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1.  Sperm competitiveness in frogs: slow and steady wins the race 
When sperm compete to fertilize available ova, selection is expected to favour ejaculate traits that contribute to a male's fertilization success. While there is much evidence to show that selection favours increased numbers of sperm, only a handful of empirical studies have examined how variation in sperm form and function contributes to competitive fertilization success. Here, we examine selection acting on sperm form and function in the externally fertilizing myobatrachid frog, Crinia georgiana. Using in vitro fertilization techniques and controlling for variation in the number of sperm contributed by males in competitive situations, we show that males with a greater proportion of motile sperm, and motile sperm with slower swimming velocities, have an advantage when competing for fertilizations. Sperm morphology and the degree of genetic similarity between putative sires and the female had no influence on competitive fertilization success. These unusual patterns of selection might explain why frog sperm typically exhibit relatively slow swimming speeds and sustained longevity.
PMCID: PMC2825793  PMID: 19710059
sperm competition; polyandry; genetic compatibility; sperm velocity; sperm length; frogs
2.  Sperm competition and the evolution of gamete morphology in frogs. 
Despite detailed knowledge of the ultrastructure of spermatozoa, there is a paucity of information on the selective pressures that influence sperm form and function. Theoretical models for both internal and external fertilizers predict that sperm competition could favour the evolution of longer sperm. Empirical tests of the external-fertilization model have been restricted to just one group, the fishes, and these tests have proved equivocal. We investigated how sperm competition affects sperm morphology in externally fertilizing myobatrachid frogs. We also examined selection acting on egg size, and covariation between sperm and egg morphology. Species were ranked according to probability of group spawning and hence risk of sperm competition. Body size, testis size and oviposition environment may also influence gamete traits and were included in our analyses. After controlling for phylogenetic relationships between the species examined, we found that an increased risk of sperm competition was associated with increased sperm head and tail lengths. Path analysis showed that sperm competition had its greatest direct effect on sperm tail length, as might be expected under selection resulting from competitive fertilization. Sperm competition did not influence egg size. Oviposition location had a strong influence on egg size and a weak influence on sperm length, with terrestrial spawners having larger gametes than aquatic spawners. Our analysis revealed significant correlated evolution between egg morphology and sperm morphology. These data provide a conclusive demonstration that sperm competition selects for increased sperm length in frogs, and evidence for evolutionary covariance between aspects of male and female gamete morphology.
PMCID: PMC1691467  PMID: 14561298
3.  Simultaneous mating with multiple males reduces fertilization success in the myobatrachid frog Crinia georgiana 
Approximately 50% of matings in the frog Crinia georgiana involve two or more males. We report reduced fertilization success as a major cost of mating with multiple males. For single-male matings, fertilization success was consistently high averaging 96%. Only 68% of eggs were fertilized when females were amplexed by two males and this dropped to 64% when females were amplexed by three to five males. Multiple regression analysis revealed the reduction in fertilization success was significantly related to the number of amplectant males but not to clutch size or three measures of water quality (depth, temperature and oxygen concentration) at the site of oviposition. The most likely cause of reduced fertilization success is struggles amongst males which interfere with effective sperm transfer.
PMCID: PMC1689823

Results 1-3 (3)