Intron dynamics and the evolution of integrin beta-subunit genes: maintenance of an ancestral gene structure in the coral, Acropora millepora.

We have determined the genomic structure of an integrin beta-subunit gene from the coral, Acropora millepora. The coding region of the gene contains 26 introns, spaced relatively uniformly, and this is significantly more than have been found in any integrin beta-subunit genes from higher animals. Twenty-five of the 26 coral introns are also found in a beta-subunit gene from at least one other phylum, indicating that the coral introns are ancestral. While there are some suggestions of intron gain or sliding, the predominant theme seen in the homologues from higher animals is extensive intron loss. The coral baseline allows one to infer that a number of introns found in only one phylum of higher animals result from frequent intron loss, as opposed to the seemingly more parsimonious alternative of isolated intron gain. The patterns of intron loss confirm results from protein sequences that most of the vertebrate genes, with the exception of beta4, belong to one of two beta subunit families. The similarity of the patterns within each of the beta1,2,7 and beta3,5,6,8 groups indicates that these gene structures have been very stable since early vertebrate evolution. Intron loss has been more extensive in the invertebrate genes, and obvious patterns have yet to emerge in this more limited data set.