Investigation of introgressive hybridization in endangered Sinohyriopsis mussels (Mollusca: Unionidae) using genome-wide data in Japan

Abstract Anthropogenic hybridization is a global phenomenon and a major concern in conservation biology. Sinohyriopsis schlegelii (Mollusca: Bivalvia: Unionidae), native to Lake Biwa in Japan, is considered endangered because of its hybridization with an introduced Chinese species, S. cumingii . Although previous research suggested hybridization among Sinohyriopsis mussels, the research has failed to (1) evaluate the prevalence of hybrids, (2) predict the genetic consequences for populations containing hybrids, and (3) genetically assess a population devoid of hybrids. This study aimed to fill the knowledge gaps. We collected samples from all persistent populations in Lake Biwa, Lake Kasumigaura, and Lake Anenuma (the latter two populations were introduced from Lake Biwa) and analyzed genome-wide single nucleotide polymorphisms (SNPs) data. Our findings demonstrate that hybrids are prevalent in Lake Biwa and Lake Kasumigaura but absent in Lake Anenuma. The proportion of S. cumingii ancestry increased over the past 20 years, indicating that the genetic material of S. schlegelii may diminish further in the future. Moreover, the S. schlegelii population in Lake Anenuma shared genetic similarities with the pre-hybridization population of Lake Biwa. Although the Lake Anenuma population exhibited lower genetic diversity, no signs of inbreeding were observed. Overall, the Lake Anenuma population remains the only genetically pure S. schlegelii population, but its low genetic diversity indicates a reduced adaptive potential to environmental changes. This study provides valuable insights into the current genetic status of S. schlegelii , ultimately informing conservation efforts for this critically endangered species.