A novel mitochondrial gene order in shorebirds (Scolopacidae, Charadriiformes)

Mitogenomic Codon Usage Patterns of Superfamily Certhioidea (Aves, Passeriformes): Insights into Asymmetrical Bias and Phylogenetic Implications

The superfamily Certhioidea currently comprises five families. Due to the rapid diversification, the phylogeny of Certhioidea is still controversial. The advent of next generation sequencing provides a unique opportunity for a mitogenome-wide study. Here, we first provided six new complete mitogenomes of Certhioidea (Certhia americana, C. familiaris, Salpornis spilonota, Cantorchilus leucotis, Pheugopedius coraya, and Pheugopedius genibarbis). We further paid attention to the genomic characteristics, codon usages, evolutionary rates, and phylogeny of the Certhioidea mitogenomes. All mitogenomes we analyzed displayed typical ancestral avian gene order with 13 protein-coding genes (PCGs), 22 tRNAs, 2 rRNAs, and one control region (CR). Our study indicated the strand-biased compositional asymmetry might shape codon usage preferences in mitochondrial genes. In addition, natural selection might be the main factor in shaping the codon usages of genes. Additionally, evolutionary rate analyses indicated all mitochondrial genes were under purifying selection. Moreover, MT-ATP8 and MT-CO1 were the most rapidly evolving gene and conserved genes, respectively. According to our mitophylogenetic analyses, the monophylies of Troglodytidae and Sittidae were strongly supported. Importantly, we suggest that Salpornis should be separated from Certhiidae and put into Salpornithidae to maintain the monophyly of Certhiidae. Our findings are useful for further evolutionary studies within Certhioidea.

Complete Mitogenomes of Polypedates Tree Frogs Unveil Gene Rearrangement and Concerted Evolution within Rhacophoridae

New developments in sequencing technology and nucleotide analysis have allowed us to make great advances in reconstructing anuran phylogeny. As a clade of representative amphibians that have radiated from aquatic to arboreal habitats, our understanding of the systematic status and molecular biology of rhacophorid tree frogs is still limited. We determined two new mitogenomes for the genus Polypedates (Rhacophoridae): P. impresus and P. mutus. We conducted comparative and phylogenetic analyses using our data and seven other rhacophorid mitogenomes. The mitogenomes of the genera Polypedates, Buergeria, and Zhangixalus were almost identical, except that the ATP8 gene in Polypedates had become a non-coding region; Buergeria maintained the legacy "LTPF" tRNA gene cluster compared to the novel "TLPF" order in the other two genera; and B. buergeri and Z. dennysi had no control region (CR) duplication. The resulting phylogenetic relationship supporting the above gene rearrangement pathway suggested parallel evolution of ATP8 gene loss of function (LoF) in Polypedates and CR duplication with concerted evolution of paralogous CRs in rhacophorids. Finally, conflicting topologies in the phylograms of 185 species reflected the advantages of phylogenetic analyses using multiple loci.

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.

Comparative mitogenomes of three species in Moenkhausia: Rare irregular gene rearrangement within Characidae

Generally, a teleostean group possesses only one type or a set of similar mitochondrial gene arrangements. However, a new type of gene arrangement has been identified in the mitochondrial genomes (mitogenomes) of Moenkhausia. Here, three newly sequenced complete mitogenomes of tetras (Characidae: Moenkhausia) are presented (M. costae, M. pittieri, and M. sanctaefilomenae). The three mitogenomes had a classical circular structure, with total lengths ranging from 15,811 to 18,435 bp. Base composition analysis indicated that the sequences were biased toward adenine (A) and thymine (T), with A + T content of 54.63% in M. costae, 58.47% in M. pittieri, and 59.98% in M. sanctaefilomenae. The gene order and organization of M. sanctaefilomenae differed from those of typical teleostean mitogenomes. The genes tRNA-Ile, tRNA-Gln, and tRNA-Pro were translocated between tRNA-Trp and tRNA-Asn. One extra tRNA-Met and an extra CR were also discovered in the mitogenome. BI and ML analyses based on sequences of 38 different mitogenomes showed that M. costae and M. pittieri were classified together, and M. sanctaefilomenae was slightly further from other fish of the same genus. These results provide insight into the gene arrangement features of Characidae mitogenomes and lay the foundation for further phylogenetic studies on Characidae.

Characterization and Comparative Analysis of Complete Mitogenomes of Three Cacatua Parrots (Psittaciformes: Cacatuidae)

Cacatua alba, Cacatua galerita, and Cacatua goffiniana are parrots of the family Cacatuidae. Wild populations of these species are declining with C. alba listed by the International Union for the Conservation of Nature and Natural Resources (IUCN) as Endangered. In this study, complete mitogenomes were sequenced for a comparative analysis among the Cacatua species, and a detailed analysis of the control region. Mitogenome lengths of C. alba,C. galerita, and C. goffiniana were 18,894, 18,900, and 19,084 bp, respectively. They included 13 protein coding genes, two ribosomal RNA genes, 24 transfer RNA genes, three degenerated genes, and two control regions. Ten conserved motifs were found in three domains within each of the two control regions. For an evolution of duplicated control regions of Cacatua, domain I and the 3′ end of domain III experienced an independent evolution, while domain II and most of the regions of domain III was subjected to a concerted evolution. Based on a phylogenetic analysis of 37 mitochondrial genes, the genus Cacatua formed a well-supported, monophyletic, crown group within the Cacatuidae. Molecular dating results showed that Cacatua diverged from other genera of Cacatuinae in the middle of Miocene.

Novel gene rearrangement in the mitochondrial genome of Muraenesox cinereus and the phylogenetic relationship of Anguilliformes

The structure and gene sequence of the fish mitochondrial genome are generally considered to be conservative. However, two types of gene arrangements are found in the mitochondrial genome of Anguilliformes. In this paper, we report a complete mitogenome of Muraenesox cinereus (Anguilliformes: Muraenesocidae) with rearrangement phenomenon. The total length of the M. cinereus mitogenome was 17,673 bp, and it contained 13 protein-coding genes, two ribosomal RNAs, 22 transfer RNA genes, and two identical control regions (CRs). The mitochondrial genome of M. cinereus was obviously rearranged compared with the mitochondria of typical vertebrates. The genes ND6 and the conjoint trnE were translocated to the location between trnT and trnP, and one of the duplicated CR was translocated to the upstream of the ND6. The tandem duplication and random loss is most suitable for explaining this mitochondrial gene rearrangement. The Anguilliformes phylogenetic tree constructed based on the whole mitochondrial genome well supports Congridae non-monophyly. These results provide a basis for the future Anguilliformes mitochondrial gene arrangement characteristics and further phylogenetic research.

New view on the organization and evolution of Palaeognathae mitogenomes poses the question on the ancestral gene rearrangement in Aves

BACKGROUND

Bird mitogenomes differ from other vertebrates in gene rearrangement. The most common avian gene order, identified first in Gallus gallus, is considered ancestral for all Aves. However, other rearrangements including a duplicated control region and neighboring genes have been reported in many representatives of avian orders. The repeated regions can be easily overlooked due to inappropriate DNA amplification or genome sequencing. This raises a question about the actual prevalence of mitogenomic duplications and the validity of the current view on the avian mitogenome evolution. In this context, Palaeognathae is especially interesting because is sister to all other living birds, i.e. Neognathae. So far, a unique duplicated region has been found in one palaeognath mitogenome, that of Eudromia elegans.

RESULTS

Therefore, we applied an appropriate PCR strategy to look for omitted duplications in other palaeognaths. The analyses revealed the duplicated control regions with adjacent genes in Crypturellus, Rhea and Struthio as well as ND6 pseudogene in three moas. The copies are very similar and were subjected to concerted evolution. Mapping the presence and absence of duplication onto the Palaeognathae phylogeny indicates that the duplication was an ancestral state for this avian group. This feature was inherited by early diverged lineages and lost two times in others. Comparison of incongruent phylogenetic trees based on mitochondrial and nuclear sequences showed that two variants of mitogenomes could exist in the evolution of palaeognaths. Data collected for other avian mitogenomes revealed that the last common ancestor of all birds and early diverging lineages of Neoaves could also possess the mitogenomic duplication.

CONCLUSIONS

The duplicated control regions with adjacent genes are more common in avian mitochondrial genomes than it was previously thought. These two regions could increase effectiveness of replication and transcription as well as the number of replicating mitogenomes per organelle. In consequence, energy production by mitochondria may be also more efficient. However, further physiological and molecular analyses are necessary to assess the potential selective advantages of the mitogenome duplications.

Characterization of three new mitochondrial genomes of Coraciiformes (Megaceryle lugubris, Alcedo atthis, Halcyon smyrnensis) and insights into their phylogenetics

Coraciiformes contains more than 200 species with great differences on external morphology and life-style. The evolutionary relationships within Coraciiformes and the phylogenetic placement of Coraciiformes in Aves are still questioned. Mitochondrial genome (mitogenome) sequences are popular markers in molecular phylogenetic studies of birds. This study presented the genome characteristics of three new mitogenomes in Coraciiformes and explored the phylogenetic relationships among Coraciiformes and other five related orders with mitogenome data of 30 species. The sizes of three mitogenomes were 17,383 bp (Alcedo atthis), 17,892 bp (Halcyon smyrnensis) and 17,223 bp (Megaceryle lugubris). Each mitogenome contained one control region and 37 genes that were common in vertebrate mitogenomes. The organization of three mitogenomes was identical to the putative ancestral gene order in Aves. Among 13 available Coraciiform mitogenomes, 12 protein coding genes showed indications of negative selection, while the MT-ND6 presented sign of positive selection or relaxed purifying selection. The phylogenetic results supported that Upupidae and Bucerotidae should be separated from Coraciiformes, and that Coraciiformes is more closely related to Piciformes than to Strigiformes, Trogoniformes and Cuculiformes. Our study provide valuable data for further phylogenetic investigation of Coraciiformes.

Resolving Phylogenetic Relationships within Passeriformes Based on Mitochondrial Genes and Inferring the Evolution of Their Mitogenomes in Terms of Duplications

Mitochondrial genes are placed on one molecule, which implies that they should carry consistent phylogenetic information. Following this advantage, we present a well-supported phylogeny based on mitochondrial genomes from almost 300 representatives of Passeriformes, the most numerous and differentiated Aves order. The analyses resolved the phylogenetic position of paraphyletic Basal and Transitional Oscines. Passerida occurred divided into two groups, one containing Paroidea and Sylvioidea, whereas the other, Passeroidea and Muscicapoidea. Analyses of mitogenomes showed four types of rearrangements including a duplicated control region (CR) with adjacent genes. Mapping the presence and absence of duplications onto the phylogenetic tree revealed that the duplication was the ancestral state for passerines and was maintained in early diverged lineages. Next, the duplication could be lost and occurred independently at least four times according to the most parsimonious scenario. In some lineages, two CR copies have been inherited from an ancient duplication and highly diverged, whereas in others, the second copy became similar to the first one due to concerted evolution. The second CR copies accumulated over twice as many substitutions as the first ones. However, the second CRs were not completely eliminated and were retained for a long time, which suggests that both regions can fulfill an important role in mitogenomes. Phylogenetic analyses based on CR sequences subjected to the complex evolution can produce tree topologies inconsistent with real evolutionary relationships between species. Passerines with two CRs showed a higher metabolic rate in relation to their body mass.

Complete mitochondrial genome of Ophichthus brevicaudatus reveals novel gene order and phylogenetic relationships of Anguilliformes

Generally, a teleostean group possesses only one type or a set of similar mitochondrial gene arrangement. However, two types of gene arrangement have been identified in the mitochondrial genomes (mitogenomes) of Anguilliformes. Here, a newly sequenced mitogenome of Ophichthus brevicaudatus (Anguilliformes; Ophichthidae) was presented. The total length of the O. brevicaudatus mitogenome was 17,773 bp, and it contained 13 protein-coding genes (PCGs), two ribosomal RNAs (rRNAs), 22 transfer RNA (tRNA) genes, and two identical control regions (CRs). The gene order differed from that of the typical vertebrate mitogenomes. The genes ND6 and the conjoint trnE were translocated to the location between trnT and trnP, and one of the duplicated CR was translocated to the upstream of the ND6. The duplication-random loss model was adopted to explain the gene rearrangement events in this mitogenome. The most comprehensive phylogenetic trees of Anguilliformes based on complete mitogenome was constructed. The non-monophyly of Congridae was well supported, whereas the non-monophyly of Derichthyidae and Chlopsidae was not supported. These results provide insight into gene arrangement features of anguilliform mitogenomes and lay the foundation for further phylogenetic studies on Anguilliformes.

New Insight into Parrots' Mitogenomes Indicates That Their Ancestor Contained a Duplicated Region

Mitochondrial genomes of vertebrates are generally thought to evolve under strong selection for size reduction and gene order conservation. Therefore, a growing number of mitogenomes with duplicated regions changes our view on the genome evolution. Among Aves, order Psittaciformes (parrots) is especially noteworthy because of its large morphological, ecological, and taxonomical diversity, which offers an opportunity to study genome evolution in various aspects. Former analyses showed that tandem duplications comprising the control region with adjacent genes are restricted to several lineages in which the duplication occurred independently. However, using an appropriate polymerase chain reaction strategy, we demonstrate that early diverged parrot groups contain mitogenomes with the duplicated region. These findings together with mapping duplication data from other mitogenomes onto parrot phylogeny indicate that the duplication was an ancestral state for Psittaciformes. The state was inherited by main parrot groups and was lost several times in some lineages. The duplicated regions were subjected to concerted evolution with a frequency higher than the rate of speciation. The duplicated control regions may provide a selective advantage due to a more efficient initiation of replication or transcription and a larger number of replicating genomes per organelle, which may lead to a more effective energy production by mitochondria. The mitogenomic duplications were associated with phenotypic features and parrots with the duplicated region can live longer, show larger body mass as well as predispositions to a more active flight. The results have wider implications on the presence of duplications and their evolution in mitogenomes of other avian groups.

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.

Mitogenomes of two neotropical bird species and the multiple independent origin of mitochondrial gene orders in Passeriformes

At least four mitogenome arrangements occur in Passeriformes and differences among them are derived from an initial tandem duplication involving a segment containing the control region (CR), followed by loss or reduction of some parts of this segment. However, it is still unclear how often duplication events have occurred in this bird order. In this study, the mitogenomes from two species of Neotropical passerines (Sicalis olivascens and Lepidocolaptes angustirostris) with different gene arrangements were first determined. We also estimated how often duplication events occurred in Passeriformes and if the two CR copies demonstrate a pattern of concerted evolution in Sylvioidea. One tissue sample for each species was used to obtain the mitogenomes as a byproduct using next generation sequencing. The evolutionary history of mitogenome rearrangements was reconstructed mapping these characters onto a mitogenome Bayesian phylogenetic tree of Passeriformes. Finally, we performed a Bayesian analysis for both CRs from some Sylvioidea species in order to evaluate the evolutionary process involving these two copies. Both mitogenomes described comprise 2 rRNAs, 22 tRNAs, 13 protein-codon genes and the CR. However, S. olivascens has 16,768 bp showing the ancestral avian arrangement, while L. angustirostris has 16,973 bp and the remnant CR2 arrangement. Both species showed the expected gene order compared to their closest relatives. The ancestral state reconstruction suggesting at least six independent duplication events followed by partial deletions or loss of one copy in some lineages. Our results also provide evidence that both CRs in some Sylvioidea species seem to be maintained in an apparently functional state, perhaps by concerted evolution, and that this mechanism may be important for the evolution of the bird mitogenome.

The mitochondrial genome of red-necked phalarope Phalaropus lobatus (Charadriiformes: Scolopacidae) and phylogeny analysis among Scolopacidae

The red-necked phalarope is a wonderful species with specific morphological characters and lifestyles. Mitochondrial genomes, encoding necessary proteins involved in the system of energy metabolism, are important for the evolution and adaption of species. In this study, we determined the complete mitogenome sequence of Phalaropus lobatus (Charadriiformes: Scolopacidae). The circular genome is 16714 bp in size, containing 13 PCGs, two ribosomal RNAs and 22 tRNAs and a high AT-rich control region. The AT skew and GC skew of major strand is positive and negative respectively. Most of PCGs are biased towards A-rich except ND1. A codon usage analysis shows that 3 start codons (ATG, GTG and ATA), 4 stop codons (TAA, TAG, AGG, AGA) and two incomplete terminate codons (T-). Twenty two transfer RNAs have the typical cloverleaf structure, and a total of ten base pairs are mismatched throughout the nine tRNA genes. The phylogenetic tree based on 13 PCGs and 2 rRNA genes indicates that monophyly of the family and genus Phalaropus is close to genus Xenus plus Tringa. The analysis of selective pressure shows 13 protein-coding genes are evolving under the purifying selection and P. lobatus is different from other Scolopacidae species on the selective pressure of gene ND4. This study helps us know the inherent mechanism of mitochondrial structure and natural selection.

Gene duplication and concerted evolution of mitochondrial DNA in crane species

The gene duplication in mitochondrial DNA (mtDNA) has been reported in diverse bird taxa so far. Although many phylogenetic and population genetic analyses of cranes were carried out based on mtDNA diversity, whether mtDNA contains duplicated regions is unknown. To address the presence or absence of gene duplication in cranes and investigate the molecular evolutionary features of crane mtDNA, we analyzed the gene organization and the molecular phylogeny of mtDNA from 13 crane species. We found that the mtDNA in 13 crane species shared a tandem duplicated region, which consists of duplicated sequence sets including cytochrome b (Cytb), NADH6, control region (CR) and three genes of tRNA. The gene order in the duplicated region was identical among all the 13 crane species, and the nucleotide sequences found within each individual showed high similarities. In addition, phylogenetic trees based on homologous sequences of CR and Cytb indicated the possibility of concerted evolution among the duplicated genes. The results suggested that the duplication event occurred in the common ancestor of crane species or some older ancestors.

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.

Rearrangement and evolution of mitochondrial genomes in parrots

Mitochondrial genome rearrangements that result in control region duplication have been described for a variety of birds, but the mechanisms leading to their appearance and maintenance remain unclear, and their effect on sequence evolution has not been explored. A recent survey of mitochondrial genomes in the Psittaciformes (parrots) found that control region duplications have arisen independently at least six times across the order. We analyzed complete mitochondrial genome sequences from 20 parrot species, including representatives of each lineage with control region duplications, to document the gene order changes and to examine effects of genome rearrangements on patterns of sequence evolution. The gene order previously reported for Amazona parrots was found for four of the six independently derived genome rearrangements, and a previously undescribed gene order was found in Prioniturus luconensis, representing a fifth clade with rearranged genomes; the gene order resulting from the remaining rearrangement event could not be confirmed. In all rearranged genomes, two copies of the control region are present and are very similar at the sequence level, while duplicates of the other genes involved in the rearrangement show signs of degeneration or have been lost altogether. We compared rates of sequence evolution in genomes with and without control region duplications and did not find a consistent acceleration or deceleration associated with the duplications. This could be due to the fact that most of the genome rearrangement events in parrots are ancient, and additionally, to an effect of body size on evolutionary rate that we found for mitochondrial but not nuclear sequences. Base composition analyses found that relative to other birds, parrots have unusually strong compositional asymmetry (AT- and GC-skew) in their coding sequences, especially at fourfold degenerate sites. Furthermore, we found higher AT skew in species with control region duplications. One potential cause for this compositional asymmetry is that parrots have unusually slow mtDNA replication. If this is the case, then any replicative advantage provided by having a second control region could result in selection for maintenance of both control regions once duplicated.

The mitochondrial genome of the Saunders's gull Chroicocephalus saundersi (Charadriiformes: Laridae) and a higher phylogeny of shorebirds (Charadriiformes)

The complete mitogenome of Chroicocephalus saundersi was characterized and compared with the 6 published Charadriiformes mitogenomes. The mitogenome of C. saundersi is a closed circular molecule 16,739 bp in size, and contains 37 genes and a control region. The AT and GC skews are positive and negative, respectively, and in agreement with those of the other Charadriiformes mitogenomes. The mitogenome of C. saundersi contains 3 start codons (ATG, GTG, and ATT), 4 stop codons (TAA, TAG, AGG, and AGA), and an incomplete stop codon (T-) in 13 PCGs. A codon usage analysis of all available Charadriiformes mitogenomes showed that the ATG (78%) and TAA (50.5%) were the most common start codon and stop codon, respectively. An unusual start codon, ATT, is commonly found in the ND3s of Charadriiformes mitogenomes, whereas the more common start codons, ATC and ATA, are rarely found. In all the Laridae species, one extra cytosine was inserted at position 174 in ND3. The control region of C. saundersi is 1180-bp long, with a nucleotide composition of 30.2% A, 28.6% T, 27.3% C, and 14.0% G. Variable numbers of tandem repeats (VNTRs) with nine copies of the 10 bp repeat sequence (AACAACAAAC) are found within the CSB domain of the control region. The ML/BI analyses, based on the amino acids of the 13 mitochondrial PCGs, strongly support the monophyly of the order Charadriiformes, with the suborder Lari considered sister to the Scolopaci, which is in turn a sister group to the suborder Charadrii.

Tandem Duplication and Random Loss for mitogenome rearrangement in Symphurus (Teleost: Pleuronectiformes)

Background

The mitochondrial genomes (mitogenomes) of flatfishes (Pleuronectiformes) exhibit highly diversified types of large-scale gene rearrangements. We have reported that the mitogenomes of Crossorhombus azureus (Bothidae), Samariscus latus (Samaridae) and Cynoglossus fishes (Cynoglossidae) show different types of gene rearrangements.

Results

In the present study, the complete mitogenomes of two Symphurus species (Cynoglossidae), Symphurus plagiusa and Symphurus orientalis, were determined. The gene order in the S. plagiusa mitogenome is the same as that of a typical vertebrate (without any gene rearrangements). Surprisingly, large-scale gene rearrangements have occurred in S. orientalis. In the rearranged fragment from the control region (CR) to the WANCY tRNA cluster (tRNA cluster of tRNA-W, tRNA-A, tRNA-N, tRNA-C and tRNA-Y) in the S. orientalis mitogenome, tRNA-V and tRNA-M have been translocated to the 3’ end of the 16S rRNA gene, with six large intergenic spacers over 20 bp in length. In addition, an origin for light-strand replication (O_L) structure that is typically located in the WANCY region was absent in both the S. plagiusa and S. orientalis mitogenomes. It is generally recognized that a sequence in the WANCY region that encodes tRNAs forms a hairpin structure (O_L-like structure) and can act as the O_L when the typical locus is lost. Moreover, an additional O_L-like structure was identified near the control region in the S. plagiusa mitogenome.

Conclusions

The positions of the intergenic spacers and the rearranged genes of the S. orientalis mitogenome strongly indicate that the mechanism underlying the rearrangement of this mitogenome was Tandem Duplication and Random Loss. Additionally, two O_L-like regions substituting for the typical locus were found in the S. plagiusa mitogenome. We speculate that the ancestral mitogenomes of S. plagiusa and S. orientalis also had this characteristic, such that if both O_L-like structures functioned during mitochondrial replication, they could initiate duplicate replications of the light strand (L-strand), leading to duplication of the region between the two structures. We consider that this mechanism may account for the gene duplication that occurred during the gene rearrangement process in the evolution of the ancestral mitogenome to the S. orientalis mitogenome.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1581-6) contains supplementary material, which is available to authorized users.

Seven complete mitochondrial genome sequences of bushtits (Passeriformes, Aegithalidae, Aegithalos): the evolution pattern in duplicated control regions

The control region (CR) of the mitochondrial DNA exhibits important functions in replication and transcription, and duplications of the CR have been reported in a wide range of animal groups. In most cases, concerted evolution is expected to explain the high similarity of duplicated CRs. In this paper, we present seven complete mitochondrial genome sequences from the bushtits (genus Aegithalos), in which we discovered two duplicated CRs, and try to survey the evolution pattern of these duplicated CRs. We also found that the duplicated CRs within one individual were almost identical, and variations were concentrated in two sections, one located between a poly-C site and a potential TAS (termination associated sequence) element, the other one located at the 3' end of the duplicated CRs. The phylogenetic analyses of paralogous CRs showed that the tree topology were depending on whether the two high variable regions at the upstream of TAS element and the 3'end of duplicated CRs: when they were concluded, the orthologous copies were closely related; when they were excluded, the paralogous copies in the same lineages were closely related. This may suggest the role of recombination in the evolution of duplicated CRs. Consequently, the recombination was detected, and the breakpoints were found at ∼120 bp (the upstream of the potential TAS element) and ∼1150 bp of the alignment of duplicated CRs. According to these results, we supposed that homologous recombination occurred between paralogous CRs from different mtDNA molecule was proposed as the most suitable mechanism for concerted evolution of the duplicated CRs, and the recombination took place in every replication cycle, so that most part of the duplicated regions remain identical within an individual, while the 5' and 3'end of the duplicated CRs were not involved in recombination, and evolved independently.

Genetic variation in nuclear and mitochondrial markers supports a large sex difference in lifetime reproductive skew in a lekking species

Sex differences in skews of vertebrate lifetime reproductive success are difficult to measure directly. Evolutionary histories of differential skew should be detectable in the genome. For example, male-biased skew should reduce variation in the biparentally inherited genome relative to the maternally inherited genome. We tested this approach in lek-breeding ruff (Class Aves, Philomachus pugnax) by comparing genetic variation of nuclear microsatellites (θ_n; biparental) versus mitochondrial D-loop sequences (θ_m; maternal), and conversion to comparable nuclear (N_e) and female (N_ef) effective population size using published ranges of mutation rates for each marker (μ). We provide a Bayesian method to calculate N_e (θ_n = 4N_eμ_n) and N_ef (θ_m = 2N_efμ_m) using 95% credible intervals (CI) of θ_n and θ_m as informative priors, and accounting for uncertainty in μ. In 96 male ruffs from one population, N_e was 97% (79–100%) lower than expected under random mating in an ideal population, where N_e:N_ef = 2. This substantially lower autosomal variation represents the first genomic support of strong male reproductive skew in a lekking species.

The complete mitochondrial genomes of sixteen ardeid birds revealing the evolutionary process of the gene rearrangements

BACKGROUND

The animal mitochondrial genome is generally considered to be under selection for both compactness and gene order conservation. As more mitochondrial genomes are sequenced, mitochondrial duplications and gene rearrangements have been frequently identified among diverse animal groups. Although several mechanisms of gene rearrangement have been proposed thus far, more observational evidence from major taxa is needed to validate specific mechanisms. In the current study, the complete mitochondrial DNA of sixteen bird species from the family Ardeidae was sequenced and the evolution of mitochondrial gene rearrangements was investigated. The mitochondrial genomes were then used to review the phylogenies of these ardeid birds.

RESULTS

The complete mitochondrial genome sequences of the sixteen ardeid birds exhibited four distinct mitochondrial gene orders in which two of them, named as "duplicate tRNA(Glu)-CR" and "duplicate tRNAThr-tRNA(Pro) and CR", were newly discovered. These gene rearrangements arose from an evolutionary process consistent with the tandem duplication--random loss model (TDRL). Additionally, duplications in these gene orders were near identical in nucleotide sequences within each individual, suggesting that they evolved in concert. Phylogenetic analyses of the sixteen ardeid species supported the idea that Ardea ibis, Ardea modesta and Ardea intermedia should be classified as genus Ardea, and Ixobrychus flavicollis as genus Ixobrychus, and indicated that within the subfamily Ardeinae, Nycticorax nycticorax is closely related to genus Egretta and that Ardeola bacchus and Butorides striatus are closely related to the genus Ardea.

CONCLUSIONS

The duplicate tRNAThr-CR gene order is found in most ardeid lineages, suggesting this gene order is the ancestral pattern within these birds and persisted in most lineages via concerted evolution. In two independent lineages, when the concerted evolution stopped in some subsections due to the accumulation of numerous substitutions and deletions, the duplicate tRNAThr-CR gene order was transformed into three other gene orders. The phylogenetic trees produced from concatenated rRNA and protein coding genes have high support values in most nodes, indicating that the mitochondrial genome sequences are promising markers for resolving the phylogenetic issues of ardeid birds when more taxa are added.

A novel model of double replications and random loss accounts for rearrangements in the Mitogenome of Samariscus latus (Teleostei: Pleuronectiformes)

BACKGROUND

Although more than one thousand complete mitochondrial DNA (mtDNA) sequences have been determined in teleostean fishes, only a few gene rearrangements have been observed, and genome-scale rearrangements are even rarer. However, flatfishes (Pleuronectiformes) have been identified as having diverse types of mitochondrial gene rearrangements. It has been reported that tongue soles and the blue flounder mitogenomes exhibit different types of large-scale gene rearrangements.

RESULTS

In the present study, the complete mitochondrial genome of another flatfish, Samariscus latus, was sequenced, and genome-scale rearrangements were observed. The genomic features of this flounder are different from those of any other studied vertebrates, including flatfish species too. The mitogenome of S. latus is characterized by the duplication and translocation of the control region (CR). The genes located between the two CRs are divided into two clusters in which their relative orders are maintained.

CONCLUSIONS

We propose a "Double Replications and Random Loss" model to explain the rearrangement events in S. latus mitogenome. This model consists of the following steps. First, the CR was duplicated and translocated. Subsequently, double replications of the mitogenome were successively initiated from the two CRs, leading to the duplication of the genes between the two CRs. Finally, one of each pair of duplicated genes was lost in a random event.

Recombination and evolution of duplicate control regions in the mitochondrial genome of the Asian big-headed turtle, Platysternon megacephalum

Complete mitochondrial (mt) genome sequences with duplicate control regions (CRs) have been detected in various animal species. In Testudines, duplicate mtCRs have been reported in the mtDNA of the Asian big-headed turtle, Platysternon megacephalum, which has three living subspecies. However, the evolutionary pattern of these CRs remains unclear. In this study, we report the completed sequences of duplicate CRs from 20 individuals belonging to three subspecies of this turtle and discuss the micro-evolutionary analysis of the evolution of duplicate CRs. Genetic distances calculated with MEGA 4.1 using the complete duplicate CR sequences revealed that within turtle subspecies, genetic distances between orthologous copies from different individuals were 0.63% for CR1 and 1.2% for CR2app:addword:respectively, and the average distance between paralogous copies of CR1 and CR2 was 4.8%. Phylogenetic relationships were reconstructed from the CR sequences, excluding the variable number of tandem repeats (VNTRs) at the 3' end using three methods: neighbor-joining, maximum likelihood algorithm, and Bayesian inference. These data show that any two CRs within individuals were more genetically distant from orthologous genes in different individuals within the same subspecies. This suggests independent evolution of the two mtCRs within each P. megacephalum subspecies. Reconstruction of separate phylogenetic trees using different CR components (TAS, CD, CSB, and VNTRs) suggested the role of recombination in the evolution of duplicate CRs. Consequently, recombination events were detected using RDP software with break points at ≈290 bp and ≈1,080 bp. Based on these results, we hypothesize that duplicate CRs in P. megacephalum originated from heterological ancestral recombination of mtDNA. Subsequent recombination could have resulted in homogenization during independent evolutionary events, thus maintaining the functions of duplicate CRs in the mtDNA of P. megacephalum.

The largest unassigned regions of the male- and female-transmitted mitochondrial DNAs in Musculista senhousia (Bivalvia Mytilidae)

Musculista senhousia is a marine mussel with doubly uniparental inheritance (DUI) of mitochondria. In this study we analyzed the largest unassigned region (LUR) of its female- and male-transmitted mitochondrial genomes, described their fine characteristics and searched for shared features. Our results suggest that both LURs contain the control region of their respective mitochondrial genomes. The female-transmitted control region is duplicated in tandem, with the two copies evolving in concert. This makes the F-mtDNA of M. senhousia the first Bivalve mitochondrial genome with this feature. We also compared M. senhousia control regions to that of other Mytilidae, and demonstrated that signals for basic mtDNA functions are retained over evolutionary times even among the fast-evolving mitochondrial genomes of DUI species. Finally, we discussed how similarities between female and male LURs may be explained in the context of DUI evolution and if the duplicated female control region might have influenced the DUI system in this species.

Intraspecific rearrangement of duplicated mitochondrial control regions in the Luzon Tarictic Hornbill Penelopides manillae (Aves: Bucerotidae)

Philippine hornbills of the genera Aceros and Penelopides (Bucerotidae) are known to possess a large tandemly duplicated fragment in their mitochondrial genome, whose paralogous parts largely evolve in concert. In the present study, we surveyed the two distinguishable duplicated control regions in several individuals of the Luzon Tarictic Hornbill Penelopides manillae, compare their characteristics within and across individuals, and report on an intraspecific mitochondrial gene rearrangement found in one single specimen, i.e., an interchange between the two control regions. To our knowledge, this is the first observation of two distinct mitochondrial genome rearrangements within a bird species. We briefly discuss a possible evolutionary mechanism responsible for this pattern, and highlight potential implications for the application of control region sequences as a marker in population genetics and phylogeography.

Multiple independent origins of mitochondrial control region duplications in the order Psittaciformes

Mitochondrial genomes are generally thought to be under selection for compactness, due to their small size, consistent gene content, and a lack of introns or intergenic spacers. As more animal mitochondrial genomes are fully sequenced, rearrangements and partial duplications are being identified with increasing frequency, particularly in birds (Class Aves). In this study, we investigate the evolutionary history of mitochondrial control region states within the avian order Psittaciformes (parrots and cockatoos). To this aim, we reconstructed a comprehensive multi-locus phylogeny of parrots, used PCR of three diagnostic fragments to classify the mitochondrial control region state as single or duplicated, and mapped these states onto the phylogeny. We further sequenced 44 selected species to validate these inferences of control region state. Ancestral state reconstruction using a range of weighting schemes identified six independent origins of mitochondrial control region duplications within Psittaciformes. Analysis of sequence data showed that varying levels of mitochondrial gene and tRNA homology and degradation were present within a given clade exhibiting duplications. Levels of divergence between control regions within an individual varied from 0-10.9% with the differences occurring mainly between 51 and 225 nucleotides 3' of the goose hairpin in domain I. Further investigations into the fates of duplicated mitochondrial genes, the potential costs and benefits of having a second control region, and the complex relationship between evolutionary rates, selection, and time since duplication are needed to fully explain these patterns in the mitochondrial genome.

Full mitochondrial genome sequences of two endemic Philippine hornbill species (Aves: Bucerotidae) provide evidence for pervasive mitochondrial DNA recombination

Background

Although nowaday it is broadly accepted that mitochondrial DNA (mtDNA) may undergo recombination, the frequency of such recombination remains controversial. Its estimation is not straightforward, as recombination under homoplasmy (i.e., among identical mt genomes) is likely to be overlooked. In species with tandem duplications of large mtDNA fragments the detection of recombination can be facilitated, as it can lead to gene conversion among duplicates. Although the mechanisms for concerted evolution in mtDNA are not fully understood yet, recombination rates have been estimated from "one per speciation event" down to 850 years or even "during every replication cycle".

Results

Here we present the first complete mt genome of the avian family Bucerotidae, i.e., that of two Philippine hornbills, Aceros waldeni and Penelopides panini. The mt genomes are characterized by a tandemly duplicated region encompassing part of cytochrome b, 3 tRNAs, NADH6, and the control region. The duplicated fragments are identical to each other except for a short section in domain I and for the length of repeat motifs in domain III of the control region. Due to the heteroplasmy with regard to the number of these repeat motifs, there is some size variation in both genomes; with around 21,657 bp (A. waldeni) and 22,737 bp (P. panini), they significantly exceed the hitherto longest known avian mt genomes, that of the albatrosses. We discovered concerted evolution between the duplicated fragments within individuals. The existence of differences between individuals in coding genes as well as in the control region, which are maintained between duplicates, indicates that recombination apparently occurs frequently, i.e., in every generation.

Conclusions

The homogenised duplicates are interspersed by a short fragment which shows no sign of recombination. We hypothesize that this region corresponds to the so-called Replication Fork Barrier (RFB), which has been described from the chicken mitochondrial genome. As this RFB is supposed to halt replication, it offers a potential mechanistic explanation for frequent recombination in mitochondrial genomes.