The origin of eye size differences in cichlid fish ecotypes
Michaels, Mary E.
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The evolutionary mechanisms that act upon brain and eye development are not well known. Here, we investigate the causes of the adult eye size difference of the two cichlid fish ecotypes from Lake Malawi, and demonstrate that the variation in size starts early in development and is due to early patterning. In brain development, the eye field splits from the forebrain, a process that marks the beginning of the development of the eyes. Genes necessary for eye development include the eye field transcription factors rx3 and pax6. Greater expression of these genes before the eye field first segregates from the forebrain could cause the eyes to be larger when they are initially formed. Of the two cichlid ecotypes, the sand-dwellers (utaka) have larger eyes as adults than the rock-dwellers (mbuna). We show that the utaka have larger eyes than mbuna at the first developmental stage when the eyes are specified, or neurulation. Differences in expression of the eye field transcription factors before neurulation is responsible for this initial size difference.