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MOST OF OUR study of gene networks has been done by comparison of related species to reconstruct network evolution and by knocking out specific genes to determine what the effects of their absence are. In a new paper Isalan and coworkers try something new, reprogramming genetic networks in Escherichia coli and examining the mutants to detect viability and any possible benefits to genetic pathway modification.
IN MANY CASES even large phenotypic changes can occur without much genetic change. However, occasionally a species will be placed in a situation in which its ancestor’s genes are insufficient, and if the species possesses a gene that can be co-opted into a new role, selection can favor evolution of new genes and a corresponding radiation of species. The Pieridae family of butterflies lay their eggs on plants in the Brassicaceae family, which contains mustards and cabbages. These plants have evolved to produce compounds that are harmless in undamaged leaves, but when a leaf is damaged are converted to a potent insecticide. The pierids evolved a deactivating protein that diverts the chemical reaction to produce nontoxic products. This gene evolved shortly after the plants themselves, and would have allowed these butterflies’ larvae to feed upon these plants with little competition.
THIS WEEK I’ve been covering some interesting instances of new gene evolution. The one I’m covering today is hard to boil down into a short title. This is a case of a new gene in hominoids as the result of retrotransposition of an aberrant mRNA transcript. Transposons showed up in the last post as well, but here they play a different role. That example involved a class II transposon, a segment of DNA that can jump around the genome. This case involves a class I transposon, a retrotransposon that transcribes itself into RNA, then copies that transcript back into DNA, and inserts it elsewhere in the genome. Here the retrotransposon accidentally retrotransposed a gene transcript instead of a retrotransposon transcript. This is not an especially rare event, but this case is unusual because the transcript itself is unusual.
I MENTIONED BEFORE the IAD model for gene synthesis–innovation in a gene producing a new secondary function, amplification of that gene by duplication, and divergence of the gene copies. However, sometimes things can happen more suddenly. In 2002 studies of a pond snail reveal the generation of two new genes by duplication of an ancestral gene, and then a intragenic inversion converting one duplicate into two new genes!
ONE QUESTION in evolutionary biology not too many years ago was where new genes came from. One researcher who was influential in this area was Ohno, who described a variety of ways in which this could occur. One of the major ones is gene duplication followed by mutation, selection, and the eventual emergence of a new function for one of the gene copies. Certainly gene duplication unequivocally occurs fairly frequently–gene products that are required in large amounts frequently have multiple gene copies, and variations in gene copy numbers in humans leads to subpopulations with different phenotypes, such as faster metabolism of certain drugs.