INTRASPECIFIC VARIATION IN SPERM SIZE CHARACTERS

Interspecific variation in quantitative characters of sperm is conside rable and can sometimes be related to aspects of sperm competition. In contrast, continuous intraspecific variation is often ignored; for ex ample, many studies concentrate on measuring a species-typical sperm s ize. However, statistically significant variation amongst males in a v ariety of sperm size characters, particularly total length and a numbe r of head characters, has been reported in at least 13 species. Furthe rmore, sperm size has been shown to influence the outcomes of processe s invoked in sperm competition in two species, with larger sperm being more successful in both cases. It is suggested here that intraspecifi c variation may be a widespread phenomena, further examination of whic h could contribute considerably to our understanding of sperm evolutio n. Size differences between the sperm of competing males could be used in controlled matings to investigate the fates of sperm within female tracts and storage organs, but care should be taken that size charact ers may not be neutral markers. Furthermore, the genetic determination of sperm size is considerable (h(2)=0.56-0.92 for total length and a number of head characters) and determinants are at least sometimes sex -linked, with the result that particular care must go into experimenta l design if genetic effects are to be fully elucidated. Studies combin ing genetical and behavioural effects of intraspecific variation in sp erm morphology should be particularly rewarding.