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PubMed2026

Admixture and Selection Driven by El Niño-Southern Oscillation Events Shape the Genetic Structure of Octopus mimus-O. hubbsorum Complex Across the Humboldt and South Equatorial Current Transition Zone.

Marine transition zones, where contrasting water masses converge, can function as natural laboratories for studying admixture and early stages of speciation. The genomic structure of the eastern Pacific Octopus mimus-O. hubbsorum complex was investigated by analyzing whole-genome sequencing data from 67 individuals sampled along the west coast of the Americas, spanning Mexico and the Peruvian coast. This includes the South Equatorial Current, the transition zone, and the Humboldt Current System. The mitochondrial genomes fell into two major genetic clades that largely corresponded to the warm-water northern (O. hubbsorum) and cold-water southern (O. mimus) lineages. Analyses of the nuclear genomes revealed the same bipartite structure but also identified a broad admixture zone characterized by two different admixed clades (Admixed-Cold and Admixed-Warm). The results suggest that episodic relaxation of oceanographic barriers during El Niño-Southern Oscillation (ENSO) events promotes secondary contact and gene flow, resulting in admixed individuals recurrently during ENSO years. However, the survival of these admixed individuals depends on the adaptive genetic composition of each organism and the prevailing environmental conditions. Outlier SNP analysis supports these findings, where the Admixed-Cold cluster shares mainly the adaptive genetic component identified as outliers in O. mimus, while Admixed-Warm is linked to those in O. hubbsorum. The O. mimus-O. hubbsorum complex is currently occupying a gray zone of speciation, in which selection and climate-driven connectivity act in tandem to shape genomic divergence.

gene flow↗
PubMed2026

Genomic separation of Salish Sea and Pacific outer coast populations of the keystone sea star Pisaster ochraceus.

Environmental boundaries shape genetic diversity through the interacting effects of geographic distance, local adaptation, and constraints on gene flow. The ochre sea star (Pisaster ochraceus), an intertidal keystone predator, has long been considered to have limited spatial genetic structure along the North American Pacific coast, likely due to its extended larval dispersal period and high potential for gene flow. Here, we characterize spatial genomic variation in Pisaster ochraceus using whole-genome sequencing data from individuals spanning nearly 3000 kilometers of coastline from Alaska to southern California. Analyses of putatively neutral SNPs demonstrate considerable mixing across the latitudinal range, but also reveal substantial structure between outer Pacific coast populations and those within the semi-enclosed Salish Sea, suggesting restricted gene flow and demographic divergence between these regions. Genomic divergence is further supported by evidence of selection, with outlier loci highlighting extended regions of low diversity in the Salish Sea, consistent with recent selective sweeps and potential local adaptation to distinct estuarine conditions. These findings support the role of oceanographic barriers and environmental heterogeneity in shaping population structure in Pisaster ochraceus, challenging earlier expectations of range-wide homogeneity and providing insight into the persistence of this keystone marine species in a rapidly changing world.

Pisaster↗