Previous studies have reported that the developmental processes of vertebrate eyes are controlled by four Pax-6 splicing variants, each modulating different downstream genes, whereas those of insect eyes are controlled by duplicated Pax-6 genes. Cephalopods belong to the Protostomes but possess a camera-type eye similar to those in vertebrates. We examined Pax-6 variations in the squid and found five types of Pax-6 splicing variants but no duplication of the Pax-6 gene. In the five splicing variants, the splicing patterns were produced by the combination of two additional exons to the ortholog and one jettisoned exon containing most of the Homeobox domain (HD). These five variants show spatio-temporal patterns of gene expression during development in the squid. Our study suggests that cephalopods acquired Pax-6 splicing variants independent of those in vertebrates and that these variants were similarly utilized in the development of the squid eye.
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