Mehlis Marion, Rahn Anna K, Bakker Theo C M
Institute for Evolutionary Biology and Ecology, University of Bonn, An der Immenburg 1, D-53121, Bonn, Germany.
BMC Evol Biol. 2015 Apr 26;15:74. doi: 10.1186/s12862-015-0353-x.
Mating 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.
The 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.
In 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.
近亲交配常常会导致个体适应性下降，这是由于有害等位基因的表达增加所致。因此，在许多动物类群中，交配前和交配后的近亲繁殖规避机制已经演化出来。在奠基者事件期间，近亲繁殖的风险增加，进而可能导致遗传变异丧失，因为在大多数情况下，只有少数个体建立新的种群。三刺鱼（Gasterosteus aculeatus）是一种小型体外受精的鱼类，面临着激烈的精子竞争。三刺鱼栖息于海洋和淡水环境，溯河洄游种群多次建立了遗传多样性较低的新淡水种群。先前的研究表明，溯河洄游的三刺鱼深受近亲繁殖衰退之苦，并且在有选择的情况下，雌性更倾向于与无亲缘关系的雄性交配。
本研究旨在探讨在精子竞争实验中是否存在仅基于精卵相互作用的交配后近亲繁殖规避机制。我们使用了来自大型、遗传异质的溯河洄游种群或小型、遗传多样性较低的淡水种群的F1个体。在配对研究设计中，对于每个种群，用同一两个雄性的精子对两个不同雌性的卵进行体外受精。在主要实验中，一个雄性是雌性的全同胞兄弟，在对照实验中所有个体均无亲缘关系。结果显示，两个种群的受精成功率均与亲缘关系无关，这表明普遍缺乏交配后近亲繁殖规避机制。相反，在竞争性受精试验中，雄性质量（即精子形态）预测了父权成功。
在三刺鱼中，没有证据表明存在交配后近亲繁殖规避现象。相反，精子形态预测了父权，因此精子质量性状受到强烈的性选择，这可能是由自然条件下精子竞争的高风险驱动的。
