Frequency and origin of triploidy in the fire ant Solenopsis invicta

A large microsatellite survey of fire ants of both social forms (monogyne and polygyne) from both the native and introduced ranges (Argentina and the U.S.A.) revealed surprisingly high levels of triploidy (12%) in non-reproductive females from an introduced polygyne population in Georgia, U.S.A. Triploid females were not detected among reproductive (egg-laying) queens from this population, among females from monogyne populations in the introduced range or among females of either social form from the native range. The results of complementary analyses with allozyme markers were highly concordant with the individual microsatellite ploidy designations and confirmed the general patterns in the distribution of triploidy observed with microsatellites. Triploidy in the Georgia polygyne population is hypothesized to be linked to the high frequency of diploid males in this population; although most such males are sterile, 2.4% were found to possess functional reproductive tracts and presumably produce diploid sperm. Mating of such males with polygyne queens is expected to give rise to triploid females at levels close to those observed. Diploid males are absent in monogyne populations and uncommon in the introduced range, thus explaining the lack of triploid females in samples from these sources. The absence of triploid reproductive queens in the Georgia polygyne population may result from subviability of triploid queens or execution of such queens by workers. Our results suggest that triploid females of social Hymenoptera may be more common than appreciated and are most likely to be found in populations with substantial numbers of fertile diploid males.