Data from: Fine-scale temporal analysis of genotype-dependent mortality at settlement in the Pacific oyster Crassostrea gigas

Settlement and metamorphosis mark a critical transition in the life cycle of marine invertebrates, during which substantial mortality occurs in both field and laboratory settings. Previous pair-crossing experiments with the Pacific oyster Crassostrea gigas have revealed significant selective or genotype-dependent mortality around the metamorphic transition, but the fine-scale nature and timing of this mortality is not known, particularly whether it occurs before, during or after metamorphosis. In this laboratory study, microsatellite marker segregation ratios were followed daily throughout the settlement and metamorphosis of an F2 cross of the Pacific oyster to examine the fine-scale patterns of genotype dependent mortality at this transition and whether settlement timing (early vs. late) might be under genetic control and affect inference of genotype dependent mortality. Settlement occurred over nine days (day 18 to day 27 post-fertilization) with 68% of individuals settling either early (day 19) or late (day 24). Tracking the survival of spat for 40 days after initial settlement revealed almost no mortality and thus no appreciable genetic mortality. Temporal genetic analysis revealed that 3/11 loci exhibited genotype dependent mortality around the metamorphic transition, one of which (Cg205) was followed throughout settlement and metamorphosis. Alternative temporal patterns of strong selection against each homozygous genotype at Cg205 revealed possible defects in both the competency pathway (inability to initiate metamorphosis) and the morphogenesis pathway (mortality during the metamorphic transition). Quantitative trait locus (QTL) mapping of settlement timing identified three individual and one epistatic QTL with significant genetic effects on this trait (29% of the variance explained in total); however, two of these loci were linked to markers exhibiting selective mortality at metamorphosis, potentially confounding their apparent association with settlement timing. Overall, the results of this study highlight the complex nature of mortality and behavior during settlement and metamorphosis in oysters and suggest that endogenous sources of mortality at settlement may play an important role in the recruitment dynamics of oysters and possibly other broadcast spawning marine invertebrates.

附着变态（settlement and metamorphosis）是海洋无脊椎动物（marine invertebrates）生命周期中的关键转折点，在此阶段野外与实验室环境中均会出现大量个体死亡。过往针对太平洋牡蛎（Crassostrea gigas）的成对杂交实验已发现，在变态转变阶段存在显著的选择性死亡或基因型依赖型死亡，但该类死亡的精细特征与发生时间仍不明晰，尤其无法确定其是发生在变态前、变态过程中还是变态后。本实验室研究以太平洋牡蛎的F2杂交群体（F2 cross）为对象，每日追踪其附着变态过程中的微卫星标记（microsatellite marker）分离比，以此解析该转变阶段基因型依赖型死亡的精细模式，并探究附着时间（早 vs. 晚）是否受遗传调控，以及该调控是否会影响对基因型依赖型死亡的推断。附着过程持续9天（受精后第18天至第27天），其中68%的个体分别于早期（受精后第19天）或晚期（受精后第24天）完成附着。对初始附着后的幼贝（spat）进行40天的存活追踪后发现，几乎未出现死亡，因此未检测到显著的遗传型死亡。时序遗传分析显示，11个基因位点中有3个在变态转变阶段存在基因型依赖型死亡，其中Cg205位点的变化被全程追踪观测。在Cg205位点上，针对两种纯合基因型的强选择呈现出不同的时序模式，这表明该位点可能同时存在发育潜能通路（无法启动变态）和形态发生通路（变态转变阶段死亡）的缺陷。对附着时间的数量性状位点（quantitative trait locus, QTL）定位分析共鉴定出3个独立数量性状位点与1个上位性数量性状位点（epistatic QTL），该类位点对附着时间性状具有显著遗传效应（总解释变异量达29%）；但其中2个位点与变态阶段表现出选择性死亡的标记连锁，这可能会干扰其与附着时间的表观关联分析。综上，本研究结果揭示了牡蛎附着变态阶段死亡与行为的复杂特性，并表明附着阶段的内源性死亡可能在牡蛎以及其他体外产卵型（broadcast spawning）海洋无脊椎动物的种群补充动力学（recruitment dynamics）中扮演重要角色。