The conundrum of the yellow crazy ant (Anoplolepis gracilipes) reproductive mode: no evidence for dependent lineage genetic caste determination

Asexual reproduction and hybridisation are often found among highly invasive plants and marine invertebrates. Recently, it has been suggested that clonality may enhance the success of invasive ants. In contrast, obligate hybridisation (dependent lineage genetic caste determination or DL GCD in ants) may decrease the chances of population persistence if one lineage is less prevalent than the other (asymmetry in lineage ratio). Genetic data available for the invasive yellow crazy ant (Anoplolepis gracilipes) suggest that it has an unconventional mode of reproduction that may involve asexual reproduction by workers or queens, or a form of genetic caste determination. Here, we investigated whether A. gracilipes reproduction involved DL GCD. The potential for worker reproduction was also assessed. We used microsatellite markers to assess the population structure of A. gracilipes workers, males, queens and sperm in queen spermathecae, from field collections in Arnhem Land. We found that a single queen lineage is present in Arnhem Land. The presence of a single lineage of queens discounts the possibility of DL GCD. Population structure separated queens and workers into different lineages, suggesting that these castes are determined genetically in A. gracilipes, or the mode of reproduction differs between workers and queens. Evidence for worker reproduction was weak. We conclude that the reproductive mode of A. gracilipes does not involve DL GCD. The resolution of the reproductive mode of A. gracilipes is complicated by a high prevalence of diploid males. The determination of the A. gracilipes reproductive mode remains a fascinating research question, and its resolution will improve our understanding of the contribution of the reproductive system to invasion success.