Individual heterozygosity may influence the expression of fitness-related traits, via genome-wide or local genetic effects. Earlier studies have shown negative relationships between heterozygosity and sperm variation, predominantly in captive, highly inbred populations. Little is known about the possible influence of variation in heterozygosity on sperm traits in wild, outbred populations. We studied two populations of the bluethroat, one from the widely distributed northern subspecies (Luscinia. s. svecica) and the other from the smaller, more patchily distributed subspecies breeding along the French coast of Brittany (L. s. namnetum). The two subspecies differed significantly in body size, plumage colour, sperm traits and the degree of genetic diversity. However, there was no evidence that sperm traits (total length and motility) were influenced by the degree of heterozygosity at the individual level. In contrast, we found that male body size was positively related to heterozygosity across both populations, indicating a possible relationship between overall genetic diversity and general vigour or ability to obtain food. We conclude that sperm traits are unrelated to levels of heterozygosity in the studied outbred and weakly genetically depauperate bluethroat populations.

Blood and sperm samples and morphology collected from free-living individuals, using mist nets. Samples processed in DNA lab and through light microscopy.

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