University of Cambridge > > Evolution and Development Seminar Series > Developmental Constraints? Why blind cavefish still have eye at embryonic stage

Developmental Constraints? Why blind cavefish still have eye at embryonic stage

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Many organisms lost their eye after living in a darkness environment. Recent data suggested that the losing eye is a result of natural selection instead of genetic drift. Therefore, the losing eye could have some advantageous effects for living in a darkness environment. Interestingly, most of blind vertebrate (could be all of them) have small vestigial eye. The blind animals had developed eye at embryonic stage, but it was degenerated at later stage. Why have they started building their eye that destroyed at later stage? Astyanax mexicanus has a surface dwelling eyed population and several cave dwelling eyeless populations. The cavefish populations also have started to develop eye structures similar as other blind vertebrate. At embryonic stage, the cavefish have developed small eye anlagen comparing with eyed surface population and the small eye anlagen have degenerated by apoptosis at later stage. Astyanax mexicanus is a good experimental model system for tackling the issue of why blind animal still have eye. Rx3 is a transcription factor containing a homeobox domain and is expressed at future eye field of anterior neural plate for initiating eye development in vertebrate. Homozygous mutants of Rx3 fail to form eye anlagen at embryonic stage and completely lost the eye tissues. We have isolated a fragment of rx3 gene from Astyanax mexicanus and compare the sequences and expression pattern among different populations of Astyanax. The cording sequences of rx3 are identical among three Astyanax populations. However, the expression patterns and amount during embryonic development are different at each population. One of cavefish population, Tinija have extremely small rx3 expression domain at future eye field comparing with the expression domain of eyed surface fish. Another cavefish population, Pachon have slightly small expression domain of rx3, and interestingly, the expression intensity is very weak in the eye field comparing with eyed surface and Tinija cavefish populations. Cell transplantation experiments confirmed the reduction of the expression intensity in Pachon cavefish could be controlled by cell-autonomous manner. These observations indicate having large eye anlagen in Astyanax is probably less favored in the cave environment. However, why have they developed small eye anlagen instead of getting ride of the whole eye anlagen, like a rx3 mutant? In zebrafish, rx3 homozygous mutant have malformation of telencephalon and jaw structure. Losing eye at early embryonic stage could have deleterious effect in the head, and it has not been selected during evolution. This phenomenon could be one of developmental constraints that a limitation on phenotypic variability caused by dynamics of the developmental system.

This talk is part of the Evolution and Development Seminar Series series.

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