Strong first sperm precedence in the freshwater hermaphroditic snail Planorbella trivolvis

摘要

Post-copulatory sexual selection, through either sperm competition or cryptic female choice, is an important aspect of sexual selection in hermaphrodites. We investigated sperm precedence, non-random fertilization of eggs by multiple partners, to determine the potential for sperm competition in the hermaphroditic freshwater snail Planorbella trivolvis. We provided snails with two sequential partners, and assessed paternity using albinism as a genetic marker. We mated albino snails first to a pigmented partner then an albino (PA; n = 26) or first to an albino then a pigmented individual (AP; n = 27) and calculated the proportion of eggs fertilized by the second partner (P2) during the 1st, 3rd, and 5th week post-mating. We collected all egg masses laid each week (similar to 300) and examined the embryos to determine paternity by the presence or absence of eye pigmentation. P2 values were quite low (0.18 +/- 0.04) indicating strong first sperm precedence and thus little opportunity for sperm competition. We also found that mating order influenced precedence; when the first mate was an albino P2 values were higher than when the first mate was pigmented, suggesting a difference in sperm transfer or viability or a preference for pigmented partners.