarily older contrasts being more strongly correlated (for the contrasts older than 30 million yr, r = -0.998, P < 0.002). The concept of evolutionary characters consolidation (ECC), previously formulated for mammals, is tested with special reference to the error fraction in the total character variance. In this test, the ECC for the nucleotypic effect cannot be proven for mammals as a whole class, but it holds for the two separate orders tested, rodents and passerine birds. An upper taxonomic limit for the ECC is suggested.

The mitochondrial genome of a sea anemone Bolocera sp. exhibits novel genetic structures potentially involved in adaptation to the deep-sea environment

The deep sea is one of the most extensive ecosystems on earth. Organisms living there survive in an extremely harsh environment, and their mitochondrial energy metabolism might be a result of evolution. As one of the most important organelles, mitochondria generate energy through energy metabolism and play an important role in almost all biological activities. In this study, the mitogenome of a deep‐sea sea anemone (Bolocera sp.) was sequenced and characterized. Like other metazoans, it contained 13 energy pathway protein‐coding genes and two ribosomal RNAs. However, it also exhibited some unique features: just two transfer RNA genes, two group I introns, two transposon‐like noncanonical open reading frames (ORFs), and a control region‐like (CR‐like) element. All of the mitochondrial genes were coded by the same strand (the H‐strand). The genetic order and orientation were identical to those of most sequenced actiniarians. Phylogenetic analyses showed that this species was closely related to Bolocera tuediae. Positive selection analysis showed that three residues (31 L and 42 N in ATP6, 570 S in ND5) of Bolocera sp. were positively selected sites. By comparing these features with those of shallow sea anemone species, we deduced that these novel gene features may influence the activity of mitochondrial genes. This study may provide some clues regarding the adaptation of Bolocera sp. to the deep‐sea environment.

The mitochondrial genome sequences of the round goby and the sand goby reveal patterns of recent evolution in gobiid fish

Background

Vertebrate mitochondrial genomes are optimized for fast replication and low cost of RNA expression. Accordingly, they are devoid of introns, are transcribed as polycistrons and contain very little intergenic sequences. Usually, vertebrate mitochondrial genomes measure between 16.5 and 17 kilobases (kb).

Results

During genome sequencing projects for two novel vertebrate models, the invasive round goby and the sand goby, we found that the sand goby genome is exceptionally small (16.4 kb), while the mitochondrial genome of the round goby is much larger than expected for a vertebrate. It is 19 kb in size and is thus one of the largest fish and even vertebrate mitochondrial genomes known to date. The expansion is attributable to a sequence insertion downstream of the putative transcriptional start site. This insertion carries traces of repeats from the control region, but is mostly novel. To get more information about this phenomenon, we gathered all available mitochondrial genomes of Gobiidae and of nine gobioid species, performed phylogenetic analyses, analysed gene arrangements, and compared gobiid mitochondrial genome sizes, ecological information and other species characteristics with respect to the mitochondrial phylogeny. This allowed us amongst others to identify a unique arrangement of tRNAs among Ponto-Caspian gobies.

Conclusions

Our results indicate that the round goby mitochondrial genome may contain novel features. Since mitochondrial genome organisation is tightly linked to energy metabolism, these features may be linked to its invasion success. Also, the unique tRNA arrangement among Ponto-Caspian gobies may be helpful in studying the evolution of this highly adaptive and invasive species group. Finally, we find that the phylogeny of gobiids can be further refined by the use of longer stretches of linked DNA sequence.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3550-8) contains supplementary material, which is available to authorized users.

Comparative mitogenomic analysis of three species of periwinkles: Littorina fabalis, L. obtusata and L. saxatilis

The flat periwinkles, Littorina fabalis and L. obtusata, offer an interesting system for local adaptation and ecological speciation studies. In order to provide genomic resources for these species, we sequenced their mitogenomes together with that of the rough periwinkle L. saxatilis by means of next-generation sequencing technologies. The three mitogenomes present the typical repertoire of 13 protein-coding genes, 22 transfer RNA genes, two ribosomal RNA genes and a putative control region. Although the latter could not be fully recovered in flat periwinkles using short-reads due to a highly repetitive fragment, in L. saxatilis this problem was overcome with additional long-reads and we were able to assemble the complete mitogenome. Both gene order and nucleotide composition are similar between the three species as well as compared to other Littorinimorpha. A large variance in divergence was observed across mitochondrial regions, with six- to ten-fold difference between the highest and the lowest divergence rates. Based on nucleotide changes on the whole molecule and assuming a molecular clock, L. fabalis and L. obtusata started to diverge around 0.8 Mya (0.4-1.1 Mya). The evolution of the mitochondrial protein-coding genes in the three Littorina species appears mainly influenced by purifying selection as revealed by phylogenetic tests based on d_N/d_S ratios that did not detect any evidence for positive selection, although some caution is required given the limited power of the dataset and the implemented approaches.

Phylogeny analysis of complete mitochondrial DNA sequences for pelagic fishes from tuna fishery

Pelagic fishes captured in the international tuna fisheries have attracted increasing attention in recent years because of declines in their populations. In this study, 58 complete mitochondrial genomes of pelagic species from the classes Teleostean and Chondrichthyans were analyzed. The length of the 58 mtDNA sequences ranged from 15,598 to 18,880 bp, and all of which contained 13 protein-coding genes, 22 tRNA genes, 2 rRNA genes, and 1 control region. Our results suggested that mitochondrial genomes could be a powerful marker for resolving the phylogeny of pelagic fishes. Phylogenetic relationships based on the complete mitochondrial among 58 species indicated that Teleostean and Chondrichthyans are well separated. However, the control region length of Mobula japonicais was much larger than the other species in this study. Additionally, the Prionace glauca was divided into the clade in the genus Carcharhinus which provided a prospective taxonomic status of P.glauca.

Characterization of the complete mitochondrial genome of Cynoglossus gracilis and a comparative analysis with other Cynoglossinae fishes

Mitochondrial genomes can provide basic information for phylogenetic analysis and evolutionary studies. We present here the mitochondrial genome of Cynoglossus gracilis, which is 16,565bp in length. Numerous distinct regions were identified, including 13 protein-coding genes (PCGs), 22 tRNA genes, two rRNA genes, a light-strand replication origin, and a control region. Interestingly, we detected rearrangement of genes in C. gracilis, including a control region translocation, tRNA(Gln) gene inversion, and tRNA(Ile) gene shuffling. Additionally, a phylogenetic analysis based on the nucleotide sequences of the 13 PCGs using maximum likelihood and Bayesian inference methods reveals that C. gracilis is closely related to Cynoglossus semilaevis. This study provides important mitogenomic data for analyzing phylogenetic relationships in the Cynoglossinae.

Comparative Mitogenomic Analysis of Species Representing Six Subfamilies in the Family Tenebrionidae

To better understand the architecture and evolution of the mitochondrial genome (mitogenome), mitogenomes of ten specimens representing six subfamilies in Tenebrionidae were selected, and comparative analysis of these mitogenomes was carried out in this study. Ten mitogenomes in this family share a similar gene composition, gene order, nucleotide composition, and codon usage. In addition, our results show that nucleotide bias was strongly influenced by the preference of codon usage for A/T rich codons which significantly correlated with the G + C content of protein coding genes (PCGs). Evolutionary rate analyses reveal that all PCGs have been subjected to a purifying selection, whereas 13 PCGs displayed different evolution rates, among which ATPase subunit 8 (ATP8) showed the highest evolutionary rate. We inferred the secondary structure for all RNA genes of Tenebrio molitor (Te2) and used this as the basis for comparison with the same genes from other Tenebrionidae mitogenomes. Some conserved helices (stems) and loops of RNA structures were found in different domains of ribosomal RNAs (rRNAs) and the cloverleaf structure of transfer RNAs (tRNAs). With regard to the AT-rich region, we analyzed tandem repeat sequences located in this region and identified some essential elements including T stretches, the consensus motif at the flanking regions of T stretch, and the secondary structure formed by the motif at the 3′ end of T stretch in major strand, which are highly conserved in these species. Furthermore, phylogenetic analyses using mitogenomic data strongly support the relationships among six subfamilies: ((Tenebrionidae incertae sedis + (Diaperinae + Tenebrioninae)) + (Pimeliinae + Lagriinae)), which is consistent with phylogenetic results based on morphological traits.

Characterization of the Complete Mitochondrial Genome Sequence of the Globose Head Whiptail Cetonurus globiceps (Gadiformes: Macrouridae) and Its Phylogenetic Analysis

The particular environmental characteristics of deep water such as its immense scale and high pressure systems, presents technological problems that have prevented research to broaden our knowledge of deep-sea fish. Here, we described the mitogenome sequence of a deep-sea fish, Cetonurus globiceps. The genome is 17,137 bp in length, with a standard set of 22 transfer RNA genes (tRNAs), two ribosomal RNA genes, 13 protein-coding genes, and two typical non-coding control regions. Additionally, a 70 bp tRNA(Thr)-tRNA(Pro) intergenic spacer is present. The C. globiceps mitogenome exhibited strand-specific asymmetry in nucleotide composition. The AT-skew and GC-skew values in the whole genome of C. globiceps were 0 and -0.2877, respectively, revealing that the H-strand had equal amounts of A and T and that the overall nucleotide composition was C skewed. All of the tRNA genes could be folded into cloverleaf secondary structures, while the secondary structure of tRNA(Ser(AGY)) lacked a discernible dihydrouridine stem. By comparing this genome sequence with the recognition sites in teleost species, several conserved sequence blocks were identified in the control region. However, the GTGGG-box, the typical characteristic of conserved sequence block E (CSB-E), was absent. Notably, tandem repeats were identified in the 3' portion of the control region. No similar repetitive motifs are present in most of other gadiform species. Phylogenetic analysis based on 12 protein coding genes provided strong support that C. globiceps was the most derived in the clade. Some relationships however, are in contrast with those presented in previous studies. This study enriches our knowledge of mitogenomes of the genus Cetonurus and provides valuable information on the evolution of Macrouridae mtDNA and deep-sea fish.

The mitochondrial genome of Atrijuglans hetaohei Yang (Lepidoptera: Gelechioidea) and related phylogenetic analyses

Complete mitochondrial genome sequences are of great importance for better understanding the genome-level characteristics and phylogenetic relationships among related species. In this study, the complete mitochondrial genome of Atrijuglans hetaohei Yang is sequenced and analyzed, which is 15,379bp in length (GenBank: KT581634) and contains a typical set of 13 protein-coding genes, 22 tRNA genes, two rRNA genes and a non-coding region (control region). Except for cox1 gene that is initiated by CGA codon, all protein-coding genes start with ATN codons and end with the stop codon T, TA or TAA. All tRNAs have a typical clover-leaf secondary structure, except for trnS1, of which the DHU arm could not form a stable stem-loop structure. The secondary structure of rrnL and rrnS consists of 49 helices and 33 helices, respectively. Phylogenetic analyses of the complete mitochondrial genome sequences and of the amino acid sequences for 13 mitochondrial protein-coding genes among related species support the view that A. hetaohei is more closely related to the Gelechioidea than Yponomeutoidea. This result is consistent with a previous classification based on morphology.

The Complete Mitochondrial Genome of two Tetragnatha Spiders (Araneae: Tetragnathidae): Severe Truncation of tRNAs and Novel Gene Rearrangements in Araneae

Mitogenomes can provide information for phylogenetic analysis and evolutionary biology. The Araneae is one of the largest orders of Arachnida with great economic importance. In order to develop mitogenome data for this significant group, we determined the complete mitogenomes of two long jawed spiders Tetragnatha maxillosa and T. nitens and performed the comparative analysis with previously published spider mitogenomes. The circular mitogenomes are 14578 bp long with A+T content of 74.5% in T. maxillosa and 14639 bp long with A+T content of 74.3% in T. nitens, respectively. Both the mitogenomes contain a standard set of 37 genes and an A+T-rich region with the same gene orientation as the other spider mitogenomes, with the exception of the different gene order by the rearrangement of two tRNAs (trnW and trnG). Most of the tRNAs lose TΨC arm stems and have unpaired amino acid acceptor arms. As interesting features, both trnS(AGN) and trnS(UCN) lack the dihydrouracil (DHU) arm and long tandem repeat units are presented in the A+T-rich region of both the spider mitogenomes. The phylogenetic relationships of 23 spider mitogenomes based on the concatenated nucleotides sequences of 13 protein-coding genes indicated that the mitogenome sequences could be useful in resolving higher-level relationship of Araneae. The molecular information acquired from the results of this study should be very useful for future researches on mitogenomic evolution and genetic diversities in spiders.

Complete mitochondrial genome sequences of thirteen globally sourced strains of fruit fly (Drosophila melanogaster) form a powerful model for mitochondrial research

The complete mitogenomes of 13 strains of the fruit fly Drosophila melanogaster were sequenced. Haplotypes varied between 19 532 and 19 537 bp in length, and followed standard dipteran mitogenome content and organization. We detected a total of 354 variable sites between all thirteen haplotypes, while single pairs of haplotypes were separated by an average of 123 variable sites. The sequenced fly strains form a powerful model for mitochondrial research, when it comes to elucidating the links between the mitochondrial genotype and the phenotype.

The mitochondrial genome of Dastarcus helophoroides (Coleoptera: Bothrideridae) and related phylogenetic analyses

The complete mitochondrial genome of Dastarcus helophoroides (Coleoptera: Bothrideridae) which consists of 13 PCGs, 22 tRNA genes, two rRNA genes and a non-coding region (D-loop), is sequenced for its nucleotide sequence of 15,878 bp (GenBank: KF811054.1). The genome has a typical gene order which is identical to other Coleoptera species. Except for COI gene generally starts with non-canonical initial codon, all protein-coding genes start with ATN codon and terminate with the stop codon TA(A) or TAG. The secondary structure of rrnL and rrnS consists of 48 helices (contains four newly proposed helices) and 35 helices (contains two newly proposed helices) respectively. All 22 tRNAs in D. helophoroides are predicted to fold into typical cloverleaf secondary structure, except trnS1 (AGN), in which the dihydrouracil arm (DHU arm) could not form stable stem-loop structure. Thirteen protein-coding genes (nucleotide dataset and nucleic acid dataset) of the available species (29 taxa) have been used to infer the phylogenetic relationships among these orders. Tenebrionoidea and Cucujoidea form a sister group, and D. helophoroides is classified into Cucujoidea (Bothrideridae). The study first research on the phylogenetic analyses involving to the D. helophoroides mitogenome, and the results strongly bolster the current morphology-based hypothesis.

The complete mitochondrial DNA of Tegillarca granosa and comparative mitogenomic analyses of three Arcidae species

To better understand the characteristics and the evolutionary dynamics of mt genomes in Arcidae, the complete mitochondrial genome of Tegillarca granosa was firstly determined and compared with other two Arcidae species (Scapharca broughtonii and Scapharca kagoshimensis). The complete mitochondrial genome of T. granosa was 31,589 bp in length, including 12 protein-coding genes, 2 rRNA genes and 23 tRNA genes, and a major non-coding region. Three tandem repeat fragments were identified in the major non-coding region and the tandem repeat motifs of these fragments can be folded into stem-loop structures. The mitochondrial genome of the three species has several common features such as the AT content, the arrangement of the protein-coding genes, the codon usage of the protein-coding genes and AT/GC skew. However, a high level of variability is presented in the size of the genome, the number of tRNA genes and the length of non-coding sequences in the three mitogenomes. According to the phylogenetic analyses, these mitogenome-level characters are correlated with their phylogenetic relationships. It is the absence of the duplicated tRNAs and large non-coding sequences that are responsible for the length divergence of mitogenomes between T. granosa and other two Arcidae species. The phylogenetic analyses were conducted based on 12 partitioned protein genes, which support the relationship at the family level: (((Pectinidae+Ostreidae)+Mytilidae)+Arcidae).

Length variation and sequence divergence in mitochondrial control region of Schizothoracine (Teleostei: Cyperinidae) species

Schizothoracine fish commonly called snow trouts inhibit the entire network of snow and spring fed cool waters of Kashmir, India. Over 10 species reported earlier, only five species have been found, these include Schizothorax niger, Schizothorax esocinus, Schizothorax plagiostomus, Schizothorax curvifrons and Schizothorax labiatus. The relationship between these species is contradicting. To understand the evolutionary relation of these species, we examined the sequence information of mitochondrial D-loop of 25 individuals representing five species. Sequence alignment showed D-loop region highly variable and length variation was observed in di-nucleotide (TA)n microsatellite between and within species. Interestingly, all these species have (TA)n microsatellite not associated with longer tandem repeats at the 3' end of the mitochondrial control region and do not show heteroplasmy. Our analysis also indicates the presence of four conserved sequence blocks (CSB), CSB-D, CSB-1, CSB-II and CSB-III, four (Termination Associated Sequence) TAS motifs and 15bp pyrimidine block within the mitochondrial control region, that are highly conserved within genus Schizothorax when compared with other species. The phylogenetic analysis carried by Maximum likelihood (ML), Neighbor Joining (NJ) and Bayesian inference (BI) generated almost identical results. The resultant BI tree showed a close genetic relationship of all the five species and supports two distinct grouping of S. esocinus species. Besides the species relation, the presence of length variation in tandem repeats is attributed to differences in predicting the stability of secondary structures. The role of CSBs and TASs, reported so far as main regulatory signals, would explain the conservation of these elements in evolution.

Control region length dynamics potentially drives amino acid evolution in tarsier mitochondrial genomes

Patterns and processes of molecular evolution critically influence inferences in phylogeny and phylogeography. Within primates, a shift in evolutionary rates has been identified as the rationale for contrasting findings from mitochondrial and nuclear DNA studies as to the position of Tarsius. While the latter now seems settled, we sequenced complete mitochondrial genomes of three Sulawesi tarsiers (Tarsius dentatus, T. lariang, and T. wallacei) and analyzed substitution rates among tarsiers and other primates to infer driving processes of molecular evolution. We found substantial length polymorphism of the D-loop within tarsier individuals, but little variation of predominant lengths among them, regardless of species. Length variation was due to repetitive elements in the CSB domain-minisatellite motifs of 35 bp length and microsatellite motifs of 6 bp length. Amino acid evolutionary rates were second highest among major primate taxa relative to nucleotide substitution rates. We observed many radical possibly function-altering amino acid changes that were rarely driven by positive selection and thus potentially slightly deleterious or neutral. We hypothesize that the observed pattern of an increased amino acid evolutionary rate in tarsier mitochondrial genomes may be caused by hitchhiking of slightly deleterious mutations with favored D-loop length variants selected for maximizing replication success within the cell or the mitochondrion.

The complete mitogenome of Apocheima cinerarius (Lepidoptera: Geometridae: Ennominae) and comparison with that of other lepidopteran insects

The complete mitochondrial genome (mitogenome) of a female flightless geometrid moth Apocheima cinerarius was found to be 15,722 bp in length, containing 13 protein-coding genes (PCGs), 22 transfer RNA (tRNA) genes, 2 ribosomal RNA (rRNA) genes, and a control region. The A+T content of the complete mitogenome is 80.83%. The AT skew value ([A-T]/[A+T]) is 0.027. The 13 PCGs of the mitogenome start with typical ATN codons, except for cox1 with the start codon CGA. All the tRNA genes have typical cloverleaf secondary structures, except for trnSer(AGN). The secondary structures of rrnL and rrnS were predicted. Six structural domains including conserved regions (IV, V) and variable regions (I, II, III, VI) were identified in the secondary structure of rrnL. The secondary structure of rrnS consists of 3 structural domains. The control region of A. cinerarius begins with conserved motifs of "ATAGA"+19-bp poly T. It also contains a microsatellite-like (TA)26, a stem-and-loop structure, and a poly-A stretch. Phylogenetic analysis showed that Geometroidea is more closely related to Bombycoidea than to Noctuoidea. A. cinerarius is more closely related to Biston panterinaria than to Phthonandria atrilineata, which is in accordance with the conventional morphology-based classification.