Introgression of mitochondrial DNA among lineages in a hybridogenetic ant

When One's Not Enough: Colony Pool-Seq Outperforms Individual-Based Methods for Assessing Introgression in Apis mellifera mellifera

The human management of honey bees (Apis mellifera) has resulted in the widespread introduction of subspecies outside of their native ranges. One well known example of this is Apis mellifera mellifera, native to Northern Europe, which has now been significantly introgressed by the introduction of C lineage honey bees. Introgression has consequences for species in terms of future adaptive potential and long-term viability. However, estimating introgression in colony-living haplodiploid species is challenging. Previous studies have estimated introgression using individual workers, individual drones, multiple drones, and pooled workers. Here, we compare introgression estimates via three genetic approaches: SNP array, individual RAD-seq, and pooled colony RAD-seq. We also compare two statistical approaches: a maximum likelihood cluster program (ADMIXTURE) and an incomplete lineage sorting model (ABBA BABA). Overall, individual approaches resulted in lower introgression estimates than pooled colonies when using ADMIXTURE. However, the pooled colony ABBA BABA approach resulted in generally lower introgression estimates than all three ADMIXTURE estimates. These results highlight that sometimes one individual is not enough to assess colony-level introgression, and future studies that do use colony pools should not be solely dependent on clustering programs for introgression estimates.

Is mimicry a diversification-driver in ants? Biogeography, ecology, ethology, genetics and morphology define a second West-Palaearctic Colobopsis species (Hymenoptera: Formicidae)

The West-Palaearctic Colobopsis ant populations have long been considered a single species (Colobopsis truncata). We studied the diversity of this species by employing a multidisciplinary approach and combining data from our surveys, museum and private collections, and citizen science platforms. As a result, we have revealed the existence of a second species, which we describe as Colobopsis imitans sp. nov., distributed allopatrically from Co. truncata and living in the Maghreb, Sicily and southern Iberia. While the pigmentation of Co. truncata is reminiscent of Dolichoderus quadripunctatus, that of Co. imitans is similar to Crematogaster scutellaris, with which Co. imitans lives in close spatial association, and whose foraging trails it habitually follows, similar to Camponotus lateralis and other ant-mimicking ants. The isolation between Co. imitans and Co. truncata seems to have occurred relatively recently because of significant, yet not extreme, morphometric differentiation, and to mtDNA polyphyly. Both Co. imitans and Co. truncata appear to employ mimicry of an unpalatable or aggressive ant species as an important defensive strategy; this ‘choice’ of a different model species is motivated by biogeographic reasons and appears to act as a critical evolutionary driver of their diversification.

Estimating hybridization as a consequence of climatic fluctuations: A case study of two geometridae species

In some cases, the phylogenetic analysis based on the mitochondrial DNA (mtDNA) and the nuclear DNA (ncDNA) are discordant. There are three major causes of the discordance within insects, including hybridization, incomplete lineage sorting (ILS) and infection by Wolbachia. In this study, we used a combination of multilocus and coalescent analyses to explore these processes occurred during the evolutionary history of Limbatochlamys rosthorni Rothschild, 1894 and Limbatochlamys pararosthorni Han and Xue, 2005. The ncDNA phylogenetic tree supported two reciprocally monophyletic species, whereas the mtDNA results failed to reveal such a structure and revealed an extensive level of admixture between two species. Because of very low Wolbachia infection rates (<20%), we firstly excluded this reason for the discordance. The fixed nucleotide differences and large genetic distances (1.5-2.5%) at the ncDNA genes suggested that the lineage sorting process between these two species is nearly complete and two species have experienced a prolonged period of independent evolution. Thus, we secondly excluded ILS. Sharing haplotypes, mtDNA gene flow occurring and the transitional samples with morphological features supported hybridization. The distribution contraction during glaciations and postglacial distribution expansion might have facilitated hybridization. Taken together, our study indicates that the current genetic structure of L. rosthorni and L. pararosthorni is the results of contraction and fragmentation into separated refugia during glaciations, followed by postglacial expansion and admixture.

Chaotic genetic structure and past demographic expansion of the invasive gastropod Tritia neritea in its native range, the Mediterranean Sea

To better predict population evolution of invasive species in introduced areas it is critical to identify and understand the mechanisms driving genetic diversity and structure in their native range. Here, we combined analyses of the mitochondrial COI gene and 11 microsatellite markers to investigate both past demographic history and contemporaneous genetic structure in the native area of the gastropod Tritia neritea, using Bayesian skyline plots (BSP), multivariate analyses and Bayesian clustering. The BSP framework revealed population expansions, dated after the last glacial maximum. The haplotype network revealed a strong geographic clustering. Multivariate analyses and Bayesian clustering highlighted the strong genetic structure at all scales, between the Black Sea and the Adriatic Sea, but also within basins. Within basins, a random pattern of genetic patchiness was observed, suggesting a superimposition of processes involving natural biological effects (no larval phase and thus limited larval dispersal) and putative anthropogenic transport of specimens. Contrary to the introduced area, no isolation-by-distance patterns were recovered in the Mediterranean or the Black Seas, highlighting different mechanisms at play on both native and introduced areas, triggering unknown consequences for species’ evolutionary trajectories. These results of Tritia neritea populations on its native range highlight a mixture of ancient and recent processes, with the effects of paleoclimates and life history traits likely tangled with the effects of human-mediated dispersal.

Cryptic lineages hybridize for worker production in the harvester ant Messor barbarus

The reproductive division of labour between queen and worker castes in social insects is a defining characteristic of eusociality and a classic example of phenotypic plasticity. Whether social insect larvae develop into queens or workers has long been thought to be determined by environmental cues, i.e. larvae are developmentally totipotent. Contrary to this paradigm, several recent studies have revealed that caste is determined by genotype in some ant species, but whether this is restricted to just a few exceptional species is still unclear. Here, we show that the Mediterranean harvester ant Messor barbarus possesses an unusual reproductive system, in which the female castes are genetically determined. Using both nuclear and mitochondrial data, we show that Iberian populations have two distinct, cryptic lineages. Workers are always inter-lineage hybrids whereas queens are always produced from pure-lineage matings. The results suggest that genetic caste determination may be more widespread in ants than previously thought, and that further investigation in other species is needed to understand the frequency and evolution of this remarkable reproductive system.

Convergent evolution of social hybridogenesis in Messor harvester ants

Sexual reproduction generally requires no more than two partners. Here, we show convergent evolution of social hybridogenesis, a reproductive system requiring three reproductive partners in harvester ants. In this unorthodox reproductive system, two distinct genetic lineages live in sympatry and queens have to mate with males of their own lineage to produce queens along with males of the alternative lineage to produce workers. Using a large transcriptomic data set of nine species, we show that social hybridogenesis evolved at least three times independently in the genus Messor. Moreover, a study of 13 populations of Messor barbarus revealed that this mode of reproduction is fixed in the whole range of this ecologically dominant species. Finally, we show that workers can produce males carrying genes of the two genetic lineages, raising the possibility of rare gene flow between lineages contributing to the long-term maintenance of pairs of interdependent lineages. These results emphasize the evolutionary importance of social hybridogenesis, a major transition possibly linked to the peculiar ecology of harvester ants.

Combined hybridization and mitochondrial capture shape complex phylogeographic patterns in hybridogenetic Cataglyphis desert ants

Some species of Cataglyphis desert ants have evolved a hybridogenetic mode of reproduction at the social scale. In hybridogenetic populations, two distinct genetic lineages coexist. Non-reproductive offspring (workers) are hybrids of the two lineages, whereas sexual offspring (males and new queens) are produced by parthenogenesis and belong to the mother queen lineage. How this unusual reproductive system affects phylogeographic patterns and speciation processes remains completely unknown to date. Using one mitochondrial and four nuclear genes, we examined the phylogenetic relationships between three species of Cataglyphis (C. hispanica, C. humeya and C. velox) where complex DNA inheritance through social hybridogenesis may challenge phylogenetic inference. Our results bring two important insights. First, our data confirm a hybridogenetic mode of reproduction across the whole distribution range of the species C. hispanica. In contrast, they do not provide support for hybridogenesis in the populations sampled of C. humeya and C. velox. This suggests that these populations are not hybridogenetic, or that hybridogenesis is too recent to result in reciprocally monophyletic lineages on nuclear genes. Second, due to mitochondrial introgression between lineages (Darras and Aron, 2015), the faster-evolving COI marker is not lineage specific, hence, unsuitable to further investigate the segregation of lineages in the species studied. Different mitochondrial haplotypes occur in each locality sampled, resulting in strongly structured populations. This micro-allopatric structure leads to over-splitting species delimitation on mitochondrial gene, as every locality could potentially be considered a putative species; haploweb analyses of nuclear markers, however, yield species delimitations that are consistent with morphology. Overall, this study highlights how social hybridogenesis varies across species and shapes complex phylogeographic patterns.

Clonal reproduction with androgenesis and somatic recombination: the case of the ant Cardiocondyla kagutsuchi

In haplodiploid insects such as ants, male sexuals develop from unfertilised haploid eggs, while female sexuals and workers develop from fertilized diploid eggs. However, some ant species do not exchange their gene pool between sexes; both male and female sexuals are clonally produced, while workers are sexually produced. To date, three ant species, Wasmannia auropunctata, Vollenhovia emeryi, and Paratrechina longicornis, have been reported to reproduce using such reproductive systems. In this study, we reveal that in one lineage of the ant Cardiocondyla kagutsuchi, male and female sexuals are also clonally produced. In contrast to the abovementioned three species, the workers were not only sexually produced but had recombinant sequences in their nuclear internal transcribed spacer regions, although the recombinant sequences were not detected in male or female sexuals. These results suggest that the lineage likely possesses a mechanism to compensate for the reduction in genetic variation due to clonal reproduction with somatic recombination that occurs within the workers.