Ancient allelism at the cytosolic chaperonin-alpha-encoding gene of the zebrafish

The T-complex protein 1, TCP1, gene codes for the CCT-alpha subunit of the group II chaperonins. The gene was first described in the house mouse, in w hich it is closely linked to the T locus at a distance of similar to 11 cM from the Mhc. In the zebrafish, Danio rerio, in which the T homolog is link ed to the class I Mhc loci, the TCP1 locus segregates independently of both the T and the Mhc loci. Despite its conservation between species, the zebr afish TCP1 locus is highly polymorphic. In a sample of 15 individuals and t he screening of a cDNA library, 12 different alleles were found, and some o f the allelic pairs were found to differ by up to nine nucleotides in a 275 -bp-long stretch of sequence. The substitutions occur in both translated an d untranslated regions, but in the former they occur predominantly at synon ymous codon sites. Phyloge- netically, the alleles fall into two groups dis tinguished also by the presence or absence of a 10-bp insertion/deletion in the 3' untranslated region. The two groups may have diverged as long as 3. 5 mya, and the polymorphic differences may have accumulated by genetic drif t in geographically isolated populations.