Abstract
Gene duplication is a major driver of evolutionary divergence. In most vertebrates a single PAX6 gene encodes a transcription factor required for eye, brain, olfactory system, and pancreas development. In zebrafish, following a postulated whole-genome duplication event in an ancestral teleost, duplicates pax6a and pax6b jointly fulfill these roles. Mapping of the homozygously viable eye mutant sunrise identified a homeodomain missense change in pax6b, leading to loss of target binding. The mild phenotype emphasizes role-sharing between the co-orthologues. Meticulous mapping of isolated BACs identified perturbed synteny relationships around the duplicates. This highlights the functional conservation of pax6 downstream (3′) control sequences, which in most vertebrates reside within the introns of a ubiquitously expressed neighbour gene, ELP4, whose pax6a-linked exons have been lost in zebrafish. Reporter transgenic studies in both mouse and zebrafish, combined with analysis of vertebrate sequence conservation, reveal loss and retention of specific cis-regulatory elements, correlating strongly with the diverged expression of co-orthologues, and providing clear evidence for evolution by subfunctionalization.
Highlights
Complex spatiotemporal control is required to regulate the expression of major tissue-specific transcription factors and signaling molecules which fulfill multiple roles in developmental patterning [1,2]
About 350 million years ago, whole genome duplication occurred in an ancestor of teleost fishes, followed by variable loss of duplicated segments, and this process has been suggested to account for extensive speciation [20,21,22,23], so that half of all vertebrate species are teleost fish
We were unable to rescue the effect of the missense mutation using capped mRNA injection into 1–2 cell stage mutant embryos, we show that increased dosage of wild type pax6b leads to a deleterious phenotype, and that the over-expression phenotype is less severe with an equal amount of mRNA encoding the L244P mutant protein, providing additional indirect evidence that this partial loss-of-function mutation is the underlying cause of the sri phenotype
Summary
Complex spatiotemporal control is required to regulate the expression of major tissue-specific transcription factors and signaling molecules which fulfill multiple roles in developmental patterning [1,2]. In addition to transgenic reporter studies in multiple species [6,7,8,9,10,11,12], human disease-associated genomic rearrangements (reviewed in refs [2,13]) and mutations [8,14], as well as natural and targeted deletions in mice [15,16,17], have contributed to our current understanding of how these regulatory elements control gene expression. Gene duplication provides an opportunity for such diversification without major penalties for loss of function [19]. Where duplicate (or triplicate) genes have been examined, they reveal functional divergence, the mechanisms have generally not been explored [25]. ¤b Current address: Center for Brain Research, Medical University of Vienna, Vienna, Austria
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