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.

固着与变态是海洋无脊椎动物生命周期中至关重要的过渡阶段，野外与实验室环境下该阶段均会发生大量死亡事件。此前针对太平洋牡蛎（Crassostrea gigas）的双列杂交实验显示，变态过渡阶段存在显著的选择性死亡或基因型依赖性死亡，但该死亡的精细特征与发生时间尚不明确，尤其是其究竟发生于变态前、变态中还是变态后。本实验室研究通过每日追踪太平洋牡蛎F2杂交系固着与变态全过程中的微卫星标记（microsatellite marker）分离比，以解析该过渡阶段基因型依赖性死亡的精细模式，并探究固着时间（早与晚）是否受遗传调控，以及是否会影响基因型依赖性死亡的推断结果。实验中个体固着过程持续9天（受精后第18天至第27天），其中68%的个体于早期（第19天）或晚期（第24天）完成固着。对初始固着后的稚贝（spat）开展40天存活追踪显示，几乎无死亡发生，因此未检测到显著的遗传死亡。时间维度遗传分析显示，11个微卫星位点中有3个在变态过渡阶段表现出基因型依赖性死亡，其中Cg205位点的全过程变化被完整追踪。针对Cg205位点，两种针对纯合基因型的强选择模式揭示其可能同时存在两类发育通路缺陷：一是感应通路缺陷（无法启动变态），二是形态发生通路缺陷（变态过渡阶段发生死亡）。固着时间的数量性状位点（quantitative trait locus, QTL）定位共鉴定出3个独立QTL与1个上位性QTL（epistatic QTL），对该性状具有显著遗传效应（总解释变异量达29%）；然而其中两个位点与变态阶段发生选择性死亡的标记存在连锁关系，这可能会混淆其与固着时间的表观关联。总体而言，本研究结果揭示了牡蛎固着与变态阶段死亡与行为的复杂本质，并表明固着阶段的内源性死亡可能在牡蛎乃至其他体外产卵型海洋无脊椎动物的种群补充动态中发挥重要作用。