Where do those genes keep going? Just a few short years ago, we were arguing about whether the human genome had 100,000 genes or maybe just 80,000. Then we published the human genome ("we" being about 2000 co-authors on 2 papers) and reported that alas, we only had 25,000-30,000 genes. What a let down. The lowly weed Arabidopsis thaliana has 25,000 genes - can't we do better than a plant? It doesn't even have a brain, for pete's sake!
Well, don't look now, but a new study by Michele Clamp and colleagues at MIT's Broad Institute, just published in PNAS, says that the number, which had been hovering at 24,500, should be reduced to 20,500. Their analysis is pretty convincing - they did careful alignments of all human genes to both mouse and dog, and were able to identify several thousand genes that didn't seem to exist in our furry friends. Wait, though, you might say: perhaps these are specific to humans, or at least to primates? No, they thought of that too, and virtually none of the "genes" they propose deleting were found in our chimpanzee cousins either. That means that either we just delete these genes, or accept the rather far-fetched hypothesis that primates have both a "prodigious rate of gene birth" (to allow for over 1100 human-specific genes) and a "ferocious rate of gene death" (to explain why we don't see a similar number of genes shared by humans and chimps but no other species).
They eliminated quite a few other genes on the way to reducing the gene count by 4000, and I must admit their methods are pretty compelling.
So it appears that we have to take ourselves down another peg. We still have more genes than fruit flies, but the gap is getting smaller. I've been telling my students that size doesn't matter - we don't have the biggest genome, we don't have the most genes, we're not the biggest creatures by a wide margin. And lately we've learned that our own bodies have more bacterial cells (by a factor of 10 or more!) than human cells.
We humans want to think we're tops at something - wait, I've got it: we have the biggest egos! But those darn cats, now, I don't know...
Field of Science
But is it possible that we (humans) have more splice variants (of the same genes) and therefore do not need that many genes compared to the species that do not have "permissible tissues" which are fertile grounds for these variants?
ReplyDeleteI think it should be soon possible to make a comparative analysis for enough genes in Human, Mouse, Drosophila etc. for such a comparison. It should be enough EST data for some estimates already...