Microsatellite DNA analysis reveals low diploid male production in a communal bee with inbreeding

The mechanism of sex determination assumed widespread in parthenogeneticall y arrhenotokous Hymenoptera is that of single locus complementary sex deter mination (CSD). Functionally sterile diploid males are produced under CSD a nd generate a genetic load, the cost of which increases with inbreeding. We quantify diploid male production (DMP, proportion of diploid individuals t hat are male) using a morphological criterion (adult fresh weight) and gene tical (microsatellite DNA) markers in a communal, sexually size-dimorphic b ee, Andrena scotica, which inbreeds. Male genotypes suggested a DMP of 0.00 3. The inbreeding coefficient, f; was significantly positive (+0.165), equi valent to 44% of matings being among full sibs (predicted DMP of 0.11). We hypothesize three non-mutually exclusive explanations to account for the la rge difference between the low observed (in males) and high expected (deriv ed from f for females) DMP: (i) multilocus CSD, (ii) 'sex allele signalling ' tied to mate selection, and (iii) sperm selection within mated Females. T he costs of inbreeding through DMP are apparently low in A. scotica. (C) 20 00 The Linnean Society of London.