Extensive mitochondrial introgression in North American Great Black-backed Gulls (Larus marinus) from the American Herring Gull (Larus smithsonianus) with little nuclear DNA impact

Patterns of cytonuclear discordance and divergence between subspecies of the scarlet macaw (Ara macao) in Central America

The scarlet macaw, Ara macao, is a neotropical parrot that contains two described subspecies with broadly discrete geographical distributions. One subspecies, A. m. macao, is found from South America north into southwestern Costa Rica, while the second subspecies, A. m. cyanoptera, is found from eastern Costa Rica north into central Mexico. Our previous research using mitochondrial data to examine phylogeographical divergence across the collective range of these two subspecies concluded that they represent distinct evolutionary entities, with minimal contemporary hybridization between them. Here we further examine phylogenetic relationships and patterns of genetic variation between these two subspecies using a dataset of genetic markers derived from their nuclear genomes. Our analyses show clear nuclear divergence between A. m. macao and A. m. cyanoptera in Central America. Collectively however, samples from this region appear genetically more similar to one another than they do to the examined South American (Brazilian) A. m. macao sample. This observation contradicts our previous assessments based on mitochondrial DNA analyses that A. m. macao in Central and South America represent a single phylogeographical group that is evolutionarily distinct from Central American A. m. cyanoptera. Nonetheless, in agreement with our previous findings, ongoing genetic exchange between the two subspecies appears limited. Rather, our analyses indicate that incomplete lineage sorting is the best supported explanation for cytonuclear discordance within these parrots. High-altitude regions in Central America may act as a reproductive barrier, limiting contemporary hybridization between A. m. macao and A. m. cyanoptera. The phylogeographic complexities of scarlet macaw taxa in this region highlight the need for additional evolutionary examinations of these populations.

Mitonuclear discordance results from incomplete lineage sorting, with no detectable evidence for gene flow, in a rapid radiation of Todiramphus kingfishers

Many organisms possess multiple discrete genomes (i.e. nuclear and organellar), which are inherited separately and may have unique and even conflicting evolutionary histories. Phylogenetic reconstructions from these discrete genomes can yield different patterns of relatedness, a phenomenon known as cytonuclear discordance. In many animals, mitonuclear discordance (i.e. discordant evolutionary histories between the nuclear and mitochondrial genomes) has been widely documented, but its causes are often considered idiosyncratic and inscrutable. We show that a case of mitonuclear discordance in Todiramphus kingfishers can be explained by extensive genome-wide incomplete lineage sorting (ILS), likely a result of the explosive diversification history of this genus. For these kingfishers, quartet frequencies reveal that the nuclear genome is dominated by discordant topologies, with none of the internal branches in our consensus nuclear tree recovered in >50% of genome-wide gene trees. Meanwhile, a lack of inter-species shared ancestry, non-significant pairwise tests for gene flow, and little evidence for meaningful migration edges between species, leads to the conclusion that gene flow cannot explain the mitonuclear discordance we observe. This lack of evidence for gene flow combined with evidence for extensive genome-wide gene tree discordance, a hallmark of ILS, leads us to conclude that the mitonuclear discordance we observe likely results from ILS, specifically deep coalescence of the mitochondrial genome. Based on this case study, we hypothesize that similar demographic histories in other 'great speciator' taxa across the Indo-Pacific likely predispose these groups to high levels of ILS and high likelihoods of mitonuclear discordance.

Introgression at the emerging secondary contact zone of magpie Pica pica subspecies (Aves: Corvidae): integrating data on nuclear and mitochondrial markers, vocalizations, and field observations

Zones of secondary contact provide a good opportunity to investigate the origin and dynamics of reproductive isolation between related populations. We analyzed genetic and phenotypic patterns and gene flow between two subspecies of the Eurasian magpie Pica pica s.l. which recently came into contact after presumably long periods of isolation. We describe the distribution of subspecies in a young contact zone at Argun’ river basin in southern Siberia where populations occur in parapatry and an older hybrid population in eastern Mongolia. Based on genome-wide SNP data, we analyzed patterns and strength of gene flow between the subspecies. Our results indicate occasional hybridization with backcrossing and asymmetric introgression along a wide range in Transbaikalia and locally in eastern Mongolia. Males of P. p. jankowskii apparently exhibit higher dispersal ability towards the west compared to P. p. leucoptera (towards the east). The former occasionally migrates to eastern Mongolia and Transbaikalia where introgression of nuclear, but not mitochondrial DNA was evident. Bioacoustic investigations showed differences between the subspecies in speed and structure of vocalization. We discovered intermediate calls of hybrid magpies and bilingual birds alternating calls that are typical for the two taxa. Furthermore, we found dramatically decreased reproductive success in hybridogeneous populations. By complementing our results with established phylogeographic patterns of P. pica s.l. based on a mitochondrial marker sequence, and considering indications of sterility of hybrids in the contact zone, we propose to elevate the two corresponding subspecies to species level: P. pica for the western form and P. serica for the eastern form.

Sea surface temperature, rather than land mass or geographic distance, may drive genetic differentiation in a species complex of highly dispersive seabirds

Seabirds, particularly Procellariiformes, are highly mobile organisms with a great capacity for long dispersal, though simultaneously showing high philopatry, two conflicting life-history traits that may lead to contrasted patterns of genetic population structure. Landmasses were suggested to explain differentiation patterns observed in seabirds, but philopatry, isolation by distance, segregation between breeding and nonbreeding zones, and oceanographic conditions (sea surface temperatures) may also contribute to differentiation patterns. To our knowledge, no study has simultaneously contrasted the multiple factors contributing to the diversification of seabird species, especially in the gray zone of speciation. We conducted a multilocus phylogeographic study on a widespread seabird species complex, the little shearwater complex, showing highly homogeneous morphology, which led to considerable taxonomic debate. We sequenced three mitochondrial and six nuclear markers on all extant populations from the Atlantic (lherminieri) and Indian Oceans (bailloni), that is, five nominal lineages from 13 populations, along with one population from the eastern Pacific Ocean (representing the dichrous lineage). We found sharp differentiation among populations separated by the African continent with both mitochondrial and nuclear markers, while only mitochondrial markers allowed characterizing the five nominal lineages. No differentiation could be detected within these five lineages, questioning the strong level of philopatry showed by these shearwaters. Finally, we propose that Atlantic populations likely originated from the Indian Ocean. Within the Atlantic, a stepping-stone process accounts for the current distribution. Based on our divergence time estimates, we suggest that the observed pattern of differentiation mostly resulted from historical and current variation in sea surface temperatures.

Sympatric occurrence of deeply diverged mitochondrial DNA lineages in Siberian geometrid moths (Lepidoptera: Geometridae): cryptic speciation, mitochondrial introgression, secondary admixture or effect of Wolbachia?

The divergent sympatric mitochondrial lineages within traditionally recognized species present a challenge regularly faced by taxonomists and evolutionary biologists. We encountered this problem when studying the Siberian geometrid moths, Alcis deversata and Thalera chlorosaria. Within each of these species we found two deeply diverged mitochondrial lineages that demonstrated a level of genetic differentiation exceeding the standard interspecific DNA barcode threshold. Using analyses of nuclear genes, morphology, ecological preferences and Wolbachia endosymbionts, we tested five hypotheses that might explain the mitochondrial pattern observed: cryptic speciation, ancestral polymorphism, interspecific mitochondrial introgression, secondary admixture of allopatrically evolved populations and an effect of intracellular Wolbachia endosymbionts. We demonstrate that in A. deversata and Th. chlorosaria the mitochondrial differences are not correlated with differences in nuclear genes, morphology, ecology and Wolbachia infection status, thus not supporting the hypothesis of cryptic species and an effect of Wolbachia. Mitochondrial introgression can lead to a situation in which one species has both its own mitochondrial lineage and the lineage obtained from another species. We found this situation in the species pair Alcis repandata and Alcis extinctaria. We conclude that the mitochondrial heterogeneity in A. deversata and Th. chlorosaria is most likely to be attributable to the secondary admixture of allopatrically evolved populations.

Genetic diversity of two populations of the tufted puffin Fratercula cirrhata (Pallas, 1769)

The tufted puffin Fratercula cirrhata (Charadriiformes: Alcidae) is distributed throughout the boreal and low Arctic areas of the North Pacific, from California, USA to Hokkaido, Japan. Few studies have investigated the genetic diversity of this species. Therefore, we analyzed the genetic diversity of two captive populations using nucleotide sequences of two mitochondrial loci (COX1 and D-loop) and one nuclear locus (RHBG). We sequenced these loci for birds from Tokyo Sea Life Park (Kasai Rinkai Suizokuen), originally from Alaska, and birds from Aqua World Oarai, originally from far eastern Russia. We found five COX1 haplotypes and 17 D-loop haplotypes for the mitochondrial loci, and obtained 14 predicted haplotypes for the nuclear RHBG locus. The major haplotypes of all three loci occurred in individuals from both populations. Thus, there were no clear genetic differences between the populations with respect to these three loci. Although the breeding range of the tufted puffin covers the boreal and low Arctic from California to Hokkaido, our results suggest that the species has not genetically diverged within its breeding range.

Complex histories of gene flow and a mitochondrial capture event in a nonsister pair of birds

Hybridization, introgression, and reciprocal gene flow during speciation, specifically the generation of mitonuclear discordance, are increasingly observed as parts of the speciation process. Genomic approaches provide insight into where, when, and how adaptation operates during and after speciation and can measure historical and modern introgression. Whether adaptive or neutral in origin, hybridization can cause mitonuclear discordance by placing the mitochondrial genome of one species (or population) in the nuclear background of another species. The latter, introgressed species may eventually have its own mtDNA replaced or “captured” by other species across its entire geographical range. Intermediate stages in the capture process should be observable. Two nonsister species of Australasian monarch‐flycatchers, Spectacled Monarch (Symposiachrus trivirgatus) mostly of Australia and Indonesia and Spot‐winged Monarch (S. guttula) of New Guinea, present an opportunity to observe this process. We analysed thousands of single nucleotide polymorphisms (SNPs) derived from ultraconserved elements of all subspecies of both species. Mitochondrial DNA sequences of Australian populations of S. trivirgatus form two paraphyletic clades, one being sister to and presumably introgressed by S. guttula despite little nuclear signal of introgression. Population genetic analyses (e.g., tests for modern and historical gene flow and selection) support at least one historical gene flow event between S. guttula and Australian S. trivirgatus. We also uncovered introgression from the Maluku Islands subspecies of S. trivirgatus into an island population of S. guttula, resulting in apparent nuclear paraphyly. We find that neutral demographic processes, not adaptive introgression, are the most likely cause of these complex population histories. We suggest that a Pleistocene extinction of S. guttula from mainland Australia resulted from range expansion by S. trivirgatus.

Complete mitochondrial genome of Chroicocephalus brunnicephalus from India: phylogeny with other Larids

The complete mitogenome sequence of the brown-headed gull, Chroicocephalus brunnicephalus was determined in this study. The 16,771 bp genome consists of 13 protein-coding genes (PCGs), two ribosomal RNA (rRNA) genes, and 22 transfer RNA (tRNA) genes, and a control region (CR). The decoded mitogenome was AT-rich (54.77%) with nine overlapping and 17 intergenic spacer regions. Most of the PCGs were started by a typical ATG initiation codon except for cox1 and nad3. Further, the usual termination codons (AGG, TAG, TAA, and AGA) were used by 11 PCGs except for cox3 and nad4. The concatenated PCGs based Bayesian phylogeny clearly discriminates all the Laridae species and reflects the sister relationship of C. brunnicephalus with C. ridibundus. The present mitogenome-based phylogeny was congruent with the earlier hypothesis and confirmed the evolutionary position of the brown-headed gull as masked species. The generated mitogenome of C. brunnicephalus is almost identical to the previously generated mitogenome from China except for two base pairs in CR. To visualize the population structure of this migratory species, we propose more sampling from different geographical locations and the generation of additional molecular data to clarify the reality.

Diversification, Introgression, and Rampant Cytonuclear Discordance in Rocky Mountains Chipmunks (Sciuridae: Tamias)

Evidence from natural systems suggests that hybridization between animal species is more common than traditionally thought, but the overall contribution of introgression to standing genetic variation within species remains unclear for most animal systems. Here, we use targeted exon-capture to sequence thousands of nuclear loci and complete mitochondrial genomes from closely related chipmunk species in the Tamias quadrivittatus group that are distributed across the Great Basin and the central and southern Rocky Mountains of North America. This recent radiation includes six overlapping, ecologically distinct species (T. canipes, T. cinereicollis, T. dorsalis, T. quadrivittatus, T. rufus, and T. umbrinus) that show evidence for widespread introgression across species boundaries. Such evidence has historically been derived from a handful of markers, typically focused on mitochondrial loci, to describe patterns of introgression; consequently, the extent of introgression of nuclear genes is less well characterized. We conducted a series of phylogenomic and species-tree analyses to resolve the phylogeny of six species in this group. In addition, we performed several population genomic analyses to characterize nuclear genomes and infer coancestry among individuals. Furthermore, we used emerging quartets-based approaches to simultaneously infer the species tree (SVDquartets) and identify introgression (HyDe). We found that, in spite of rampant introgression of mitochondrial genomes between some species pairs (and sometimes involving up to three species), there appears to be little to no evidence for nuclear introgression. These findings mirror other genomic results where complete mitochondrial capture has occurred between chipmunk species in the absence of appreciable nuclear gene flow. The underlying causes of recurrent massive cytonuclear discordance remain unresolved in this group but mitochondrial DNA appears highly misleading of population histories as a whole. Collectively, it appears that chipmunk species boundaries are largely impermeable to nuclear gene flow and that hybridization, while pervasive with respect to mtDNA, has likely played a relatively minor role in the evolutionary history of this group.

Integrative genomic phylogeography reveals signs of mitonuclear incompatibility in a natural hybrid goby population

Hybridization between divergent lineages generates new allelic combinations. One mechanism that can hinder the formation of hybrid populations is mitonuclear incompatibility, that is, dysfunctional interactions between proteins encoded in the nuclear and mitochondrial genomes (mitogenomes) of diverged lineages. Theoretically, selective pressure due to mitonuclear incompatibility can affect genotypes in a hybrid population in which nuclear genomes and mitogenomes from divergent lineages admix. To directly and thoroughly observe this key process, we de novo sequenced the 747-Mb genome of the coastal goby, Chaenogobius annularis, and investigated its integrative genomic phylogeographics using RNA-sequencing, RAD-sequencing, genome resequencing, whole mitogenome sequencing, amplicon sequencing, and small RNA-sequencing. Chaenogobius annularis populations have been geographically separated into Pacific Ocean (PO) and Sea of Japan (SJ) lineages by past isolation events around the Japanese archipelago. Despite the divergence history and potential mitonuclear incompatibility between these lineages, the mitogenomes of the PO and SJ lineages have coexisted for generations in a hybrid population on the Sanriku Coast. Our analyses revealed accumulation of nonsynonymous substitutions in the PO-lineage mitogenomes, including two convergent substitutions, as well as signals of mitochondrial lineage-specific selection on mitochondria-related nuclear genes. Finally, our data implied that a microRNA gene was involved in resolving mitonuclear incompatibility. Our integrative genomic phylogeographic approach revealed that mitonuclear incompatibility can affect genome evolution in a natural hybrid population.

Multiple species delimitation approaches applied to the avian lark genus Alaudala

Species delimitation has advanced from a purely phenotypic exercise to a branch of science that integrates multiple sources of data to identify independently evolving lineages that can be treated as species. We here test species limits in the avian Lesser Short-toed Lark Alaudala rufesens-Sand Lark A. raytal complex, which has an intricate taxonomic history, ranging from a single to three recognised species, with different inclusiveness in different treatments. Our integrative taxonomic approach is based on a combination of DNA sequences, plumage, biometrics, songs, song-flights, geographical distributions, habitat, and bioclimatic data, and using various methods including a species delimitation program (STACEY) based on the multispecies coalescent model. We propose that four species should be recognised: Lesser Short-toed Lark A. rufescens (sensu stricto), Heine's Short-toed Lark A. heinei, Asian Short-toed Lark A. cheleensis and Sand Lark A. raytal. There is also some evidence suggesting lineage separation within A. cheleensis and A. raytal, but additional data are required to evaluate this. The species delimitation based on STACEY agrees well with the non-genetic data. Although computer-based species delimitation programs can be useful in identifying independently evolving lineages, we stress that whenever possible, species hypotheses proposed by these programs should be tested by independent, non-genetic data. Our results highlight the difficulty and subjectivity of delimiting lineages and species, especially at early stages in the speciation process.

Ultraconserved yet informative for species delimitation: Ultraconserved elements resolve long-standing systematic enigma in Central European bees

Accurate and testable species hypotheses are essential for measuring, surveying and managing biodiversity. Taxonomists often rely on mitochondrial DNA barcoding to complement morphological species delimitations. Although COI-barcoding has largely proven successful in assisting identifications for most animal taxa, there are nevertheless numerous cases where mitochondrial barcodes do not reflect species hypotheses. For instance, what is regarded as a single species can be associated with two distinct DNA barcodes, which can point either to cryptic diversity or to within-species mitochondrial divergences without reproductive isolation. In contrast, two or more species can share barcodes, for instance due to mitochondrial introgression. These intrinsic limitations of DNA barcoding are commonly addressed with nuclear genomic markers, which are expensive, may have low repeatability and often require high-quality DNA. To overcome these limitations, we examined the use of ultraconserved elements (UCEs) as a quick and robust genomic approach to address such problematic cases of species delimitation in bees. This genomic method was assessed using six different species complexes suspected to harbour cryptic diversity, mitochondrial introgression or mitochondrial paraphyly. The sequencing of UCEs recovered between 686 and 1,860 homologous nuclear loci and provided explicit species delimitation in all investigated species complexes. These results provide strong evidence for the suitability of UCEs as a fast method for species delimitation even in recently diverged lineages. Furthermore, we provide the first evidence for both mitochondrial introgression among distinct bee species, and mitochondrial paraphyly within a single bee species.

Genome-wide data reveal discordant mitonuclear introgression in the intermediate horseshoe bat (Rhinolophus affinis)

Closely related taxa often exhibit mitonuclear discordance attributed to introgression of mitochondrial DNA (mtDNA), yet few studies have considered the underlying causes of mtDNA introgression. Here we test for demographic versus adaptive processes as explanations for mtDNA introgression in three subspecies of the intermediate horseshoe bat (Rhinolophus affinis). We generated sequences of 1692 nuclear genes and 13 mitochondrial protein-coding genes for 48 individuals. Phylogenetic reconstructions based on 320 exon sequences and 2217 single nucleotide polymorphisms (SNPs) both revealed conflicts between the species tree and mtDNA tree. These results, together with geographic patterns of mitonuclear discordance, and shared identical or near-identical mtDNA sequences, suggest extensive introgression of mtDNA between the two parapatric mainland subspecies. Under demographic hypotheses, we would also expect to uncover traces of ncDNA introgression, however, population structure and gene flow analyses revealed little nuclear admixture. Furthermore, we found inconsistent estimates of the timing of population expansion and that of the most recent common ancestor for the clade containing introgressed haplotypes. Without a clear demographic explanation, we also examined whether introgression likely arises from adaptation. We found that five mtDNA genes contained fixed amino acid differences between introgressed and non-introgressed individuals, including putative positive selection found in one codon, although this did not show introgression. While our evidence for rejecting demographic hypotheses is arguably stronger than that for rejecting adaptation, we find no definitive support for either explanation. Future efforts will focus on larger-scale resequencing to decipher the underlying causes of discordant mitonuclear introgression in this system.

Identifying mechanisms of genetic differentiation among populations in vagile species: historical factors dominate genetic differentiation in seabirds

Elucidating the factors underlying the origin and maintenance of genetic variation among populations is crucial for our understanding of their ecology and evolution, and also to help identify conservation priorities. While intrinsic movement has been hypothesized as the major determinant of population genetic structuring in abundant vagile species, growing evidence indicates that vagility does not always predict genetic differentiation. However, identifying the determinants of genetic structuring can be challenging, and these are largely unknown for most vagile species. Although, in principle, levels of gene flow can be inferred from neutral allele frequency divergence among populations, underlying assumptions may be unrealistic. Moreover, molecular studies have suggested that contemporary gene flow has often not overridden historical influences on population genetic structure, which indicates potential inadequacies of any interpretations that fail to consider the influence of history in shaping that structure. This exhaustive review of the theoretical and empirical literature investigates the determinants of population genetic differentiation using seabirds as a model system for vagile taxa. Seabirds provide a tractable group within which to identify the determinants of genetic differentiation, given their widespread distribution in marine habitats and an abundance of ecological and genetic studies conducted on this group. Herein we evaluate mitochondrial DNA (mtDNA) variation in 73 seabird species. Lack of mutation-drift equilibrium observed in 19% of species coincided with lower estimates of genetic differentiation, suggesting that dynamic demographic histories can often lead to erroneous interpretations of contemporary gene flow, even in vagile species. Presence of land across the species sampling range, or sampling of breeding colonies representing ice-free Pleistocene refuge zones, appear to be associated with genetic differentiation in Tropical and Southern Temperate species, respectively, indicating that long-term barriers and persistence of populations are important for their genetic structuring. Conversely, biotic factors commonly considered to influence population genetic structure, such as spatial segregation during foraging, were inconsistently associated with population genetic differentiation. In light of these results, we recommend that genetic studies should consider potential historical events when identifying determinants of genetic differentiation among populations to avoid overestimating the role of contemporary factors, even for highly vagile taxa.

Ancient hybridization and mtDNA introgression behind current paternal leakage and heteroplasmy in hybrid zones

Hybridization between heterospecific individuals has been documented as playing a direct role in promoting paternal leakage and mitochondrial heteroplasmy in both natural populations and laboratory conditions, by relaxing the egg-sperm recognition mechanisms. Here, we tested the hypothesis that hybridization can lead to mtDNA heteroplasmy also indirectly via mtDNA introgression. By using a phylogenetic approach, we showed in two reproductively isolated beetle species, Ochthebius quadricollis and O. urbanelliae, that past mtDNA introgression occurred between them in sympatric populations. Then, by developing a multiplex allele-specific PCR assay, we showed the presence of heteroplasmic individuals and argue that their origin was through paternal leakage following mating between mtDNA-introgressed and pure conspecific individuals. Our results highlight that mtDNA introgression can contribute to promote paternal leakage, generating genetic novelty in a way that has been overlooked to date. Furthermore, they highlight that the frequency and distribution of mtDNA heteroplasmy can be deeply underestimated in natural populations, as i) the commonly used PCR-Sanger sequencing approach can fail to detect mitochondrial heteroplasmy, and ii) specific studies aimed at searching for it in populations where mtDNA-introgressed and pure individuals co-occur remain scarce, despite the fact that mtDNA introgression has been widely documented in several taxa and populations.

Unexpected low genetic variation in the South American hystricognath rodent Lagostomus maximus (Rodentia: Chinchillidae)

The South American plains vizcacha, Lagostomus maximus inhabits primarily the Pampean and adjoining Espinal, Monte and Chaquenean regions of Argentina. In order to study the population genetic structure of L. maximus, a fragment of 560 bp of the mitochondrial DNA hypervariable region 1from 90 individuals collected from the 3 subspecies and 8 groups along Argentina was amplified and analyzed. We found 9 haplotypes. The haplotype network did not show an apparent phylogeographical signal. Although low levels of genetic variation were found in all the subspecies and groups analyzed, a radiation of L. maximus would have occurred from the North and Center of the Pampean region toward the rest of its geographic range in Argentina. Low levels of genetic diversity, the existence of a single genetically distinct population in Argentina and changes of its effective size indicate that metapopulation processes and changes in human population dynamics during the late-Holocene were important factors shaping the population genetic structure of L. maximus in Argentina.

Phylotranscriptomic Insights into the Diversification of Endothermic Thunnus Tunas

Birds, mammals, and certain fishes, including tunas, opahs and lamnid sharks, are endothermic, conserving internally generated, metabolic heat to maintain body or tissue temperatures above that of the environment. Bluefin tunas are commercially important fishes worldwide, and some populations are threatened. They are renowned for their endothermy, maintaining elevated temperatures of the oxidative locomotor muscle, viscera, brain and eyes, and occupying cold, productive high-latitude waters. Less cold-tolerant tunas, such as yellowfin tuna, by contrast, remain in warm-temperate to tropical waters year-round, reproducing more rapidly than most temperate bluefin tuna populations, providing resiliency in the face of large-scale industrial fisheries. Despite the importance of these traits to not only fisheries but also habitat utilization and responses to climate change, little is known of the genetic processes underlying the diversification of tunas. In collecting and analyzing sequence data across 29,556 genes, we found that parallel selection on standing genetic variation is associated with the evolution of endothermy in bluefin tunas. This includes two shared substitutions in genes encoding glycerol-3 phosphate dehydrogenase, an enzyme that contributes to thermogenesis in bumblebees and mammals, as well as four genes involved in the Krebs cycle, oxidative phosphorylation, β-oxidation, and superoxide removal. Using phylogenetic techniques, we further illustrate that the eight Thunnus species are genetically distinct, but found evidence of mitochondrial genome introgression across two species. Phylogeny-based metrics highlight conservation needs for some of these species.

Past lake shore dynamics explain present pattern of unidirectional introgression across a habitat barrier

Introgression patterns between divergent lineages are often characterized by asymmetry in the direction and among-marker variation in the extent of gene flow, and therefore inform on the mechanisms involved in differentiation and speciation. In the present study, we test the hypothesis that unidirectional introgression between two phenotypically and genetically distinct lineages of the littoral, rock-dwelling cichlid fish Tropheus moorii across a wide sandy bay is linked to observed differences in mate preferences between the two lineages. This hypothesis predicts bi-directional nuclear gene flow and was rejected by congruent patterns of introgression in mtDNA, AFLP and microsatellite markers, with admixture confined to the populations west of the bay. This pattern can be explained on the basis of habitat changes in the course of lake level fluctuations, which first facilitated the development of a symmetric admixture zone including the area corresponding to the present sand bay and then shaped asymmetry by causing local extinctions and cessation of gene flow when this area became once more inhabitable. This conforms with previous assumptions that habitat dynamics are a primary determinant of population-level evolution in Tropheus. In this respect, Tropheus may be representative of species whose preferred habitat is subject to frequent re-structuring.

Contrasting phylogeographic histories between the continent and islands of East Asia: Massive mitochondrial introgression and long-term isolation of hares (Lagomorpha: Lepus)

Hares of the genus Lepus are distributed worldwide, and introgressive hybridization is thought to be pervasive among species, leading to reticulate evolution and taxonomic confusion. Here, we performed phylogeographic analyses of the following species of hare across East Asia: L. timidus, L. mandshuricus, L. coreanus, and L. brachyurus collected from far-eastern Russia, South Korea, and Japan. Nucleotide sequences of one mitochondrial DNA and eight nuclear gene loci were examined, adding sequences of hares in China from databases. All nuclear DNA analyses supported the clear separation of three phylogroups: L. timidus, L. brachyurus, and the L. mandshuricus complex containing L. coreanus. On the other hand, massive mitochondrial introgression from two L. timidus lineages to the L. mandshuricus complex was suggested in continental East Asia. The northern population of the L. mandshuricus complex was mainly associated with introgression from the continental lineage of L. timidus, possibly since the last glacial period, whereas the southern population of the L. mandshuricus complex experienced introgression from another L. timidus lineage related to the Hokkaido population, possibly before the last glacial period. In contrast to continental hares, no evidence of introgression was found in L. brachyurus in the Japanese Archipelago, which showed the oldest divergence amongst East Asian hare lineages. Our findings suggest that glacial-interglacial climate changes in the circum-Japan Sea region promoted distribution shifts and introgressive hybridization among continental hare species, while the geographic structure of the region contributed to long-term isolation of hares on the islands, preventing inter-species gene flow.

Population Connectivity and Traces of Mitochondrial Introgression in New Zealand Black-Billed Gulls (Larus bulleri)

Black-billed gulls (Larus bulleri) are endemic to New Zealand and are suspected to be undergoing substantial population declines. They primarily breed on open gravel beds in braided rivers of the South Island—a habitat that is diminishing and becoming increasingly modified. Although management of this species is increasing, little has been published on their movements and demographics. In this study, both mitochondrial DNA (mtDNA) control region domain I and nuclear single nucleotide polymorphisms (SNPs) were examined to help understand the connectivity and population structure of black-billed gulls across the country and to help inform management decisions. Mitochondrial DNA showed no population structure, with high haplotype and low nucleotide diversity, and analyses highlighted mitochondrial introgression with the closely related red-billed gulls (Larus novaehollandiae scopulinus). Nuclear DNA analyses, however, identified two groups, with Rotorua birds in the North Island being distinct from the rest of New Zealand, and isolation-by-distance evident across the South Island populations. Gene flow primarily occurs between nearby colonies with a stepwise movement across the landscape. The importance from a genetic perspective of the more isolated North Island birds (1.6% of total population) needs to be further evaluated. From our results, we infer that the South Island black-billed gull management should focus on maintaining several populations within each region rather than focusing on single specific colonies or river catchments. Future study is needed to investigate the genetic structure of populations at the northern limit of the species’ range, and identify the mechanisms behind, and extent of, the hybridisation between red-billed and black-billed gulls.

Revisiting the Distribution of Thaumetopoea pityocampa (Lepidoptera: Notodontidae) and T. pityocampa ENA Clade in Greece

In the present work, we sampled individuals of the processionary pine moth, Thaumetopoea pityocampa (Denis and Schiffermüller; Lepidoptera: Notodontidae) from different areas of Greece between 2014 and 2016. These samples were sequenced for a 760-bp long mtDNA COI locus and the haplotypes retrieved clearly showed that the occurrence of T. pityocampa in Greece is being considerably restricted, with only 8 individuals out of the 221 exhibiting T. pityocampa haplotypes and the rest being identified as T. pityocampa ENA clade haplotypes. To that, one haplotype in particular exhibited the highest abundance and broadest geographic distribution, occurring both in mainland and on islands. Our data suggest a rather recent and rapid population expansion of the ENA clade in Greece and a concomitant recent displacement of T. pityocampa. It thus seems that the relation between T. pityocampa and T. pityocampa ENA clade needs to be further and thorough analyzed before the taxonomic status of T. pityocampa ENA clade can be concluded with confidence.

Historical introgression among the species of Rodgersia (Saxifragaceae) in mountainous forests of southwest China

In the present study, we used genetic data and ecological niche modelling to explore possible historical introgressions among the species of Rodgersia (Saxifragaceae) in central-southwest China. Markedly differentiated chloroplast haplotypes were found in R. aesculifolia, R. sambucifolia and the Lijiang (LJ) population of R. pinnata, respectively, and differentiated chloroplast haplotypes within each of them showed the closest relationships with haplotypes from different species. ITS cloning did not reveal any shared ribotype between R. aesculifolia and the remaining species. Historical introgression between R. aesculifolia and R. sambucifolia (or R. pinnata) seems to be the most plausible explanation according to the geographical pattern and derivative status of putative introgressed chloroplast haplotypes, and also from morphological evidence. Introgressions were also found among R. sambucifolia, R. pinnata, and R. henricii from Yunnan. Frequent gene exchanges may have promoted the diversity of leaf shapes in this genus. Ecological niche modelling indicated that past secondary contact following range shifts during Pleistocene cold periods may have provided opportunities for ancient introgression between R. aesculifolia and adjacent species.

Human-mediated secondary contact of two tortoise lineages results in sex-biased introgression

Human-mediated secondary contact of recently diverged taxa offers valuable opportunities for studying the evolutionary mechanisms involved in the establishment and maintenance of genetic boundaries between taxa. We used mitochondrial and microsatellite markers to examine a recently introduced population of the spur-thighed tortoise (Testudo graeca) of mixed origin in the Doñana National Park (SW Spain). The earliest records of tortoises in Doñana trace back to the 18th century, but several population reinforcements in the 20th century with animals from Morocco are well-documented. Consequently, different genetic lineages, which represent distinct subspecies, are thought to co-exist there. Our results confirmed the presence of distinct lineages by revealing that tortoises of the subspecies T. g. marokkensis were introduced into a local allochthonous T. g. graeca population. Unexpectedly, T. g. marokkensis haplotypes exclusively appeared in males, and admixture levels were statistically sex-biased toward males. The sex ratio of the population deviated from parity, with males being 2.36-fold more abundant than females. Our results indicated that population reinforcements had a strong effect on the genetic composition of this population and aggravated its sex ratio deviation. We predict that this sex-biased pattern of introgression is ephemeral and advocated to the near loss of T. g. marokkensis haplotypes.

The on-again, off-again relationship between mitochondrial genomes and species boundaries

The study of reproductive isolation and species barriers frequently focuses on mitochondrial genomes and has produced two alternative and almost diametrically opposed narratives. On one hand, mtDNA may be at the forefront of speciation events, with co-evolved mitonuclear interactions responsible for some of the earliest genetic incompatibilities arising among isolated populations. On the other hand, there are numerous cases of introgression of mtDNA across species boundaries even when nuclear gene flow is restricted. We argue that these seemingly contradictory patterns can result from a single underlying cause. Specifically, the accumulation of deleterious mutations in mtDNA creates a problem with two alternative evolutionary solutions. In some cases, compensatory or epistatic changes in the nuclear genome may ameliorate the effects of mitochondrial mutations, thereby establishing coadapted mitonuclear genotypes within populations and forming the basis of reproductive incompatibilities between populations. Alternatively, populations with high mitochondrial mutation loads may be rescued by replacement with a more fit, foreign mitochondrial haplotype. Coupled with many nonadaptive mechanisms of introgression that can preferentially affect cytoplasmic genomes, this form of adaptive introgression may contribute to the widespread discordance between mitochondrial and nuclear genealogies. Here, we review recent advances related to mitochondrial introgression and mitonuclear incompatibilities, including the potential for cointrogression of mtDNA and interacting nuclear genes. We also address an emerging controversy over the classic assumption that selection on mitochondrial genomes is inefficient and discuss the mechanisms that lead lineages down alternative evolutionary paths in response to mitochondrial mutation accumulation.

Introgression and selection shaped the evolutionary history of sympatric sister-species of coral reef fishes (genus: Haemulon)

Closely related marine species with large overlapping ranges provide opportunities to study mechanisms of speciation, particularly when there is evidence of gene flow between such lineages. Here, we focus on a case of hybridization between the sympatric sister-species Haemulon maculicauda and H. flaviguttatum, using Sanger sequencing of mitochondrial and nuclear loci, as well as 2422 single nucleotide polymorphisms (SNPs) obtained via restriction site-associated DNA sequencing (RADSeq). Mitochondrial markers revealed a shared haplotype for COI and low divergence for CytB and CR between the sister-species. On the other hand, complete lineage sorting was observed at the nuclear loci and most of the SNPs. Under neutral expectations, the smaller effective population size of mtDNA should lead to fixation of mutations faster than nDNA. Thus, these results suggest that hybridization in the recent past (0.174-0.263 Ma) led to introgression of the mtDNA, with little effect on the nuclear genome. Analyses of the SNP data revealed 28 loci potentially under divergent selection between the two species. The combination of mtDNA introgression and limited nuclear DNA introgression provides a mechanism for the evolution of independent lineages despite recurrent hybridization events. This study adds to the growing body of research that exemplifies how genetic divergence can be maintained in the presence of gene flow between closely related species.

Dynamics of mtDNA introgression during species range expansion: insights from an experimental longitudinal study

Introgressive hybridization represents one of the long-lasting debated genetic consequences of species range expansion. Mitochondrial DNA has been shown to heavily introgress between interbreeding animal species that meet in new sympatric areas and, often, asymmetric introgression from local to the colonizing populations has been observed. Disentangling among the evolutionary and ecological processes that might shape this pattern remains difficult, because they continuously act across time and space. In this context, long-term studies can be of paramount importance. Here, we investigated the dynamics of mitochondrial introgression between two mosquito species (Aedes mariae and Ae. zammitii ) during a colonization event that started in 1986 after a translocation experiment. By analyzing 1,659 individuals across 25 years, we showed that introgression occurred earlier and at a higher frequency in the introduced than in the local species, showing a pattern of asymmetric introgression. Throughout time, introgression increased slowly in the local species, becoming reciprocal at most sites. The rare opportunity to investigate the pattern of introgression across time during a range expansion along with the characteristics of our study-system allowed us to support a role of demographic dynamics in determining the observed introgression pattern.

Recurrent hybridization and recent origin obscure phylogenetic relationships within the 'white-headed' gull (Larus sp.) complex

Species complexes that have undergone recent radiations are often characterized by extensive allele sharing due to recent ancestry and (or) introgressive hybridization. This can result in discordant evolutionary histories of genes and heterogeneous genomes, making delineating species limits difficult. Here we examine the phylogenetic relationships among a complex group of birds, the white-headed gulls (Aves: Laridae), which offer a unique window into the speciation process due to their recent evolutionary history and propensity to hybridize. Relationships were examined among 17 species (61 populations) using a multilocus approach, including mitochondrial and nuclear intron DNA sequences and microsatellite genotype information. Analyses of microsatellite and intron data resulted in some species-based groupings, although most species were not represented by a single cluster. Considerable allele and haplotype sharing among white-headed gull species was observed; no locus contained a species-specific clade. Despite this, our multilocus approach provided better resolution among some species than previous studies. Interestingly, most clades appear to correspond to geographic locality: our BEAST analysis recovered strong support for a northern European/Icelandic clade, a southern European/Russian clade, and a western North American/canus clade, with weak evidence for a high latitude clade spanning North America and northwestern Europe. This geographical structuring is concordant with behavioral observations of pervasive hybridization in areas of secondary contact. The extent of allele and haplotype sharing indicates that ecological and sexual selection are likely not strong enough to complete reproductive isolation within several species in the white-headed gull complex. This suggests that just a few genes are driving the speciation process.

Speciation in Cloudless Sulphurs Gleaned from Complete Genomes

For 200 years, zoologists have relied on phenotypes to learn about the evolution of animals. A glance at the genotype, even through several gene markers, revolutionized our understanding of animal phylogeny. Recent advances in sequencing techniques allow researchers to study speciation mechanisms and the link between genotype and phenotype using complete genomes. We sequenced and assembled a complete genome of the Cloudless Sulphur (Phoebis sennae) from a single wild-caught specimen. This genome was used as reference to compare genomes of six specimens, three from the eastern populations (Oklahoma and north Texas), referred to as a subspeciesPhoebis sennae eubule, and three from the southwestern populations (south Texas) known as a subspeciesPhoebis sennae marcellina While the two subspecies differ only subtly in phenotype and mitochondrial DNA, comparison of their complete genomes revealed consistent and significant differences, which are more prominent than those between tiger swallowtailsPterourus canadensisandPterourus glaucus The two sulphur taxa differed in histone methylation regulators, chromatin-associated proteins, circadian clock, and early development proteins. Despite being well separated on the whole-genome level, the two taxa show introgression, with gene flow mainly fromP. s. marcellinatoP. s. eubule Functional analysis of introgressed genes reveals enrichment in transmembrane transporters. Many transporters are responsible for nutrient uptake, and their introgression may be of selective advantage for caterpillars to feed on more diverse food resources. Phylogenetically, complete genomes place family Pieridae away from Papilionidae, which is consistent with previous analyses based on several gene markers.

Molecular and ecological signs of mitochondrial adaptation: consequences for introgression?

The evolution of the mitochondrial genome and its potential adaptive impact still generates vital debates. Even if mitochondria have a crucial functional role, as they are the main cellular energy suppliers, mitochondrial DNA (mtDNA) introgression is common in nature, introducing variation in populations upon which selection may act. Here we evaluated whether the evolution of mtDNA in a rodent species affected by mtDNA introgression is explained by neutral expectations alone. Variation in one mitochondrial and six nuclear markers in Myodes glareolus voles was examined, including populations that show mtDNA introgression from its close relative, Myodes rutilus. In addition, we modelled protein structures of the mtDNA marker (cytochrome b) and estimated the environmental envelopes of mitotypes. We found that massive mtDNA introgression occurred without any trace of introgression in the analysed nuclear genes. The results show that the native glareolus mtDNA evolved under past positive selection, suggesting that mtDNA in this system has selective relevance. The environmental models indicate that the rutilus mitotype inhabits colder and drier habitats than the glareolus one that can result from local adaptation or from the geographic context of introgression. Finally, homology models of the cytochrome b protein revealed a substitution in rutilus mtDNA in the vicinity of the catalytic fraction, suggesting that differences between mitotypes may result in functional changes. These results suggest that the evolution of mtDNA in Myodes may have functional, ecological and adaptive significance. This work opens perspective onto future experimental tests of the role of natural selection in mtDNA introgression in this system.