Gonad and ejaculate allocation in alternative reproductive tactics of salmon and trout with reference to sperm competition

Detta är en avhandling från Stockholm : Department of Zoology, Stockholm University

Sammanfattning: Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., exhibit alternative male maturation phenotypes, anadromous and precociously mature parr, which commonly endure in sperm competition at the spawning grounds. Sperm competition provides subtle means by which selection acts on the level of individual gametes, which simultaneously compete for fertilization of eggs spawned by a single female. This type of conflict may be resolved by two, not mutually exclusive mechanisms: by the fair raffle, when spermatozoa compete numerically, and by the loaded raffle, when the asymmetry in mating roles or in ejaculate quality determines the outcome of the competition. This thesis tests some predictions from the sperm competition theory by examining allocation strategies in primary and secondary sexual traits of the alternative male phenotypes of Atlantic salmon and brown trout.Salmon and trout eggs are fertile for a longer period of time than usually assumed when inseminated by pooled milt from several males. Precociously mature parr of both species produce more sperm dense ejaculates than anadromous males. When compared to brown trout, Atlantic salmon males invest relatively more in gonads (GSI) and salmon precocious parr produce ejaculates with greater ATP concentrations per ejaculate unit, which have prolonged motility in both parr and anadromous males. These findings are discussed in the context of a pronounced semelparity of Atlantic salmon males.No difference in sperm morphology between alternative reproductive salmon males was found. Longer mitochondria produce more ATP, longer tails have a greater energy charge, and a longer terminal tail piece causes greater fertilization success than shorter spermatozoa. Mature Atlantic salmon parr invest relatively more in gonad tissue and produce sperm with greater ATP concentrations in ejaculates of greater vigour than anadromous counterparts. As a consequence, their gametes were superior in the numerically unloaded sperm competition with anadromous male gametes.Variance in allocation of surplus energy in body size and sexual traits in dependence of the life history ontogeny is well explained by non-linear polynomial modelling, revealing inherent differences in quality among anadromous males. Males that reach greater body size as smolt tend to return earlier to spawn, and fish that produce smaller gonads and smaller secondary sexual traits produce ejaculates of greater quality. In conclusion, the findings presented in this thesis give credence to the compensatory loaded raffle sperm competition mechanism.

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