Pleiotropy and eye degeneration in cavefish

Abstract

In an article recently published in Heredity, Wilkens (2010) reviews the mechanisms of regressive evolution in the cavefish, Astyanax mexicanus. A. mexicanus consists of a surface-dwelling morph with eyes and pigmentation, and many cave-dwelling morphs that have lost or have a reduced form of both traits. The two morphs are capable of interbreeding, allowing the mechanisms of regressive evolution to be probed by genetic analysis. In the past, three theories have been proposed to explain the loss of eyes in cave organisms: (1) neutral mutation and genetic drift, (2) positive selection against eyes due to energy conservation or their possible liability and (3) indirect selection against eyes based on increase in beneficial traits that are negatively linked to optic development by pleiotropy. The first idea was favored until about the turn of this century (Culver and Wilkens, 2000), but since then new genetic and developmental evidence has tipped the scale toward the third idea—pleiotropy (Jeffery, 2005; Protas et al., 2008). In this review, Wilkens (2010) re-evaluates the genetic and developmental data and concludes that there is no validity for the second or third (selectionist) theories, reverting to neutral mutation as the most plausible explanation for eye degeneration. Here, I contest some of the facts and interpretations presented in the Wilkens (2010) review and call attention to published developmental data implicating pleiotropy in A. mexicanus eye regression (Yamamoto et al., 2009) that were apparently not considered in drawing its conclusions.