Abstract
Hybrid zones are natural laboratories for investigating the dynamics of gene flow, reproductive isolation, and speciation. A predominant marine hybrid (or suture) zone encompasses Christmas Island (CHR) and Cocos (Keeling) Islands (CKE), where 15 different instances of interbreeding between closely related species from Indian and Pacific Oceans have been documented. Here, we report a case of hybridization between genetically differentiated Pacific and Indian Ocean lineages of the three‐spot dascyllus, Dascyllus trimaculatus (Rüppell, 1829). Field observations indicate there are subtle color differences between Pacific and Indian Ocean lineages. Most importantly, population densities of color morphs and genetic analyses (mitochondrial DNA and SNPs obtained via RADSeq) suggest that the pattern of hybridization within the suture zone is not homogeneous. At CHR, both color morphs were present, mitochondrial haplotypes of both lineages were observed, and SNP analyses revealed both pure and hybrid genotypes. Meanwhile, in CKE, the Indian Ocean color morphs were prevalent, only Indian Ocean mitochondrial haplotypes were observed, and SNP analysis showed hybrid individuals with a large proportion (~80%) of their genotypes assigning to the Indian Ocean lineage. We conclude that CHR populations are currently receiving an influx of individuals from both ocean basins, with a greater influence from the Pacific Ocean. In contrast, geographically isolated CKE populations appear to be self‐recruiting and with more influx of individuals from the Indian Ocean. Our research highlights how patterns of hybridization can be different at scales of hundreds of kilometers, due to geographic isolation and the history of interbreeding between lineages.
Highlights
Hybridization, or the interbreeding between species or subpopulations, is common in areas where allopatric lineages overlap (Hewitt, 1988)
Two major marine suture zones have been recognized for coral reef fishes: the well-studied zone that encompasses Christmas Island (CHR) and Cocos (Keeling) Islands (CKE) in the eastern Indian Ocean (Hobbs & Allen, 2014; Hobbs, Frisch, Allen, & Van Herwerden, 2008), and the less explored zone in the Socotra Archipelago (DiBattista et al, 2015). In the former, glaciations led to sea level drops that exposed the Sunda Shelf, restricting water between the tropical Indian and Pacific Oceans, which resulted in divergence of many marine populations and species (Barber, Palumbi, Erdmann, & Moosa, 2000; Gaither & Rocha, 2013; Ludt & Rocha, 2015)
The present study aims to elucidate the genomic divergence of the Pacific and Indian Ocean lineages of D. trimaculatus, and their interaction in the Cocos–Christmas suture zone using a combination of genomic and field approaches
Summary
Hybridization, or the interbreeding between species or subpopulations, is common in areas where allopatric lineages overlap (Hewitt, 1988). Two major marine suture zones have been recognized for coral reef fishes: the well-studied zone that encompasses Christmas Island (CHR) and Cocos (Keeling) Islands (CKE) in the eastern Indian Ocean (Hobbs & Allen, 2014; Hobbs, Frisch, Allen, & Van Herwerden, 2008), and the less explored zone in the Socotra Archipelago (DiBattista et al, 2015) In the former, glaciations led to sea level drops that exposed the Sunda Shelf, restricting water (and gene flow) between the tropical Indian and Pacific Oceans, which resulted in divergence of many marine populations and species (Barber, Palumbi, Erdmann, & Moosa, 2000; Gaither & Rocha, 2013; Ludt & Rocha, 2015). Our study provides an example on the importance of suture zones for genomic exchange between closely related lineages, while highlighting how dynamics of hybridization can be different at small geographic scales (hundreds of kilometers)
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