Evolution of hybridogenetic lineages in Cataglyphis ants

Ecological diversification preceded geographical expansion during the evolutionary radiation of Cataglyphis desert ants

Biological diversity often arises as organisms adapt to new ecological conditions (i.e., ecological opportunities) or colonize suitable areas (i.e., spatial opportunities). Cases of geographical expansion followed by local ecological divergence are well described; they result in clades comprising ecologically heterogeneous subclades. Here, we show that the desert ant genus Cataglyphis likely originated in open grassland habitats in the Middle East ∼18 million years ago and became a taxon of diverse species specializing in prey of different masses. The genus then colonized the Mediterranean Basin around 9 million years ago. The result was the rapid accumulation of species, and the appearance of local assemblages containing species from different lineages that still displayed ancestral foraging specialties. These findings highlight that, in Cataglyphis, ecological diversification preceded geographical expansion, resulting in a clade composed of ecologically homogeneous subclades.

Detection of Cryptic Sex in Automictic Populations: Theoretical Expectations and a Case Study in Cataglyphis Desert Ants

Reproductive strategies are diverse and a whole continuum of mixed systems lies between strict sexuality and strict clonality (apomixis), including automixis, a parthenogenetic mode of reproduction involving a meiosis and increasing homozygosity over generations. These various systems impact the genetic structure of populations, which can therefore be used to infer reproductive strategies in natural populations. Here, we first develop a mathematical model, validated by simulations, to predict heterozygosity and inbreeding in mixed sexual-automictic populations. It highlights the predominant role of the rate of heterozygosity loss experienced during automixis (γ), which is locus dependent. When γ is low, mixed populations behave like purely sexual ones until sex becomes rare. In contrast, when γ is high, the erosion of genetic diversity is tightly correlated to the rate of sex, so that the individual inbreeding coefficient can inform on the ratio of sexual/asexual reproduction. In the second part of this study, we used our model to test the presence of cryptic sex in a hybridogenetic Cataglyphis ant where new queens are produced parthenogenetically, leaving males with an apparent null fitness while they are essential to colony development as sperm is required to produce workers. Occasional sexual production of queens could resolve this paradox by providing males some fertile progeny. To determine whether this occurs in natural populations, we simulated genotypic datasets in a population under various regimes of sexual vs. asexual reproduction for queen production and compared the distribution of inbreeding, expected heterozygosity and inter-individual relatedness coefficients with those observed in a natural population of Cataglyphis mauritanica using microsatellites. Our simulations show that the distribution of inter-individual relatedness coefficients was particularly informative to assess the relative rate of sexual/asexual reproduction, and our dataset was compatible with pure parthenogenesis but also with up to 2% sexual reproduction. Our approach, implemented in an R script, should be useful to assess reproductive strategies in other biological models.

To have and not to have sex: When multiple evolutions of conditional use of sex elegantly solve the question in the ant genus Cataglyphis

Organisms use an amazingly large diversity of mechanisms to pass on their genes to the next generation. Sex is ancestral in eukaryotes, where it remains the most widespread way of reproduction. By combining one's genes with those of a partner, sex entails a dilution of one's genes at each generation. Evolution has been particularly creative in devising mechanisms allowing females to avoid this dilution, from classical parthenogenesis to the elimination of male genes after fertilization (Bell, 1982). Moreover, the term parthenogenesis includes various forms. Parthenogenesis can be used for female (thelytoky) or male (arrhenotoky) production and it can be associated with different cytological mechanisms, from strict clonality to meiotic division with the fusion of two of the four products of meiosis to restore diploidy (Suomalainen, Saura, & Lokki, 1987). Understanding the evolution of these diverse reproductive systems remains one of the most exciting and longstanding questions in evolutionary biology. By characterizing the reproductive systems of 11 species from the thermophilic ant genus Cataglyphis, in this issue of Molecular Ecology, Kuhn, Darras, Paknia, and Aron (2020) show the high lability of parthenogenesis, with multiple independent evolution of facultative thelytoky from sexual ancestors. The diversity of life history traits and social characteristics of this genus (e.g., mode of colony foundation, female polyandry) provides a unique and exciting opportunity to investigate the social and environmental factors driving the evolution of reproductive systems in social Hymenoptera.