Abstract
Hybrid zones provide natural experiments where new combinations of genotypes and phenotypes are produced. Studying the reshuffling of genotypes and remodeling of phenotypes in these zones is of particular interest to document the building of reproductive isolation and the possible emergence of transgressive phenotypes that can be a source of evolutionary novelties. Here, we specifically investigate the morphological variation patterns associated with introgressive hybridization between two species of sole, Solea senegalensis and Solea aegyptiaca. The relationship between genetic composition at nuclear loci and individual body shape variation was studied in four populations sampled across the hybrid zone located in northern Tunisia. A strong correlation between genetic and phenotypic variation was observed among all individuals but not within populations, including the two most admixed ones. Morphological convergence between parental species was observed close to the contact zone. Nevertheless, the samples taken closest to the hybrid zone also displayed deviant segregation of genotypes and phenotypes, as well as transgressive phenotypes. In these samples, deviant body shape variation could be partly attributed to a reduced condition index, and the distorted genetic composition was most likely due to missing allelic combinations. These results were interpreted as an indication of hybrid breakdown, which likely contributes to postmating reproductive isolation between the two species.
Highlights
Secondary contacts between closely related species, resulting from either natural processes or anthropogenic activity, often lead to the formation of hybrid zones in which populations with divergent genomes have the potential to exchange genes
Depending on the traits considered, introgressive hybridization may lead to morphological convergence of parental populations if the underlying divergent genes behave neutrally and readily introgress upon secondary contact (Grant & Grant, 2002)
Our analysis of genetic variation across the hybrid zone between S. senegalensis and S. aegyptiaca is in good agreement with previous
Summary
Secondary contacts between closely related species, resulting from either natural processes or anthropogenic activity, often lead to the formation of hybrid zones in which populations with divergent genomes have the potential to exchange genes. As a first step toward understanding the evolutionary constraints limiting gene flow across a species boundary, we explore body shape variation patterns along a natural hybrid zone transect between two species of sole, Solea senegalensis and Solea aegyptiaca These two taxa, which geographical distributions partially overlap in the Mediterranean, are recognized as distinct sister species (Borsa & Quignard, 2001; Vachon, Chapleau, & Desoutter-Meniger, 2008). Introgressive hybridization occurs and was evidenced using a few genetic markers in previous studies based on allozymes (She et al, 1987) and intron length polymorphisms (Ouanes et al, 2011) This phenomenon predominantly occurs in the large lagoon of Bizerte that appears to be the main habitat in which hybrids are found. We use body shape variation in admixed populations to evaluate the consequence of gene flow on the condition of introgressed individuals
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