Subterranean Fishes of the World

3074

2015

AIM 2015

Loss of phenotypic characters is a widespread phenomenon that affects nearly every living organism, yet the genetic mechanisms underlying loss remain largely unknown. Cave animals represent excellent models to examine regressive evolution, owing to the common loss of eyes and pigmentation among lineages evolving in the subterranean habitat. The blind Mexican cavefish, Astyanax mexicanus, is a particularly powerful system because “ancestral” surface-dwelling forms are available, allowing for direct comparisons with cave-dwelling morphs. To date, only two genes (Oca2-albinism and Mc1r-brown) have been linked to specific pigmentation phenotypes in cavefish. However, pigment cell (melanophore) number is a complex trait governed by multiple genes, and the identities of these loci remain unidentified. To uncover these genes, we assembled a high-resolution, second-generation linkage map. We adapted an automated phenotypic scoring strategy, in which melanophore number is quantified across seven body regions in a surface x Pachón cave F2 pedigree. QTL mapping analyses yielded several markers strongly associated with melanophore number variation in the dorsal mid-lateral stripe area and in the superior head region, which anchor to specific regions of the Astyanax draft genome and the zebrafish genome. Within these syntenic regions we identified two candidate genes, Tyrp1b and Pmela, with known roles in pigmentationrelated processes based on Gene Ontology annotation. Mutant forms of these candidate genes in other systems cause global and regional pigmentation losses, respectively. Indepth analyses of these genes reveal coding sequence alterations and reduced expression in Pachón cavefish compared to surface morphs. This work uniquely identifies Tyrp1b and Pmela as genes involved with complex aspects of Astyanax pigmentation, and provides insight to genetic mechanisms governing regressive phenotypic change.

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