Evidence of two deeply divergent co-existing mitochondrial genomes in the Tuatara reveals an extremely complex genomic organization

Genomic data support reticulate evolution in whiptail lizards from the Brazilian Caatinga

Species relationships have traditionally been represented by phylogenetic trees, but not all evolutionary histories fit into bifurcating divergence models. Introgressive hybridization challenges this assumption by sometimes [or maybe often] leading to mitochondrial introgression, wherein one species' mitochondrial genome is entirely replaced by another's (mitochondrial capture). Such processes result in mitonuclear discrepancies, complicating species delimitation and phylogenetic inference. In our study, we used ultraconserved elements (UCE) and mitogenomic data to investigate the evolutionary history of the Ameivula ocellifera complex, a group of South American whiptail lizards widely distributed in semiarid environments of the Caatinga Domain in Brazil. We examine mitonuclear discordances, assessing reticulate evolution, evaluating species limits, and testing for adaptive mitochondrial capture that could explain higher introgression in the mitochondrial genome compared to nuclear DNA. Our findings support the occurrence of an ancient reticulation event during the diversification of these lizards, driven by introgressive hybridization, leading to mitochondrial capture, and explaining mitonuclear discrepancies. Overall, we did not find clear evidence of positive selection across mitochondrial protein-coding genes suggesting adaptive mitochondrial capture of individuals with introgressed mtDNA. Thus, the genetic diversification and mitogenome evolution could be neutral, with selection against hybridization in the autosomal loci only, or even mediated by mitonuclear incompatibilities. Analyses of mtDNA genomes alongside network and species delimitation methods were crucial for identifying and validating individuals with introgressed mtDNA as a distinct species, demonstrating the potential of genome sampling, and using innovative analytical techniques for elucidating speciation processes in the presence of introgressive hybridization.

Testing ultraconserved elements (UCEs) for phylogenetic inference across bivalves (Mollusca: Bivalvia)

Bivalves constitute an important resource for fisheries and as cultural objects. Bivalve phylogenetics has had a long tradition using both morphological and molecular characters, and genomic resources are available for a good number of commercially important species. However, relationships among bivalve families have been unstable and major conflicting results exist between mitogenomics and results based on Sanger-based amplicon sequencing or phylotranscriptomics. Here we design and test an ultraconserved elements probe set for the class Bivalvia with the aim to use hundreds of loci without the need to sequence full genomes or transcriptomes, which are expensive and complex to analyze, and to open bivalve phylogenetics to museum specimens. Our probe set successfully captured 1,513 UCEs for a total of 263,800 bp with an average length of 174.59 ± 3.44 per UCE (ranging from 28 to 842 bp). Phylogenetic testing of this UCE probe set across Bivalvia and within the family Donacidae using different data matrices and methods for phylogenetic inference shows promising results at multiple taxonomic levels. In addition, our probe set was able to capture large numbers of UCEs for museum specimens collected before 1900 and from DNAs properly stored, of which many museums and laboratories are well stocked. Overall, this constitutes a novel and useful resource for bivalve phylogenetics.

The Phylogenetic Relationships of Major Lizard Families Using Mitochondrial Genomes and Selection Pressure Analyses in Anguimorpha

Anguimorpha, within the order Squamata, represents a group with distinct morphological and behavioral characteristics in different ecological niches among lizards. Within Anguimorpha, there is a group characterized by limb loss, occupying lower ecological niches, concentrated within the subfamily Anguinae. Lizards with limbs and those without exhibit distinct locomotor abilities when adapting to their habitats, which in turn necessitate varying degrees of energy expenditure. Mitochondria, known as the metabolic powerhouses of cells, play a crucial role in providing approximately 95% of an organism's energy. Functionally, mitogenomes (mitochondrial genomes) can serve as a valuable tool for investigating potential adaptive evolutionary selection behind limb loss in reptiles. Due to the variation of mitogenome structures among each species, as well as its simple genetic structure, maternal inheritance, and high evolutionary rate, the mitogenome is increasingly utilized to reconstruct phylogenetic relationships of squamate animals. In this study, we sequenced the mitogenomes of two species within Anguimorpha as well as the mitogenomes of two species in Gekkota and four species in Scincoidea. We compared these data with the mitogenome content and evolutionary history of related species. Within Anguimorpha, between the mitogenomes of limbless and limbed lizards, a branch-site model analysis supported the presence of 10 positively selected sites: Cytb protein (at sites 183 and 187), ND2 protein (at sites 90, 155, and 198), ND3 protein (at site 21), ND5 protein (at sites 12 and 267), and ND6 protein (at sites 72 and 119). These findings suggested that positive selection of mitogenome in limbless lizards may be associated with the energy requirements for their locomotion. Additionally, we acquired data from 205 mitogenomes from the NCBI database. Bayesian inference (BI) and Maximum Likelihood (ML) trees were constructed using the 13 mitochondrial protein-coding genes (PCGs) and two rRNAs (12S rRNA and 16S rRNA) from 213 mitogenomes. Our phylogenetic tree and the divergence time estimates for Squamata based on mitogenome data are consistent with results from previous studies. Gekkota was placed at the root of Squamata in both BI and ML trees. However, within the Toxicofera clade, due to long-branch attraction, Anguimorpha and (Pleurodonta + (Serpentes + Acrodonta)) were closely related groupings, which might indicate errors and also demonstrate that mitogenome-based phylogenetic trees may not effectively resolve long-branch attraction issues. Additionally, we reviewed the origin and diversification of Squamata throughout the Mesozoic era, suggesting that Squamata originated in the Late Triassic (206.05 Mya), with the diversification of various superfamilies occurring during the Cretaceous period. Future improvements in constructing squamate phylogenetic relationships using mitogenomes will rely on identifying snake and acrodont species with slower evolutionary rates, ensuring comprehensive taxonomic coverage of squamate diversity, and increasing the number of genes analyzed.

The invasive land flatworm Arthurdendyus triangulatus has repeated sequences in the mitogenome, extra-long cox2 gene and paralogous nuclear rRNA clusters

Using a combination of short- and long-reads sequencing, we were able to sequence the complete mitochondrial genome of the invasive 'New Zealand flatworm' Arthurdendyus triangulatus (Geoplanidae, Rhynchodeminae, Caenoplanini) and its two complete paralogous nuclear rRNA gene clusters. The mitogenome has a total length of 20,309 bp and contains repetitions that includes two types of tandem-repeats that could not be solved by short-reads sequencing. We also sequenced for the first time the mitogenomes of four species of Caenoplana (Caenoplanini). A maximum likelihood phylogeny associated A. triangulatus with the other Caenoplanini but Parakontikia ventrolineata and Australopacifica atrata were rejected from the Caenoplanini and associated instead with the Rhynchodemini, with Platydemus manokwari. It was found that the mitogenomes of all species of the subfamily Rhynchodeminae share several unusual structural features, including a very long cox2 gene. This is the first time that the complete paralogous rRNA clusters, which differ in length, sequence and seemingly number of copies, were obtained for a Geoplanidae.

Evidence for hybridization-driven heteroplasmy maintained across generations in a ricefish endemic to a Wallacean ancient lake

Heteroplasmy, the presence of multiple mitochondrial DNA (mtDNA) haplotypes within cells of an individual, is caused by mutation or paternal leakage. However, heteroplasmy is usually resolved to homoplasmy within a few generations because of germ-line bottlenecks; therefore, instances of heteroplasmy are limited in nature. Here, we report heteroplasmy in the ricefish species Oryzias matanensis, endemic to Lake Matano, an ancient lake in Sulawesi Island, in which one individual was known to have many heterozygous sites in the mitochondrial NADH dehydrogenase subunit 2 (ND2) gene. In this study, we cloned the ND2 gene for some additional individuals with heterozygous sites and demonstrated that they are truly heteroplasmic. Phylogenetic analysis revealed that the extra haplotype within the heteroplasmic O. matanensis individuals clustered with haplotypes of O. marmoratus, a congeneric species inhabiting adjacent lakes. This indicated that the heteroplasmy originated from paternal leakage due to interspecific hybridization. The extra haplotype was unique and contained two non-synonymous substitutions. These findings demonstrate that this hybridization-driven heteroplasmy was maintained across generations for a long time to the extent that the extra mitochondria evolved within the new host.

Novel mitochondrial genome rearrangements including duplications and extensive heteroplasmy could underlie temperature adaptations in Antarctic notothenioid fishes

Mitochondrial genomes are known for their compact size and conserved gene order, however, recent studies employing long-read sequencing technologies have revealed the presence of atypical mitogenomes in some species. In this study, we assembled and annotated the mitogenomes of five Antarctic notothenioids, including four icefishes (Champsocephalus gunnari, C. esox, Chaenocephalus aceratus, and Pseudochaenichthys georgianus) and the cold-specialized Trematomus borchgrevinki. Antarctic notothenioids are known to harbor some rearrangements in their mt genomes, however the extensive duplications in icefishes observed in our study have never been reported before. In the icefishes, we observed duplications of the protein coding gene ND6, two transfer RNAs, and the control region with different copy number variants present within the same individuals and with some ND6 duplications appearing to follow the canonical Duplication-Degeneration-Complementation (DDC) model in C. esox and C. gunnari. In addition, using long-read sequencing and k-mer analysis, we were able to detect extensive heteroplasmy in C. aceratus and C. esox. We also observed a large inversion in the mitogenome of T. borchgrevinki, along with the presence of tandem repeats in its control region. This study is the first in using long-read sequencing to assemble and identify structural variants and heteroplasmy in notothenioid mitogenomes and signifies the importance of long-reads in resolving complex mitochondrial architectures. Identification of such wide-ranging structural variants in the mitogenomes of these fishes could provide insight into the genetic basis of the atypical icefish mitochondrial physiology and more generally may provide insights about their potential role in cold adaptation.

The mitochondrial genome of the bioluminescent fish Malacosteus niger Ayres, 1848 (Stomiidae, Actinopterygii) is large and complex, and contains an inverted-repeat structure

We determined the complete mitogenome sequence of the bioluminescent fish Malacosteus niger using long-read sequencing technologies. The 21,263 bp mitogenome features a complex structure with two copies of a 1198-bp inverted-repeat and a region of 2616-bp containing alternating copies of 16 and 26 bp repeat elements. Whole mitogenome phylogenies inferred from both nucleotide and amino-acid datasets place M. niger among Melanostomiinae. The need for additional complete mitogenome sequences from the subfamily Malacosteinae is discussed.

Genome Evolution and the Future of Phylogenomics of Non-Avian Reptiles

Non-avian reptiles comprise a large proportion of amniote vertebrate diversity, with squamate reptiles-lizards and snakes-recently overtaking birds as the most species-rich tetrapod radiation. Despite displaying an extraordinary diversity of phenotypic and genomic traits, genomic resources in non-avian reptiles have accumulated more slowly than they have in mammals and birds, the remaining amniotes. Here we review the remarkable natural history of non-avian reptiles, with a focus on the physical traits, genomic characteristics, and sequence compositional patterns that comprise key axes of variation across amniotes. We argue that the high evolutionary diversity of non-avian reptiles can fuel a new generation of whole-genome phylogenomic analyses. A survey of phylogenetic investigations in non-avian reptiles shows that sequence capture-based approaches are the most commonly used, with studies of markers known as ultraconserved elements (UCEs) especially well represented. However, many other types of markers exist and are increasingly being mined from genome assemblies in silico, including some with greater information potential than UCEs for certain investigations. We discuss the importance of high-quality genomic resources and methods for bioinformatically extracting a range of marker sets from genome assemblies. Finally, we encourage herpetologists working in genomics, genetics, evolutionary biology, and other fields to work collectively towards building genomic resources for non-avian reptiles, especially squamates, that rival those already in place for mammals and birds. Overall, the development of this cross-amniote phylogenomic tree of life will contribute to illuminate interesting dimensions of biodiversity across non-avian reptiles and broader amniotes.

Signatures of positive selection in the mitochondrial genome of neotropical freshwater stingrays provide clues about the transition from saltwater to freshwater environment

Neotropical freshwater stingrays (subfamily Potamotrygoninae) are carnivorous bottom feeder batoids widely distributed in most river basins of South America. They represent the unique extant group of elasmobranchs that evolved to live exclusively in freshwater environments. These species are exploited either by commercial fisheries (e.g., for food or ornamental industry) or by indigenous communities allocated along with their natural range. Restrictive life history characteristics coupled with habitat degradation make Potamotrygoninae species highly vulnerable to human impacts and highlight the necessity of studies to inform basic biological aspects, from ecology to genetics, to guide their conservation and clarify the molecular basis of adaptation to the freshwater environment. We used available and newly assembled Potamotrygon spp. mitogenomes to perform a comparative investigation of their molecular evolution. A phylogenetic estimation using the mitogenome of Potamotrygon falkneri and other Elasmobranchii supports monophyly for Potamotrygonidae and indicates a close relationship to Dasyatidae. A synteny analysis comprising 3 Potamotrygon and other 51 batoids revealed a highly conserved mitogenomic context. We detected various amino acid sites under positive selection exclusively in Potamotrygon spp., within the sequences of ND4, ND5, ND6, and COXII genes. Positively selected mutational events in key genes of energetic metabolism may be related to the physiological adaptation of Potamotrygon spp. during the ancient incursion into freshwater. This broad comparative mitogenomic study provides novel insights into the evolutionary history of neotropical freshwater stingrays and their relatives and stands out as a valuable resource to aid in current and future research on elasmobranch molecular evolution.

Inheritance through the cytoplasm

Most heritable information in eukaryotic cells is encoded in the nuclear genome, with inheritance patterns following classic Mendelian segregation. Genomes residing in the cytoplasm, however, prove to be a peculiar exception to this rule. Cytoplasmic genetic elements are generally maternally inherited, although there are several exceptions where these are paternally, biparentally or doubly-uniparentally inherited. In this review, we examine the diversity and peculiarities of cytoplasmically inherited genomes, and the broad evolutionary consequences that non-Mendelian inheritance brings. We first explore the origins of vertical transmission and uniparental inheritance, before detailing the vast diversity of cytoplasmic inheritance systems across Eukaryota. We then describe the evolution of genomic organisation across lineages, how this process has been shaped by interactions with the nuclear genome and population genetics dynamics. Finally, we discuss how both nuclear and cytoplasmic genomes have evolved to co-inhabit the same host cell via one of the longest symbiotic processes, and all the opportunities for intergenomic conflict that arise due to divergence in inheritance patterns. In sum, we cannot understand the evolution of eukaryotes without understanding hereditary symbiosis.

Comparative genomic analysis of vertebrate mitochondrial reveals a differential of rearrangements rate between taxonomic class

Vertebrate mitochondrial genomes have been extensively studied for genetic and evolutionary purposes, these are normally believed to be extremely conserved, however, different cases of gene rearrangements have been reported. To verify the level of rearrangement and the mitogenome evolution, we performed a comparative genomic analysis of the 2831 vertebrate mitochondrial genomes representing 12 classes available in the NCBI database. Using a combination of bioinformatics methods, we determined there is a high number of errors in the annotation of mitochondrial genes, especially in tRNAs. We determined there is a large variation in the proportion of rearrangements per gene and per taxonomic class, with higher values observed in Actinopteri, Amphibia and Reptilia. We highlight that these are results for currently available vertebrate sequences, so an increase in sequence representativeness in some groups may alter the rearrangement rates, so in a few years it would be interesting to see if these rates are maintained or altered with the new mitogenome sequences. In addition, within each vertebrate class, different patterns in rearrangement proportion with distinct hotspots in the mitochondrial genome were found. We also determined that there are eleven convergence events in gene rearrangement, nine of which are new reports to the scientific community.

Genome and cuticular hydrocarbon-based species delimitation shed light on potential drivers of speciation in a Neotropical ant species complex

Geographic separation that leads to the evolution of reproductive isolation between populations generally is considered the most common form of speciation. However, speciation may also occur in the absence of geographic barriers due to phenotypic and genotypic factors such as chemical cue divergence, mating signal divergence, and mitonuclear conflict. Here, we performed an integrative study based on two genome-wide techniques (3RAD and ultraconserved elements) coupled with cuticular hydrocarbon (CHC) and mitochondrial (mt) DNA sequence data, to assess the species limits within the Ectatomma ruidum species complex, a widespread and conspicuous group of Neotropical ants for which heteroplasmy (i.e., presence of multiple mtDNA variants in an individual) has been recently discovered in some populations from southeast Mexico. Our analyses indicate the existence of at least five distinct species in this complex: two widely distributed across the Neotropics, and three that are restricted to southeast Mexico and that apparently have high levels of heteroplasmy. We found that species boundaries in the complex did not coincide with geographic barriers. We therefore consider possible roles of alternative drivers that may have promoted the observed patterns of speciation, including mitonuclear incompatibility, CHC differentiation, and colony structure. Our study highlights the importance of simultaneously assessing different sources of evidence to disentangle the species limits of taxa with complicated evolutionary histories.

Long-read sequencing reveals atypical mitochondrial genome structure in a New Zealand marine isopod

Most animal mitochondrial genomes are small, circular and structurally conserved. However, recent work indicates that diverse taxa possess unusual mitochondrial genomes. In Isopoda, species in multiple lineages have atypical and rearranged mitochondrial genomes. However, more species of this speciose taxon need to be evaluated to understand the evolutionary origins of atypical mitochondrial genomes in this group. In this study, we report the presence of an atypical mitochondrial structure in the New Zealand endemic marine isopod, Isocladus armatus. Data from long- and short-read DNA sequencing suggest that I. armatus has two mitochondrial chromosomes. The first chromosome consists of two mitochondrial genomes that have been inverted and fused together in a circular form, and the second chromosome consists of a single mitochondrial genome in a linearized form. This atypical mitochondrial structure has been detected in other isopod lineages, and our data from an additional divergent isopod lineage (Sphaeromatidae) lends support to the hypothesis that atypical structure evolved early in the evolution of Isopoda. Additionally, we find that an asymmetrical site previously observed across many species within Isopoda is absent in I. armatus, but confirm the presence of two asymmetrical sites recently reported in two other isopod species.

Bivalve molluscs as model systems for studying mitochondrial biology

The class Bivalvia is a highly successful and ancient taxon including ∼25,000 living species. During their long evolutionary history bivalves adapted to a wide range of physicochemical conditions, habitats, biological interactions, and feeding habits. Bivalves can have strikingly different size, and despite their apparently simple body plan, they evolved very different shell shapes, and complex anatomic structures. One of the most striking features of this class of animals is their peculiar mitochondrial biology: some bivalves have facultatively anaerobic mitochondria that allow them to survive prolonged periods of anoxia/hypoxia. Moreover, more than 100 species have now been reported showing the only known evolutionarily stable exception to the strictly maternal inheritance of mitochondria in animals, named doubly uniparental inheritance. Mitochondrial activity is fundamental to eukaryotic life, and thanks to their diversity and uncommon features, bivalves represent a great model system to expand our knowledge about mitochondrial biology, so far limited to a few species. We highlight recent works studying mitochondrial biology in bivalves at either genomic or physiological level. A link between these two approaches is still missing, and we believe that an integrated approach and collaborative relationships are the only possible ways to be successful in such endeavour.