Abstract
BackgroundMating between close relatives often leads to a reduction of an individual’s fitness, due to an increased expression of deleterious alleles. Thus, in many animal taxa pre- as well as postcopulatory inbreeding avoidance mechanisms have evolved. An increased risk of inbreeding and hence a loss of genetic variation may occur during founder events as in most cases only few individuals establish a new population. The threespine stickleback (Gasterosteus aculeatus) is a small externally fertilizing fish species subject to strong sperm competition. Sticklebacks inhabit both marine and freshwater environments and anadromous populations have repeatedly established new genetically less diverse freshwater populations. Previous studies showed that anadromous sticklebacks strongly suffer from inbreeding depression and when given the choice females prefer to mate with unrelated males.ResultsThe present study aimed to address whether there exists a postcopulatory inbreeding avoidance mechanism solely based on sperm-egg interactions in sperm competition experiments. We used F1 individuals that originated either from a large, genetically heterogeneous anadromous population or from a small, genetically less diverse freshwater population. For each population, eggs of two different females were in vitro fertilized by the same two males’ sperm in a paired study design. In the main experiment one male was the female’s full-sib brother and in the control experiment all individuals were unrelated. The results revealed that fertilization success was independent of relatedness in both populations suggesting a general lack of a postcopulatory inbreeding avoidance mechanism. Instead, male quality (i.e. sperm morphology) predicted paternity success during competitive fertilization trials.ConclusionIn sticklebacks, there is no evidence for postcopulatory inbreeding avoidance. Sperm morphology predicted paternity instead, thus sperm quality traits are under strong sexual selection, presumably driven by the high risk of sperm competition under natural conditions.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0353-x) contains supplementary material, which is available to authorized users.
Highlights
Mating between close relatives often leads to a reduction of an individual’s fitness, due to an increased expression of deleterious alleles
Given the severe consequences of incestuous matings (e.g. [45]) the present study aimed to investigate whether there is a postcopulatory inbreeding avoidance mechanism solely based on sperm-egg interactions in sperm competition experiments
Based on the fact that there is huge between- and within-male variation in sperm morphology in the threespine stickleback, we addressed the variation of sperm size in relation to fertilization success in the present study
Summary
Mating between close relatives often leads to a reduction of an individual’s fitness, due to an increased expression of deleterious alleles. The threespine stickleback (Gasterosteus aculeatus) is a small externally fertilizing fish species subject to strong sperm competition Sticklebacks inhabit both marine and freshwater environments and anadromous populations have repeatedly established new genetically less diverse freshwater populations. Postcopulatory preferences for genetically similar mating partners have been documented in the Arctic charr (Salvelinus alpinus) [30], the Atlantic salmon (Salmo salar) [31] and in the Peron’s tree frog (Litoria peronii) [32]. These studies stress that a high possibility of hybridization in their study species accounts for the observed results [30,31,32]. This is in accordance with recent theory, as incestuous matings provide a possibility to spread genes identical by descent [37]
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