plement - was Urbilateria a monster?
All animals have lost genes, but it does not follow that Urbilateria was a monster in terms of gene content. Bilaterian animals typically have around 20,000 genes (range 11,500 to 28,000;Meloidogyne to Tetraodon), but a substantial fraction of these are taxonomically restricted at some level. Many of these taxonomically restricted genes are paralogs or highly diverged members of large gene families, generated by duplication events that have occurred at all levels. On the basis of the currently available data, the core bilaterian gene set probably contained fewer than 10,000 genes, the caveat being that the available data are rather limited. Taxonomic gaps need to be plugged, and more data for non-bilaterians in particular will be critical in revealing the genomic makings of Urbilateria. Far too few whole-genome sequences are yet available for firm estimates to be made, but it is clear that there is no need to invoke monsters - either hopeful or hopeless.
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