Frequency of multiple insemination in a natural population of Drosophila montana

The frequency of multiple insemination was studied in a boreal Drosophila montana population using mother-offspring data for a sex-linked allozyme locus. Mating with respect to the marker studied was random. In crowded laboratory cultures the heterozygous offspring had higher viability than homozygotes, but no deviations from Hardy-Weinberg proportions were found in natural populations. In multiply sired progenies the males did not contribute equally to the progeny, but the proportion sired by the second male was 0.76. The estimated frequency of multiple inseminated females was 1.19 +/- 0.31, indicating that practically all the females carried sperm of at least two males. The estimate is the largest ever reported in natural populations of Drosophila. However, the interspecific comparisons may not be relevant, because the frequency of multiple insemination does not necessarily reflect the real lifetime frequency of multiple matings. The effects of local ecology and life history characters, e.g., the uniform age structure and the temporal patterning of matings, on the high degree of detected multiple inseminations are discussed.