Data from: Extreme fertilisation bias towards freshly inseminated sperm in a species exhibiting prolonged female sperm storage

The storage of sperm by females across successive reproductive cycles is well documented in internal fertilisers, yet the fate of stored sperm when they compete with ‘new’ sperm to fertilise a female’s eggs has rarely been considered. This gap in our understanding is likely due to the logistical difficulties of controlling behavioural interactions during or after mating, which in turn may influence how many sperm are inseminated and how stored sperm are ultimately utilised during successive bouts of sperm competition with freshly inseminated sperm. Here, we use artificial insemination (AI) in guppies (Poecilia reticulata), a polyandrous livebearing poeciliid fish exhibiting prolonged sperm storage by females, to overcome these challenges. The use of AI enables us to control potential differential maternal effects (e.g. behaviourally mediated cryptic female choice) and specifically test for post-copulatory paternity biases that favour either stored or fresh sperm when they compete to fertilise eggs. Our paternity analyses revealed the almost complete dominance of freshly inseminated sperm over stored sperm, supporting previous studies reporting similar patterns following natural matings across successive brood cycles. However, our use artificial inseminations, which excluded behavioural interactions between males and females, most likely generated a far stronger pattern of fresh sperm precedence compared to those reported in previous studies, possibly implicating ‘cryptic’ forms of selection by females that may sometimes bolster the success of stored sperm.

雌性在连续繁殖周期中储存精子的现象，在体内受精物种中已有充分记载，但当储存的精子与“新鲜”精子竞争以使雌性卵子受精时，储存精子的最终命运却极少被关注。我们对此认知的这一空白，很可能源于交配过程中及交配后对行为交互进行控制的实操困难——这类困难又会影响每次精子竞争事件中，新授精精子的数量，以及储存精子在后续与新授精精子的竞争中最终被利用的方式。本研究以孔雀鱼（Poecilia reticulata）——一种雌性可长期储存精子的一妻多夫卵胎生花鳉科鱼类——为研究对象，采用人工授精（AI）的方法以克服上述难题。人工授精的使用使我们能够控制潜在的差异化母体效应（例如通过行为介导的隐蔽雌性选择），并专门检验当储存精子与新鲜精子竞争受精时，倾向于其中一方的交配后父权偏向。我们的父权分析结果显示，新鲜授精精子几乎完全占据了竞争优势，这与此前针对自然交配下连续育雏周期的同类研究结果一致。不过，由于本研究采用的人工授精排除了雌雄个体间的行为交互，其产生的新鲜精子优先性模式，很可能比此前研究报告的更为显著——这或许意味着雌性存在某些隐蔽选择机制，有时反而能提升储存精子的受精成功率。