Most have abandoned Haeckel’s old chestnut that ontogeny recapitulates phylogeny, but when two organisms actually appear to have identical embryonic development, how close are the genetic programs that underlie each. In a wide ranging symposium on evolutionary genetics at the Society for Developmental Biology 67th annual meeting, Itai Yanai from Craig Hunter’s lab at Harvard looked at two nematode worms that are practically indistinguishable, the lab workhorse Caenorhabditis elegans and Caenorhabditis briggsae from which it diverged some 80 to 100 million years ago. Evolutionarily, that puts them about as distant as humans and mice, but morphologically they’re practically indistinguishable. So, what can Yanai say about how these organisms use those different genes during what he calls the “200 most exciting minutes in the life of the worm”? This is the time in which the worm goes from four cells to 190, and genes are turned on and off in what one would assume is a tightly controlled regulatory regimen program. Yanai constructed microarrays to look at gene expression in the two species of worm. Specifically he looked at expression of genes that are so-called one-to-one orthologs (meaning the genes look a lot alike in both organisms and haven’t been duplicated in either species since they diverged).
There are about 12,000 orthologs, and 3,500 of these gave a good profile. Of these, roughly a quarter of the genes are highly divergent in their expression. To see that much divergence during this crucial developmental period was sort of shocking, but Yanai presents some potential explanations. Shuffling of the genome, for example, could put a gene necessary for development next to one not really needed at the time. So, less necessary genes go along for the ride. It serves as a reminder that gene expression is a very rough tool for extrapolating function. Yanai noted that there’s actually quite a lot of variability in expression for different strains of C. elegans. Someone in the audience asked if that means that there’s considerable flexibility and tolerance for misexpression during development, such that there may be no canonical set of genes required. Yanai answered that to the contrary, such comparative work may help to whittle down to those genes that really are necessary. He plans to continue study in two related species of the frog Xenopus.