Mitochondrial genomes of Columbicola feather lice are highly fragmented, indicating repeated evolution of minicircle-type genomes in parasitic lice

Mitochondrial genome fragmentation is correlated with increased rates of molecular evolution

While mitochondrial genome content and organization is quite diverse across all Eukaryotes, most bilaterian animal mitochondrial genomes (mitogenomes) exhibit highly conserved gene content and organisation, with genes typically encoded on a single circular chromosome. However, many species of parasitic lice (Insecta: Phthiraptera) are among the notable exceptions, having mitogenomes fragmented into multiple circular chromosomes. To better understand the process of mitogenome fragmentation, we conducted a large-scale genomic study of a major group of lice, Amblycera, with extensive taxon sampling. Analyses of the evolution of mitogenome structure across a phylogenomic tree of 90 samples from 53 genera revealed evidence for multiple independent origins of mitogenome fragmentation, some inferred to have occurred less than five million years ago. We leveraged these many independent origins of fragmentation to compare the rates of DNA substitution and gene rearrangement, specifically contrasting branches with fragmented and non-fragmented mitogenomes. We found that lineages with fragmented mitochondrial genomes had significantly higher rates of mitochondrial sequence evolution. In addition, lineages with fragmented mitochondrial genomes were more likely to have mitogenome gene rearrangements than those with single-chromosome mitochondrial genomes. By combining phylogenomics and mitochondrial genomics we provide a detailed portrait of mitogenome evolution across this group of insects with a remarkably unstable mitogenome structure, identifying processes of molecular evolution that are correlated with mitogenome fragmentation.

Mitochondrial Genome Fragmentation Occurred Multiple Times Independently in Bird Lice of the Families Menoponidae and Laemobothriidae

Mitochondrial (mt) genome fragmentation has been discovered in all five parvorders of parasitic lice (Phthiraptera). To explore whether minichromosomal characters derived from mt genome fragmentation are informative for phylogenetic studies, we sequenced the mt genomes of 17 species of bird lice in Menoponidae and Laemobothriidae (Amblycera). Four species of Menoponidae (Actornithophilus sp. 1 ex [pied oystercatcher], Act. sp. 2 ex [masked lapwing], Austromenopon sp. 2 ex [sooty tern and crested tern], Myr. sp. 1 ex [satin bowerbird]) have fragmented mt genomes, whereas the other 13 species retain the single-chromosome mt genomes. The two Actornithophilus species have five and six mt minichromosomes, respectively. Aus. sp. 2 ex [sooty tern and crested tern] has two mt minichromosomes, in contrast to Aus. sp. 1 ex [sooty shearwater], which has a single mt chromosome. Myr. sp. 1 ex [satin bowerbird] has four mt minichromosomes. When mapped on the phylogeny of Menoponidae and Laemobothriidae, it is evident that mt genome fragmentation has occurred multiple times independently among Menoponidae and Laemobothriidae species. We found derived mt minichromosomal characters shared between Myrsidea species, between Actornithophilus species, and between and among different ischnoceran genera, respectively. We conclude that while mt genome fragmentation as a general feature does not unite all the parasitic lice that have this feature, each independent mt genome fragmentation event does produce minichromosomal characters that can be informative for phylogenetic studies of parasitic lice at different taxonomic levels.

A Novel Mitochondrial Genome Fragmentation Pattern in the Buffalo Louse Haematopinus tuberculatus (Psocodea: Haematopinidae)

Sucking lice are obligate ectoparasites of mammalian hosts, causing serious public health problems and economic losses worldwide. It is well known that sucking lice have fragmented mitochondrial (mt) genomes, but many remain undetermined. To better understand patterns of mt genome fragmentation in the sucking lice, we sequenced the mt genome of the buffalo louse Haematopinus tuberculatus using next-generation sequencing (NGS). The mt genome of H. tuberculatus has ten circular minichromosomes containing a total of 37 genes. Each minichromosome is 2.9-5.0 kb long and carries one to eight genes plus one large non-coding region. The number of mt minichromosomes of H. tuberculatus (ten) is different from those of congeneric species (horse louse H. asini, domestic pig louse H. suis and wild pig louse H. apri) and other sucking lice. Two events (gene translocation and merger of mt minichromosome) are observed in Haematopinus. Compared to other studies, our phylogeny generated from mt genome datasets showed a different topology, suggesting that inclusion of data other than mt genomes would be required to resolve phylogeny of sucking lice. To our knowledge, this is the first report of a ten mt minichromosomes genome in sucking lice, which opens a new outlook into unexplored mt genome fragmentation patterns in sucking lice.

Genomic Approaches to Uncovering the Coevolutionary History of Parasitic Lice

Simple Summary

New sequencing technologies have now made it possible to sequence entire genomes for a diversity of life on earth. Parasites comprise nearly half of all species. Lice are one important group of parasites of birds and mammals, including humans. Genome sequencing approaches have been applied to this group of parasites to uncover patterns of diversification. These patterns can be compared to the patterns of diversification in their hosts. Key findings from these studies have revealed that parasitic lice likely originated on birds and then switched to mammals multiple times. Within groups of birds and mammals, the evolutionary trees of lice match those for mammal hosts more than those for birds. Genomic approaches have also revealed that individual birds and mammals harbor distinct populations of lice. Thus, these new techniques allow for the study of patterns of diversification at a wide variety of scales.

Abstract

Next-generation sequencing technologies are revolutionizing the fields of genomics, phylogenetics, and population genetics. These new genomic approaches have been extensively applied to a major group of parasites, the lice (Insecta: Phthiraptera) of birds and mammals. Two louse genomes have been assembled and annotated to date, and these have opened up new resources for the study of louse biology. Whole genome sequencing has been used to assemble large phylogenomic datasets for lice, incorporating sequences of thousands of genes. These datasets have provided highly supported trees at all taxonomic levels, ranging from relationships among the major groups of lice to those among closely related species. Such approaches have also been applied at the population scale in lice, revealing patterns of population subdivision and inbreeding. Finally, whole genome sequence datasets can also be used for additional study beyond that of the louse nuclear genome, such as in the study of mitochondrial genome fragmentation or endosymbiont function.

Independent evolution of highly variable, fragmented mitogenomes of parasitic lice

The mitochondrial genomes (mitogenomes) of bilaterian animals are highly conserved structures that usually consist of a single circular chromosome. However, several species of parasitic lice (Insecta: Phthiraptera) possess fragmented mitogenomes, where the mitochondrial genes are present on separate, circular chromosomes. Nevertheless, the extent, causes, and consequences of this structural variation remain poorly understood. Here, we combined new and existing data to better understand the evolution of mitogenome fragmentation in major groups of parasitic lice. We found strong evidence that fragmented mitogenomes evolved many times within parasitic lice and that the level of fragmentation is highly variable, including examples of heteroplasmic arrangements. We also found a significant association between mitochondrial fragmentation and signatures of relaxed selection. Mitochondrial fragmentation was also associated with changes to a lower AT%, possibly due to differences in mutation biases. Together, our results provide a significant advance in understanding the process of mitogenome fragmentation and provide an important perspective on mitochondrial evolution in eukaryotes.

The Complete Mitochondrial Genome of the Chicken Body Louse, Menacanthus cornutus, and Evolutionary Patterns of Extensive Gene Rearrangements in the Mitochondrial Genomes of Amblycera (Psocodea: Phthiraptera)

Animal mitochondrial (mt) genomes are typically double-strand circular DNA molecules, but diverse structural variations have been widely found in multiple groups. In parasitic lice (Phthiraptera), the structure of mt genomes varies remarkably across all five suborders. In this study, we reported the complete mt genome of a chicken body louse, Menacanthus cornutus, which has a typical single circular mt chromosome and drastic mt gene rearrangements. This mt genome is 15,693 bp in length, consisting of 13 protein-coding genes, 23 tRNA genes, 2 rRNA genes, and a control region. A comparison with a typical insect mt genome suggested that two highly similar trnM are present in the mt genome of M. cornutus. Moreover, almost every single gene was rearranged, and over half of mt genes were inverted. Phylogenetic analyses inferred from the mt genome sequences supported the monophyly and position of Amblycera. Mapped over the phylogenetic relationships of Amblycera, we identified two inversion events for the conserved gene blocks in Boopidae and Menoponidae. The inverted ND4L-ND4 was likely a synapomorphic rearrangement in Menoponidae. Our study demonstrated the importance of sequencing mt genomes for additional taxa to uncover the mechanism underlying the structural evolution of the mt genome in parasitic lice.

Fragmentation in mitochondrial genomes in relation to elevated sequence divergence and extreme rearrangements

Background

A single circular mitochondrial (mt) genome is a common feature across most metazoans. The mt-genome includes protein-coding genes involved in oxidative phosphorylation, as well as RNAs necessary for translation of mt-RNAs, whose order and number are highly conserved across animal clades, with few known exceptions of alternative mt-gene order or mt-genome architectures. One such exception consists of the fragmented mitochondrial genome, a type of genome architecture where mt-genes are split across two or more mt-chromosomes. However, the origins of mt-genome fragmentation and its effects on mt-genome evolution are unknown. Here, we investigate these origin and potential mechanisms underlying mt-genome fragmentation, focusing on a genus of booklice, Liposcelis, which exhibits elevated sequence divergence, frequent rearrangement of mt-gene order, and fragmentation of the mt genome, and compare them to other Metazoan clades.

Results

We found this genus Liposcelis exhibits very low conservation of mt-gene order across species, relative to other metazoans. Levels of gene order rearrangement were, however, unrelated to whether or not mt-genomes were fragmented or intact, suggesting mitochondrial genome fragmentation is not affecting mt-gene order directly. We further investigated possible mechanisms underpinning these patterns and revealed very high conservation of non-coding sequences at the edges of multiple recombination regions across populations of one particular Liposcelis species, supportive of a hypothesis that mt-fragmentation arises from recombination errors between mt-genome copies. We propose these errors may arise as a consequence of a heightened mutation rate in clades exhibiting mt-fragmentation. Consistent with this, we observed a striking pattern across three Metazoan phyla (Arthropoda, Nematoda, Cnidaria) characterised by members exhibiting high levels of mt-gene order rearrangement and cases of mt-fragmentation, whereby the mt-genomes of species more closely related to species with fragmented mt-genomes diverge more rapidly despite experiencing strong purifying selection.

Conclusions

We showed that contrary to expectations, mt-genome fragmentation is not correlated with the increase in mt-genome rearrangements. Furthermore, we present evidence that fragmentation of the mt-genome may be part of a general relaxation of a natural selection on the mt-genome, thus providing new insights into the origins of mt-genome fragmentation and evolution.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01218-7.

Frequent tRNA gene translocation towards the boundaries with control regions contributes to the highly dynamic mitochondrial genome organization of the parasitic lice of mammals

Background

The typical single-chromosome mitochondrial (mt) genome of animals has fragmented into multiple minichromosomes in the lineage Mitodivisia, which contains most of the parasitic lice of eutherian mammals. These parasitic lice differ from each other even among congeneric species in mt karyotype, i.e. the number of minichromosomes, and the gene content and gene order in each minichromosome, which is in stark contrast to the extremely conserved single-chromosome mt genomes across most animal lineages. How fragmented mt genomes evolved is still poorly understood. We use Polyplax sucking lice as a model to investigate how tRNA gene translocation shapes the dynamic mt karyotypes.

Results

We sequenced the full mt genome of the Asian grey shrew louse, Polyplax reclinata. We then inferred the ancestral mt karyotype for Polyplax lice and compared it with the mt karyotypes of the three Polyplax species sequenced to date. We found that tRNA genes were entirely responsible for mt karyotype variation among these three species of Polyplax lice. Furthermore, tRNA gene translocation observed in Polyplax lice was only between different types of minichromosomes and towards the boundaries with the control region. A similar pattern of tRNA gene translocation can also been seen in other sucking lice with fragmented mt genomes.

Conclusions

We conclude that inter-minichromosomal tRNA gene translocation orientated towards the boundaries with the control region is a major contributing factor to the highly dynamic mitochondrial genome organization in the parasitic lice of mammals.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07859-w.

Mitochondrial genome of Geomydoecus aurei, a pocket-gopher louse

Parasitic lice demonstrate an unusual array of mitochondrial genome architectures and gene arrangements. We characterized the mitochondrial genome of Geomydoecus aurei, a chewing louse (Phthiraptera: Trichodectidae) found on pocket gophers (Rodentia: Geomyidae) using reads from both Illumina and Oxford Nanopore sequencing coupled with PCR, cloning, and Sanger sequencing to verify structure and arrangement for each chromosome. The genome consisted of 12 circular mitochondrial chromosomes ranging in size from 1,318 to 2,088 nucleotides (nt). Total genome size was 19,015 nt. All 37 genes typical of metazoans (2 rRNA genes, 22 tRNA genes, and 13 protein-coding genes) were present. An average of 26% of each chromosome was composed of non-gene sequences. Within the non-gene region of each chromosome, there was a 79-nt nucleotide sequence that was identical among chromosomes and a conserved sequence with secondary structure that was always followed by a poly-T region. We hypothesize that these regions may be important in the initiation of transcription and DNA replication, respectively. The G. aurei genome shares 8 derived gene clusters with other chewing lice of mammals, but in G. aurei, genes on several chromosomes are not contiguous.

Mitochondrial Genomic Landscape: A Portrait of the Mitochondrial Genome 40 Years after the First Complete Sequence

Notwithstanding the initial claims of general conservation, mitochondrial genomes are a largely heterogeneous set of organellar chromosomes which displays a bewildering diversity in terms of structure, architecture, gene content, and functionality. The mitochondrial genome is typically described as a single chromosome, yet many examples of multipartite genomes have been found (for example, among sponges and diplonemeans); the mitochondrial genome is typically depicted as circular, yet many linear genomes are known (for example, among jellyfish, alveolates, and apicomplexans); the chromosome is normally said to be “small”, yet there is a huge variation between the smallest and the largest known genomes (found, for example, in ctenophores and vascular plants, respectively); even the gene content is highly unconserved, ranging from the 13 oxidative phosphorylation-related enzymatic subunits encoded by animal mitochondria to the wider set of mitochondrial genes found in jakobids. In the present paper, we compile and describe a large database of 27,873 mitochondrial genomes currently available in GenBank, encompassing the whole eukaryotic domain. We discuss the major features of mitochondrial molecular diversity, with special reference to nucleotide composition and compositional biases; moreover, the database is made publicly available for future analyses on the MoZoo Lab GitHub page.

Highly rearranged mitochondrial genome in Falcolipeurus lice (Phthiraptera: Philopteridae) from endangered eagles

Background

Fragmented mitochondrial (mt) genomes and extensive mt gene rearrangements have been frequently reported from parasitic lice (Insecta: Phthiraptera). However, relatively little is known about the mt genomes from the family Philopteridae, the most species-rich family within the suborder Ischnocera.

Methods

Herein, we use next-generation sequencing to decode the mt genome of Falcolipeurus suturalis and compare it with the mt genome of F. quadripustulatus. Phylogenetic relationships within the family Philopteridae were inferred from the concatenated 13 protein-coding genes of the two Falcolipeurus lice and members of the family Philopteridae using Bayesian inference (BI) and maximum likelihood (ML) methods.

Results

The complete mt genome of F. suturalis is a circular, double-stranded DNA molecule 16,659bp in size that contains 13 protein-coding genes, 22 transfer RNA genes, two ribosomal RNA genes, and three non-coding regions. The gene order of the F. suturalis mt genome is rearranged relative to that of F. quadripustulatus, and is radically different from both other louse species and the putative ancestral insect. Phylogenetic analyses revealed clear genetic distinctiveness between F. suturalis and F. quadripustulatus (Bayesian posterior probabilities=1.0 and bootstrapping frequencies=100), and that the genus Falcolipeurus is sister to the genus Ibidoecus (Bayesian posterior probabilities=1.0 and bootstrapping frequencies=100).

Conclusions

These datasets help to better understand gene rearrangements in lice and the phylogenetic position of Falcolipeurus and provide useful genetic markers for systematic studies of bird lice.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04776-5.

How are the mitochondrial genomes reorganized in Hexapoda? Differential evolution and the first report of convergences within Hexapoda

The evolution of the Hexapoda mitochondrial genome has been the focus of several genetic and evolutionary studies over the last decades. However, they have concentrated on certain taxonomic orders of economic or health importance. The recent increase of mitochondrial genomes sequencing of diverse taxonomic orders generates an important opportunity to clarify the evolution of this group of organisms. However, there is no comparative study that investigates the evolution of the Hexapoda mitochondrial genome. In order to verify the level of rearrangement and the mitochondrial genome evolution, we performed a comparative genomic analysis of the Hexapoda mitochondrial genome available in the NCBI database. Using a combination of bioinformatics methods to carefully examine the mitochondrial gene rearrangements in 1198 Hexapoda species belonging to 32 taxonomic orders, we determined that there is a great variation in the rate of rearrangement by gene and by taxonomic order. A higher rate of genetic reassortment is observed in Phthiraptera, Thysanoptera, Protura, and Hymenoptera; compared to other taxonomic orders. Twenty-four events of convergence in the genetic order between different taxonomic orders were determined, most of them not previously reported; which proves the great evolutionary dynamics within Hexapoda.

A novel fragmented mitochondrial genome in the protist pathogen Toxoplasma gondii and related tissue coccidia

Mitochondrial genome content and structure vary widely across the eukaryotic tree of life, with protists displaying extreme examples. Apicomplexan and dinoflagellate protists have evolved highly reduced mitochondrial genome sequences, mtDNA, consisting of only three cytochrome genes and fragmented rRNA genes. Here, we report the independent evolution of fragmented cytochrome genes in Toxoplasma and related tissue coccidia and evolution of a novel genome architecture consisting minimally of 21 sequence blocks (SBs) totaling 5.9 kb that exist as nonrandom concatemers. Single-molecule Nanopore reads consisting entirely of SBs ranging from 0.1 to 23.6 kb reveal both whole and fragmented cytochrome genes. Full-length cytochrome transcripts including a divergent coxIII are detected. The topology of the mitochondrial genome remains an enigma. Analysis of a cob point mutation reveals that homoplasmy of SBs is maintained. Tissue coccidia are important pathogens of man and animals, and the mitochondrion represents an important therapeutic target. The mtDNA sequence has been elucidated, but a definitive genome architecture remains elusive.

HERMES: An improved method to test mitochondrial genome molecular synapomorphies among clades

Mitochondrial chromosomes have diversified among eukaryotes and many different architectures and features are now acknowledged for this genome. Here we present the improved HERMES index, which can measure and quantify the amount of molecular change experienced by mitochondrial genomes. We test the improved approach with ten molecular phylogenetic studies based on complete mitochondrial genomes, representing six bilaterian Phyla. In most cases, HERMES analysis spotted out clades or single species with peculiar molecular synapomorphies, allowing to identify phylogenetic and ecological patterns. The software presented herein handles linear, circular, and multi-chromosome genomes, thus widening the HERMES scope to the complete eukaryotic domain.

Structure, gene order, and nucleotide composition of mitochondrial genomes in parasitic lice from Amblycera

Parasitic lice have unique mitochondrial (mt) genomes characterized by rearranged gene orders, variable genome structures, and less AT content compared to most other insects. However, relatively little is known about the mt genomes of Amblycera, the suborder sister to all other parasitic lice. Comparing among nine different genera (including representative of all seven families), we show that Amblycera have variable and highly rearranged mt genomes. Some genera have fragmented genomes that vary considerably in length, whereas others have a single mt chromosome. Notably, these genomes are more AT-biased than most other lice. We also recover genus-level phylogenetic relationships among Amblycera that are consistent with those reported from large nuclear datasets, indicating that mt sequences are reliable for reconstructing evolutionary relationships in Amblycera. However, gene order data cannot reliably recover these same relationships. Overall, our results suggest that the mt genomes of lice, already know to be distinctive, are even more variable than previously thought.

Variation of mitochondrial minichromosome composition in Hoplopleura lice (Phthiraptera: Hoplopleuridae) from rats

Background

The family Hoplopleuridae contains at least 183 species of blood-sucking lice, which widely parasitize both mice and rats. Fragmented mitochondrial (mt) genomes have been reported in two rat lice (Hoplopleura kitti and H. akanezumi) from this family, but some minichromosomes were unidentified in their mt genomes.

Methods

We sequenced the mt genome of the rat louse Hoplopleura sp. with an Illumina platform and compared its mt genome organization with H. kitti and H. akanezumi.

Results

Fragmented mt genome of the rat louse Hoplopleura sp. contains 37 genes which are on 12 circular mt minichromosomes. Each mt minichromosome is 1.8–2.7 kb long and contains 1–5 genes and one large non-coding region. The gene content and arrangement of mt minichromosomes of Hoplopleura sp. (n = 3) and H. kitti (n = 3) are different from those in H. akanezumi (n = 3). Phylogenetic analyses based on the deduced amino acid sequences of the eight protein-coding genes showed that the Hoplopleura sp. was more closely related to H. akanezumi than to H. kitti, and then they formed a monophyletic group.

Conclusions

Comparison among the three rat lice revealed variation in the composition of mt minichromosomes within the genus Hoplopleura. Hoplopleura sp. is the first species from the family Hoplopleuridae for which a complete fragmented mt genome has been sequenced. The new data provide useful genetic markers for studying the population genetics, molecular systematics and phylogenetics of blood-sucking lice.

Fragmented mitochondrial genomes evolved in opposite directions between closely related macaque louse Pedicinus obtusus and colobus louse Pedicinus badii

We report for the first time the fragmented mitochondrial (mt) genomes of two Pedicinus species: Pedicinus obtusus and Pedicinus badii, and compared them with the lice of humans and chimpanzees. Despite being congeneric, the two monkey lice are distinct from each other in mt karyotype. The variation in mt karyotype between the two Pedicinus lice is the most pronounced among the congeneric species of sucking lice observed to date and is attributable to the opposite directions between them in mt karyotype evolution. Two of the inferred ancestral mt minichromosomes of the higher primate lice merged as one in the macaque louse whereas one of the ancestral minichromosomes split into two in the colobus louse after these two species diverged from their most recent common ancestor. Our results showed that mt genome fragmentation was a two-way process in the higher primate lice, and minichromosome merger was more common than previously thought.