Astyanax mexicanus, the Blind Mexican Cave Fish: A Model for Studies in Development and Morphology.

INTRODUCTIONThe perpetual darkness of caves has two important consequences for permanent inhabitants. First, eyes and pigmentation lose their primary functions. Second, in the absence of photosynthesis, food is rare. For these reasons, cave-adapted species typically have reduced eyes and pigmentation and increased or more efficient metabolisms. Additionally, other senses are usually augmented to compensate for the loss of vision. Identifying the genetic bases underlying these phenotypic changes will enhance our understanding of the specific pathways involved in control of these phenotypes and, in general, the evolutionary process. Unfortunately, the genetics of most cave animals cannot be studied because they are not easily bred. Blind Mexican tetras, Astyanax mexicanus, are the valuable exception to this rule because fish from the various cave populations are fully interfertile with one another and with eyed sister forms still living in nearby surface streams. Hybrids between surface and cave forms permit genetic analysis of their differences, and study of the pure forms as well as of hybrids allows study of their developmental differences. Quantitative trait loci (QTL) analysis has already identified some specific genes responsible for differences between cave and surface forms as well as other likely candidates; more will be added in the future. This system is a valuable addition to the array of existing models for the study of developmental and evolutionary genetics because cave populations are repositories of numerous naturally occurring mutations affecting development of the eyes and other senses, pigmentation, bone structure, metamerism, and metabolism. These alleles have been prescreened by natural selection for high viability, which simplifies their study. In contrast, new alleles obtained through mutagenesis in other model species are typically burdened with lower viability.