Zoology - Theses
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ItemSexual selection in cephalopods: multiple mating and sperm competition in dumpling squid (Euprymna tasmanica)( 2013)Sex differences in reproductive investment play a crucial role in sexual conflict. One intriguing aspect is conflict over mating frequency. In this regard, the evolution of female multiple mating (polyandry) has received particular attention, especially in systems where females receive no obvious direct benefits from males, and where mating is costly. Here, theory predicts that polyandrous females can increase their reproductive success by taking advantage of the genetic benefits of mating with multiple males. Cephalopods provide an interesting system for addressing this question because the great majority of species that have been studied mate multiply, with females storing sperm from multiple males, and sperm of many species have remarkable longevities. Mating is also likely to be costly in many species. Using the dumpling squid, Euprymna tasmanica, I examined differences in reproductive success between singly mated (monandrous), multiply mated (to the same male) and polyandrous (mated to two different males) females, controlling for recent male mating history and mating frequency. I found polyandrous females produced eggs faster and had larger hatchlings relative to egg mass than females mated once with a single male. These benefits are likely to outweigh the costs associated with mating and help to explain how multiple mating has evolved (or is maintained) in this group. In order to assess the natural level of multiple paternity and sperm use patterns in E. tasmanica, I developed five novel polymorphic microsatellites for this species, and measured the level of multiple paternity in clutches collected directly from the field, and from a series of clutches laid in the laboratory. All clutches measured had multiple paternity, with 2 – 4 sires per clutch, concordant with levels reported in other cephalopods. Clutches collected from the field had significantly higher levels of multiple paternity than those laid over an extended period in the laboratory, suggesting that females mate between laying bouts in the field. In order to understand sperm precedence patterns in this species I genotyped offspring produced from polyandrous females. Here I found that the last male to mate gains an advantage, siring up to 75 % of eggs at the beginning of the laying period. This level decreases to 54 % by the end of the laying period, suggesting that sperm are stratified within the female sperm storage organ at the beginning of the laying period and are more mixed by the end. Patterns of sperm precedence varied markedly among females and this variation was not correlated to any trait measured including male mass, copulation duration or the genetic relatedness of the mating pairs. The fact that all clutches had multiple sires might indicate that genetic diversity of offspring is an important driver of polyandry in this system. When examining mating behaviour in dumpling squid I found that male E. tasmanica perform more sperm displacement behaviour if the female has recently been mated, showing that they are able to determine the recent mating history of females. I found that sperm can be stored for a large proportion of their lifetime, up to 145 days. The data presented here represent a significant advancement in the knowledge of the evolution of polyandry and sperm use patterns in E. tasmanica and for cephalopods more generally.