Shedding genomic ballast: extensive parallel loss of ancestral gene families in animals.

Loss of ancestral gene families has played an important role in genomic specialization in animals. An examination of the pattern of gene family loss in completely sequenced animal genomes revealed that the same gene families have been lost independently in different lineages to a far greater extent than expected if gene loss occurred at random. This result implies that certain ancestral gene families-and thus the biological functions they encode-have been more expendable than others over the radiation of the animal phyla.