Data from: The consequences of facultative sex in a prey adapting to predation

A species reproductive mode, along with its associated costs and benefits, can play a significant role in its evolution and survival. Facultative sexuality, being able to reproduce both sexually and asexually, has been deemed evolutionary favourable as the benefits of either mode may be fully realized. In fact, many studies have focused on identifying the benefits of sex and/or the forces selecting for increased rates of sex using facultative sexual species. The costs of either mode however can also have a profound impact on a population's evolutionary trajectory. Here we used experimental evolution and fitness assays to investigate the consequences of facultative sexuality in prey adapting to predation. Specifically, we compared the adaptive response of algal prey populations exposed to constant rotifer predation and which had alternating cycles of asexual and sexual reproduction where sexual episodes were either facultative (sexual and asexual progeny simultaneously propagated) or obligate (only sexual progeny propagated). We found that prey populations with facultative sexual episodes reached a lower final relative fitness and suffered a greater trade-off in traits under selection, i.e. defense and competitive ability, as compared to prey populations with obligate sexual episodes. Our results suggest that costs associated with sexual reproduction (germination time) and asexual reproduction (selection interference) were amplified in the facultative sexual prey populations, leading to a reduction in the net advantage of sexuality. Additionally, we found evidence that the cost of sex was reduced in the obligate sexual prey populations because increased selection for sex was observed via the spontaneous production of sexual cells. These results show that certain costs associated with facultative sexuality can affect an organism's evolutionary trajectory.物种的生殖模式及其相关的成本与收益，对其演化与存活具有至关重要的作用。兼性有性生殖（Facultative sexuality）——即同时可进行有性与无性生殖的生殖方式——被认为在演化上具有优势，因为其能充分利用两种生殖模式的收益。事实上，诸多研究均以兼性有性生殖物种为对象，旨在揭示有性生殖的优势，或是筛选出推动有性生殖频率提升的演化动力。不过，两种生殖模式各自的成本，同样会对种群的演化轨迹产生深远影响。本研究通过实验演化与适合度测定（fitness assays），探究了兼性有性生殖在猎物适应捕食过程中的演化后果。具体而言，我们对比了两类藻类猎物种群的适应性响应：一类持续暴露于轮虫捕食压力下，且其生殖周期为无性与有性交替循环，其中有性生殖阶段为兼性（同时产生有性与无性后代）；另一类的有性生殖阶段为专性（仅产生有性后代）。研究发现，与专性有性生殖阶段的猎物种群相比，兼性有性生殖阶段的猎物种群最终相对适合度更低，且在选择压力下的性状权衡（trade-off）更为显著——即防御能力与竞争能力之间的权衡。我们的结果表明，与有性生殖相关的成本（萌发时间）与无性生殖相关的成本（选择干扰）在兼性有性生殖的猎物种群中被放大，进而降低了有性生殖的净优势。此外，我们发现专性有性生殖的猎物种群中，有性生殖的成本有所降低：因为通过自发产生有性细胞的方式，可观察到对有性生殖的选择压力有所提升。上述结果表明，与兼性有性生殖相关的特定成本，能够影响生物体的演化轨迹。