![]() This admixture results in significant heterozygote deficiencies at numerous loci due to Wahlund effects. Some of the cave populations receive migrants from the surface and exchange migrants with one another, especially when geographically close. We found lower genetic diversity in cave than in surface populations correlated with their smaller effective population sizes, probably the result of food and space limitations. The cave populations we studied span the full range of the cave forms in three separate geographic regions and have at least five separate evolutionary origins. We found that surface populations are similar to one another, despite their relatively large geographic separation, whereas the cave populations are better differentiated. Results To assess the genetic structure within populations and the relationships among them we genotyped individuals at 26 microsatellite loci. We studied 11 cave and 10 surface populations of Astyanax mexicanus in order to better understand the evolutionary origins of the cave forms, the basic genetic structuring of both cave and surface populations, and the degree to which present day migration among them affects their genetic divergence. Together, these data suggest evolutionary changes in the fms pathway or fms requirements, and identify changes in cellular interactions as a likely mechanism of evolutionary change in Danio pigment patterns.Ībstract (provisional) Background Cave animals converge evolutionarily on a suite of troglomorphic traits, the best known of which are eyelessness and depigmentation. ![]() Rather, our results suggest an alternative model in which evolutionary changes in pigment cell interactions themselves have contributed to stripe loss, and we test this model by manipulating melanophore numbers in interspecific hybrids. albolineatus results from a loss of fms-dependent xanthophores and their interactions with melanophores. These findings exclude the simplest model in which stripe loss in D. rerio, and in stark contrast to fms mutant D. albolineatus and enhanced xanthophore development compared with wild-type D. Yet, we also find persistent fms expression in D. albolineatus, we demonstrate increased rates of melanophore death and decreased melanophore migration, different from wild-type D. albolineatus and test the simplest model to explain this transformation, a loss of fms activity in D. We next examine the cellular bases for the evolutionary loss of stripes in D. Here, we survey additional species and demonstrate marked variation in the fms-dependence of hybrid pigment patterns, suggesting interspecific variation in the fms pathway or fms requirements during pigment pattern formation. albolineatus exhibits a uniform pattern of melanophores, and previous interspecific complementation tests identified fms as a potential contributor to this difference between species. By contrast, the closely related species D. rerio, alternating light and dark horizontal stripes develop, in part, owing to interactions between melanophores and cells of the xanthophore lineage that depend on the fms receptor tyrosine kinase zebrafish fms mutants lack xanthophores and have disrupted melanophore stripes. These patterns result from several classes of pigment cells including black melanophores and yellow xanthophores, which differentiate during metamorphosis from latent stem cells of presumptive neural crest origin. ![]() An emerging system for studying such variation is the adult pigment pattern expressed by Danio fishes. The developmental bases for species differences in adult phenotypes remain largely unknown.
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