Data from: The developmental plasticity and functional significance of an additional sperm storage compartment in female yellow dung flies

1. The mechanistic basis for, and adaptive significance of variation in female sperm storage organs is important for a range of questions concerning sexual selection and speciation, as such variation influences the evolutionary trajectories of male fertilization related traits and may facilitate speciation through its effects on gamete recognition. 2. Female yellow dung flies (Scathophaga stercoraria) usually develop three sperm storage compartments, and this subdivision may be an adaptation for sorting sperm during post-copulatory choice. 3. Using lines artificially selected to express four spermathecae (4s), we explored the fitness consequences of the novel phenotype relative to the naturally prevalent three-spermatheca (3s) phenotype by manipulating the opportunity for post-copulatory sexual selection (females mated either with three or only one male prior to oviposition). In addition, we examined the developmental plasticity of spermathecal number in response to different larval food environments and estimated its genetic correlation with growth rate. 4. Mating treatments with and without the opportunity for post-copulatory sexual selection revealed no significant fitness differences between alternative spermathecal phenotypes within selection lines despite overall benefits associated with multiple mating, and moderate egg-to-adult survival costs in response to artificial selection for 4s. Manipulations of the larval food environment revealed that the expression of 4s is highly plastic and tightly linked to environmental conditions promoting fast somatic growth and development. Likewise, siblings with fast intrinsic (genetic) growth were more likely to express 4s within and across food environments. 5. The present results highlight a great potential for rapid evolutionary change in female sperm storage morphology through indirect selection on life-history traits, and further suggest genetic assimilation as a potential mechanism facilitating phylogenetic transitions in spermatheca number as frequently observed within the Dipterans.

1. 雌性精子储存器官变异的机制基础与适应意义，对于一系列涉及性选择与物种形成的研究问题具有重要价值：此类变异会影响雄性受精相关性状的演化轨迹，并可通过对配子识别的作用推动物种形成。
2. 雌性黄粪蝇（Scathophaga stercoraria）通常具有3个精子储存室，这种结构分化可能是其在交配后选择过程中对精子进行分选的一种适应策略。
3. 本研究利用人工选育得到的表达4个受精囊（spermathecae，简称4s）的品系，通过操控交配后性选择的机会（雌虫在产卵前分别与3头或仅1头雄虫交配），探究了该新型表型相较于自然普遍存在的3个受精囊（3s）表型的适合度后果。此外，我们还检测了受精囊数量对不同幼虫食物环境的发育可塑性，并估算了其与生长速率的遗传相关关系。
4. 有无交配后性选择机会的交配处理实验显示，尽管多配偶交配整体上具有适合度收益，且针对4s的人工选育会带来中等程度的卵到成虫存活代价，但选育品系内不同受精囊表型间并未出现显著的适合度差异。对幼虫食物环境的操控实验表明，4s表型的表达具有高度可塑性，且与促进快速躯体生长发育的环境条件紧密相关。同样，在不同食物环境下，具有较快内在（遗传）生长速率的同窝个体更易表达4s表型。
5. 本研究结果表明，通过对生活史性状的间接选择，雌性精子储存形态存在快速演化的巨大潜力；同时进一步提示，遗传同化是推动受精囊数量发生系统发育转变的潜在机制，这一现象在双翅目（Dipterans）昆虫中较为常见。