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

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.