Quantitative genetics of phosphorus content in the freshwater herbivore, Daphnia pulicaria

1. Phosphorus (P) is essential for growth of all organisms, and P content is correlated with growth in most taxa. Although P content was initially considered to be a trait fixed at the species level, there is growing evidence for considerable intraspecific variation. Selection on such variation can thus alter the rates at which P fluxes through food webs.

2. Nevertheless, prior work describing the sources and extent of intraspecific variation in P content were not genetically explicit, confounded by unknown genetic background and evolutionary history. We constructed an F2 recombinant population of the dominant freshwater grazer, Daphnia pulicaria to mitigate such issues.

3. F2 recombinants exhibited considerable variation in growth rate, P content (0.49% to 1.97%), P use efficiency (PUE; 51 to 208 mg biomass/mg P), and correlated traits such as hatching time of resting eggs, in common garden conditions.

4. These results clearly demonstrate the scope of genetic recombination in generating variation in ecologically-relevant traits. The absence of environmental selection is a likely component driving such variation not observed in natural settings.

5. Although phosphoglucose isomerase (PGI) genotype was significantly associated with variation in hatching time of resting eggs, contrary to prior work with less rigorous designs, allelic variation at the PGI locus did not explain variation in P content and PUE of Daphnia, indicating that such quantitative traits are under polygenic control.

6. Together, these results suggest that although there is considerable genetic scope for variation in key ecologically-relevant traits, such as P content and efficiency of P use, these traits are likely under strong stabilizing selection, most likely due to selection on growth rate and size. Importantly, our observations suggest that anthropogenic alterations to P supply due to eutrophication could alter selection on these traits, thereby rapidly altering the role Daphnia plays in the P cycle of lakes.

1. 磷（Phosphorus, P）是所有生物生长所必需的元素，且多数类群的磷含量与生长速率呈显著相关。尽管最初学界认为磷含量是物种水平上固定的性状，但越来越多的证据表明其存在显著的种内变异。对这类变异的选择，能够改变磷在食物网中的流通速率。
2. 然而，此前针对磷含量种内变异的来源与程度的研究，并未明确其遗传基础，且受未知遗传背景与演化历史的干扰。为解决这一问题，我们构建了优势淡水植食者——水蚤（Daphnia pulicaria）的F2重组群体。
3. 在同质园实验条件下，该F2重组群体展现出多性状的显著变异：包括生长速率、磷含量（0.49%~1.97%）、磷利用效率（P use efficiency, PUE；51~208 mg生物量/mg P），以及休眠卵孵化时间等相关性状。
4. 本研究结果清晰证实了遗传重组能够催生大量生态相关性状的变异。本实验中无环境选择压力的条件，可能是催生这类自然环境中难以观测到的变异的关键因素之一。
5. 尽管磷酸葡萄糖异构酶（phosphoglucose isomerase, PGI）基因型与休眠卵孵化时间的变异显著相关，但这与此前设计不够严谨的研究结果相悖；而PGI基因座的等位基因变异无法解释水蚤的磷含量与PUE变异，这表明这类数量性状受多基因调控。
6. 综合来看，本研究结果表明：尽管磷含量、磷利用效率等关键生态相关性状存在可观的遗传变异潜力，但这类性状很可能受到强烈的稳定选择作用，其驱动因素大概率与生长速率和体型的选择压力相关。值得注意的是，我们的观测结果显示，因富营养化导致的人为磷供给改变，能够改变这类性状所受的选择压力，进而快速改变水蚤在湖泊磷循环中所扮演的生态角色。