Question

1

We see an enormous diversity of eyes in Kingdom Animalia. Until recently it was postulated that eyes must have evolved at least 40 times independently. Explain how the discovery of Pax6 (and its homologs such as eyeless ) has overturned this idea in favor of genes for eye development being present in an ancestor to all bilateral organisms.

2

Using the mouse Pax6 gene as a baseline, determine the similarity of the homologous genes from the unknown species.

For each unknown species, determine the number of nucleotides that are different from the nucleotides found in the mouse sequence. There are 60 nucleotides in each sequence. To determine the percent similarity, subtract the number of different nucleotides from 60, divide the result by 60, and multiply by 100.

Answer 1

Pax6 is a transcription factor which is very important during embryonic development . This encoded protein contains two different binding sites that are known to bind DNA and function as regulators of gene transcription. It is a key regulatory gene of eye and brain development. Mutations of the Pax genes are known to cause vaarious disorders like absence of the iris [ aniridia] and Peter's anomaly [ thinning and clouding of the cornea] .

PAX6 protein function is highly conserved across all bilaterian species. This has been proved by experiments which showed that PAX6 of the mouse can trigger eye development in Drosophila melanogaster. Also, human and mouse PAX6 have identical aminoacid sequences.

However, genomic organisation of PAX6 locus varies considerably among different species including number and distribution of exons, transcription start sites.

The genomic organisation of PAX6 is best known in mouse locus. It consists of 2 confirmed promoters [ P0 and P1] , 16 exons and at least 6 enhancers.

Of the 4 Drosophila PAX6 orthologues, it is thought that the eyeless [ey] and twin of eyeless[toy] gene products share functional homology with the vertebrate canonical PAX6 isoforms. The eyegone [eyg] and twin of eyegone [toe] gene products share functional homology with the vertebrate Pax6 [5a] isoform.

So, the function and location of the PAX6 can be traced back to the bilateral ancestor. Orthologus genes to PAX group exist throughout the Metazoa.