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

The kin selection theory of genomic imprinting and modes of reproduction in the eusocial Hymenoptera

Genomic imprinting is known from flowering plants and mammals but has not been confirmed for the Hymenoptera even though the eusocial Hymenoptera are prime candidates for this peculiar form of gene expression. Here, the kin selection theory of genomic imprinting is reviewed and applied to the eusocial Hymenoptera. The evidence for imprinting in eusocial Hymenoptera with the typical mode of reproduction, involving the sexual production of diploid female offspring, which develop into workers or gynes, and the arrhenotokous parthenogenesis of haploid males, is also reviewed briefly. However, the focus of this review is how atypical modes of reproduction, involving thelytokous parthenogenesis, hybridisation and androgenesis, may also select for imprinting. In particular, naturally occurring hybridisation in several genera of ants may provide useful tests of the role of kin selection in the evolution of imprinting. Hybridisation is expected to disrupt the coadaptation of antagonistically imprinted loci, and thus affect the phenotypes of hybrids. Some of the limited data available on hybrid worker reproduction and on colony sex ratios support predictions about patterns of imprinting derived from kin selection theory.

Genetic analysis reveals the putative native range and widespread double-clonal reproduction in the invasive longhorn crazy ant

Clonal reproduction can provide an advantage for invasive species to establish as it can circumvent inbreeding depression which often plagues introduced populations. The world's most widespread invasive ant, Paratrechina longicornis, was previously found to display a double-clonal reproduction system, whereby both males and queens are produced clonally, resulting in separate male and queen lineages, while workers are produced sexually. Under this unusual reproduction mode, inbreeding is avoided in workers as they carry hybrid interlineage genomes. Despite the ubiquitous distribution of P. longicornis, the significance of this reproductive system for the ant's remarkable success remains unclear, as its prevalence is still unknown. Further investigation into the controversial native origin of P. longicornis is also required to reconstruct the evolutionary histories of double-clonal lineages. Here, we examine genetic variation and characterize the reproduction mode of P. longicornis populations sampled worldwide using microsatellites and mitochondrial DNA sequences to infer the ant's putative native range and the distribution of the double-clonal reproductive system. Analyses of global genetic variations indicate that the Indian subcontinent is a genetic diversity hotspot of this species, suggesting that P. longicornis probably originates from this geographical area. Our analyses revealed that both the inferred native and introduced populations exhibit double-clonal reproduction, with queens and males around the globe belonging to two separate, nonrecombining clonal lineages. By contrast, workers are highly heterozygous because they are first-generation interlineage hybrids. Overall, these data indicate a worldwide prevalence of double clonality in P. longicornis and support the prediction that the unusual genetic system may have pre-adapted this ant for global colonization by maintaining heterozygosity in the worker force and alleviating genetic bottlenecks.

Population genomics supports multiple hybrid zone origins of socially hybridogenetic lineages of Pogonomyrmex harvester ants

Reproductive division of labor in the social insects is typically determined by environmental cues; however, genetic effects on caste have been discovered in a growing set of ant taxa. An extreme form of genetic caste determination is "social hybridogenesis," in which co-occurring genetic lineages obligately interbreed to produce workers, whereas daughter queens are of pure-lineage ancestry. In this study, we tested the hypothesis that social hybridogenesis in the genus Pogonomyrmex resulted from one or more interspecific hybridization events, and if so, whether individual lineages were of hybrid ancestry. We reconstructed evolutionary relationships of four lineage pairs to populations of two closely related non-hybridogenetic species, Pogonomyrmex barbatus and Pogonomyrmex rugosus, using nuclear SNP loci and mitochondrial sequencing. The nuclear phylogeny supported a hybridization hypothesis, with one member of each pair nested within P. rugosus, whereas the other was nested within P. barbatus. The source populations corresponded with two distinct geographic areas at the eastern and western edges of a zone of contact. Relatively little gene flow was detected between interbreeding lineages, either historically or currently. This suggests that shifts in reproductive caste determination may reinforce reproductive incompatibility, in a manner similar to the evolution of hybridogenesis in nonsocial systems.

Hybridization enables the fixation of selfish queen genotypes in eusocial colonies

A eusocial colony typically consists of two main castes: queens that reproduce and sterile workers that help them. This division of labor, however, is vulnerable to genetic elements that favor the development of their carriers into queens. Several factors, such as intracolonial relatedness, can modulate the spread of such caste-biasing genotypes. Here we investigate the effects of a notable yet understudied ecological setting: where larvae produced by hybridization develop into sterile workers. Using mathematical modeling, we show that the coevolution of hybridization with caste determination readily triggers an evolutionary arms race between nonhybrid larvae that increasingly develop into queens, and queens that increasingly hybridize to produce workers. Even where hybridization reduces worker function and colony fitness, this race can lead to the loss of developmental plasticity and to genetically hard-wired caste determination. Overall, our results may help understand the repeated evolution toward remarkable reproductive systems (e.g., social hybridogenesis) observed in several ant species.

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.

Sex-biased dispersal creates spatial genetic structure in a parthenogenetic ant with a dependent-lineage reproductive system

Reproduction and dispersal are key aspects of species life history that influence spatial genetic structure in populations. Several ant species in the genus Cataglyphis have evolved a unique breeding system in which new reproductives (that is, queens and males) are produced asexually by parthenogenesis; in contrast, non-reproductives (that is, workers) are produced via sexual reproduction by mates from distinct genetic lineages. We investigated how these two coexisting reproductive methods affect population-level spatial genetic structure using the ant Cataglyphis mauritanica as a model. We obtained genotypes for queens and their male mates from 338 colonies, and we found that the two lineages present in the study population occurred with equal frequency. Furthermore, analysis of spatial genetic structure revealed strong sex-biased dispersal. Because queens were produced by parthenogenesis and because they dispersed over short distances, there was an extreme level of spatial structuring: a mosaic of patches composed of clonal queens was formed. Males, on the other hand, dispersed over several hundred metres and, thus, across patches, ensuring successful interlineage mating.