Last-Male Sperm Priority in a Haplogyne Spider (Araneae: Pholcidae): Correlations between Female Morphology and Patterns of Sperm Usage

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Abstract
In species where the female mates more than once, sperm priority patterns have important implications for the mating behavior of both sexes. The shape of female sperm storage organs may play a role in determining priority patterns, although the extent of this effect has been controversial. Spiders provide an interesting test of this hypothesis because 2 phylogenetically distinct groups differ in the structure of sperm storage organs. In entelegyne spiders, the female sperm-storage organs have separate ducts for sperm entrance and exit. The male inseminates the female through the external fertilization duct. Sperm then travel through a “conduit” and are released from an internal fertilization duct to fertilize eggs. This arrangement has been hypothesized to lead to 1st-male priority. Six species in 3 entelegyne families have been shown to have 1st-male sperm priority with some mixing; 1 entelegyne species, however, shows last-male priority. Haplogyne females, in contrast, have a spermstorage area with only a single opening, which may be more likely to lead to last-male sperm priority because the last sperm to enter the storage area would be likely to be the 1st to leave. Few data are available for this group. We used a balanced sterile-male design to test the sperm priority pattern in females mated once with each of a pair of males of a haplogyne spider, Holocnemus pluchei Scopoli. Egg sacs from 72.5% of females were sired predominantly by 2nd males, 7.5% were predominantly sired by 1st males, and 20% showed mixed paternity (n = 40); these frequencies differed significantly from random expectations. The average percentage of eggs fathered by the 2nd male, or P 2, was 74%. These data clearly differ from the predominantly 1st-male pattern generally found in entelegynes, thereby supporting the hypothesized relationship between female reproductive morphology and sperm use patterns. Mating behavior of spiders that exhibit 2nd-male priority should differ from that exhibiting 1st-male priority, and evidence from H. pluchei and other haplogynes supports this inference.

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