Abstract
Natural selection and genetic drift are both important for understanding the geographic distribution and environmental responses of marine species. In this study, we revealed distinct genetic variation in Crassostrea ariakensis by comparing the COI gene segment sequence in northern and southern oysters separated by the Yangtze River estuary. Reciprocal hybridization and intrapopulation crosses clarified their taxonomic status as the same species. There was no heterosis in the survival and growth of the hybrids, while the maternal effect was observed in larvae from eggs in the native habitat that showed higher fitness. Both the northern and southern F1 progenies exhibited positive performance in fitness traits, including survivorship, respiration rate, and growth, in their native habitats compared to that in their non-native habitats, indicating a strong signature of local adaptation. The oysters dwelling in the warm/southern habitats evolved a higher thermotolerance of LT50, while the oysters inhabiting the high-salinity/northern habitats had a 2.43‰ higher LS50 than that of their southern counterparts. After strong natural selection in the northern environments, the higher survival of the F1 progenies from the southern oysters under heat shock indicates an evolved genetic basis for its higher thermal tolerance. Strong environmental gradients, especially for temperature and salinity, and genetic drift of the interaction between coastal currents and the Yangtze River estuary potentially contribute to shaping the distribution pattern and adaptive divergence of C. ariakensis in China.
Highlights
Understanding geographic distribution and the causes that drive the spatiotemporal patterns of species or populations, including selection and isolation, can provide fundamental information on organismal responses and adaptive capacity in the face of rapid global change, as well as further direct implications of marine resources and fisheries management
We proposed that strong environmental gradients and the interaction between coastal currents and the Yangtze River estuary potentially contribute to shaping the geographic distribution and adaptive divergence of C. ariakensis in China
The PCR amplicon melting curves could distinguish all of the C. ariakensis individuals as well as the C. hongkongensis oysters, which showed three independent homozygote peaks, and the peaks of the southern and northern C. ariakensis oyster groups were separated according to their different melting temperatures (Figure 2A)
Summary
Understanding geographic distribution and the causes that drive the spatiotemporal patterns of species or populations, including selection and isolation, can provide fundamental information on organismal responses and adaptive capacity in the face of rapid global change, as well as further direct implications of marine resources and fisheries management. Ni et al (2017) comprehensively summarized the effects of the Yangtze River as well as its interaction with coastal currents on the genetic diversity of many marine species, such as fishes, crabs, mollusks, etc., Most of the marine species exhibited divergent genetic structures between the northern and southern populations separated by the Yangtze River, while coastal currents facilitate internal gene flows within the northern or southern populations (Xu et al, 2009; Xiao et al, 2016; Li L. et al, 2018) These findings highlight the significant role of stochastic geographic isolation following environmental selection in ecological, evolutionary, and taxonomic studies
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