Genetic differentiation between the ant Myrmica rubra and its microgynous social parasite

Abstract

Hymenopteran inquiline species have been proposed to originate by sympatric speciation through intraspecific social parasitism. One such parasite, Myrmica microrubra, was recently synonymized with its Myrmica rubra host, because comparisons across Europe indicated insufficient genetic differentiation. Here, we use micro- satellite markers to study genetic differentiation more precisely in a sample of Finnish M. rubra and its inquilines collected at two localities, supplemented with mitochon- drial DNA sequences. The parasite had much lower genetic variation than the host at three of the four loci studied. Genetic differentiation between the host populations was moderate (FST = 0.089), whereas the parasite populations were more strongly subdivided (FST = 0.440). The host and parasite were highly genetically differentiated both across populations (FST = 0.346) and in strict sympatry (0.327, 0.364), a result that remained robust both in a haplotype network and in PCA ordination. Individual assignments of genotypes indicated that gene flow between sympatric host and inquiline populations is reduced by about an order of magnitude relative to the gene flow within the morphs. Our results suggest that the parasitic morph of M. rubra may be an incipient species, but it remains unclear to what extent the observed genetic dif- ferentiation between host and inquiline is due to possible assortative mating and selection against hybrids or to recurrent bottlenecking and genetic drift. We conclude that an explicitly functional species concept would be unam- biguous in treating this inquiline as a full species, as it begets its own kind and maintains its integrity in spite of occasional interbreeding with the host.