Widespread hybridization and bidirectional introgression in sympatric species of coral reef fish

Abstract

Coral reefs are highly diverse ecosystems, where numerous closely related species often coexist. How new species arise and are maintained in these high geneflow environments have been long-standing conundrums. Hybridization and patterns of introgression between sympatric species provide a unique insight into the mechanisms of speciation and the maintenance of species boundaries. In this study, we investigate the extent of hybridization between two closely related species of coral reef fish: the common coral trout (Plectropomus leopardus) and the bar-cheek coral trout (Plectropomus maculatus). Using a complementary set of 25 microsatellite loci, we distinguish pure genotype classes from first- and later-generation hybrids, identifying 124 interspecific hybrids from a collection of 2,991 coral trout sampled in inshore and mid-shelf reefs of the southern Great Barrier Reef. Hybrids were ubiquitous among reefs, fertile and spanned multiple generations suggesting both ecological and evolutionary processes are acting to maintain species barriers. We elaborate on these finding to investigate the extent of genomic introgression and admixture from 2,271 SNP loci recovered from a ddRAD library of pure and hybrid individuals. An analysis of genomic clines on recovered loci indicates that 261 SNP loci deviate from a model of neutral introgression, of which 132 indicate a pattern of introgression consistent with selection favouring both hybrid and parental genotypes. Our findings indicate genome-wide, bidirectional introgression between two sympatric species of coral reef fishes and provide further support to a growing body of evidence for the role of hybridization in the evolution of coral reef fishes.

Genomic islands of divergence in the Yellow Tang and the Brushtail Tang Surgeonfishes

“…Indeed, the relative abundance of Z. scopas, Z. flavescens, and their hybrids was estimated at 90.9%, 5.6%, and 3.5%, respectively (authors’ unpublished data recorded from 2013 to 2016, based on 102 visual transects over 32 sites, total number of Zebrasoma individuals = 428). A pattern of directional introgression, as was observed here, that may be predicted by skewed abundances of parental species, is not unusual, as was reported in several other coral reef fish species (Crow et al., ; Harrison et al., ; Montanari, Hobbs, Pratchett, Bay, & Van Herwerden, ; Yaakub, Bellwood, Herwerden, & Walsh, ).…”

“…However, based on outlier loci (where species are distinguishable), we did not find evidence of introgression in Z. flavescens individuals from Hawaii. We also found that Z. scopas from Moorea hardly contain any genetic material assigned to Z. flavescens (Figure 3 observed here, that may be predicted by skewed abundances of parental species, is not unusual, as was reported in several other coral reef fish species (Crow et al, 2007;Harrison et al, 2017;Montanari, Hobbs, Pratchett, Bay, & Van Herwerden, 2017;Yaakub, Bellwood, Herwerden, & Walsh, 2006).…”

An examination of introgression and incomplete lineage sorting among three closely related species of chocolate‐dipped damselfish (genus: Chromis)

“…For example, hybridization between eight pairs of reef fish species in the Cocos–Christmas suture zone was first identified based on intermediate coloration and was later supported by observations of heterospecific breeding pairs, heterospecific social groups, and genetic data (DiBattista et al, ; Hobbs et al, ; Marie, Herwerden, Choat, & Hobbs, ; Montanari, Hobbs, Pratchett, Bay, & Herwerden, ; Payet et al, ; Yaakub, Bellwood, & Herwerden, ; Yaakub, Bellwood, Herwerden, & Walsh, ). In some cases, hybridization is not detected using coloration because hybridization occurred in distant past (Koblmüller, Egger, Sturmbauer, & Sefc, ; Kuriiwa, Hanzawa, Yoshino, Kimura, & Nishida, ), or hybrids are rare and/or because backcrossed individuals look like the parent species (Harrison et al, ). Thus, genetic markers are useful for revealing cryptic and historical cases of hybridization in our oceans.…”

“…Suture zones are phylogenetically important as hybridization can have significant consequences to speciation processes (Abbott, Hegarty, Hiscock, & Brennan, 2010;Mallet, 2007). Hybrids may (a) be unfit and favor reproductive isolation between species (Via, 2009;Wu, 2001), (b) be fit and facilitate recurring introgression and reverse speciation (Harrison et al, 2017;Kleindorfer et al, 2014), or (c) even generate new species and radiation events (Mallet, 2007;Seehausen, 2004).…”

Homoploid hybridization of plants in the Hengduan mountains region

“…Overall, these cases suggest that incomplete postzygotic barriers may still have impact on the fitness of hybrid offspring, and this process may be responsible for the maintenance of species boundaries despite frequent hybridization in the HMR. Sufficient fertile F 1 hybrids can produce backcrosses and act as potential genetic bridge between both parental species for introgression (Cannon & Scher, 2017;Harrison et al, 2017;Twyford, Kidner, & Ennos, 2015;Yatabe, Kane, Scotti-Saintagne, & Rieseberg, 2007). Hence, frequent backcrossing may lead to introgression of genetic material to one or both parents.…”