UCLA

The unique habitats of the Northern Gulf of California are home to many endemic vertebrate and invertebrate taxa. Owing to their limited geographic range, these taxa are often vulnerable to extinction. Located at the tip of the Northern Gulf is the Colorado River Delta. Upstream damming and water diversions in the past century have turned the Delta into an inverse estuary. The resulting rise in salinity and environmental impacts in the region have raised concerns for the local endangered fauna. Among these is the endemic silverside fish, Colpichthys hubbsi, which is narrowly restricted to the tidal channels within the Delta. Its sister species, Colpichthys regis, inhabits lagoons and estuarine channels on both sides of the Gulf. The two species share a region of syntopy at the southwestern edge of the Delta where hybridization is suspected to occur. Combining phenotypic and molecular data, this study shows evidence for introgressive hybridization between C. regis and C. hubbsi. While we find no evidence for recent population decline in C. hubbsi, the reduction of river outflow may lead to diminished ecological separation between the sister species, putting C. hubbsi at risk of extinction by introgression. Future work on this system will need to focus on identifying genes under divergent selection and assessing the full extent of genomic introgression using high-throughput sequencing approaches. Molecular analyses reveal a surprising lack of genetic diversity in the less geographically restricted species (C. regis). Results from neutrality tests based on mitochondrial DNA suggest that both species of Colpichthys may have undergone recent demographic expansion. Recent studies have demonstrated profound effects of Pleistocene climate fluctuations on population structure of estuarine fish, prompting the need for additional investigations to elucidate the complex demographic histories of these taxa.