CLONING OF MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) GENES FROM THREESPINE STICKLEBACK, GUSTEROSTEUS-ACULEATUS

The threespine stickleback Gasterosteus aculeatus is an important mode l in evolutionary and ethologic studies. Its utility would greatly be increased by the availability of molecular markers distinguishing indi viduals and populations. Such markers can be provided by the major his tocompatibility complex (Mhc) genes, which are well known for their ex tensive polymorphism. In the present study, both class I and class II B Mhc genes have been identified and sequenced. Fifteen distinct class I exon 2 and exon 3 sequences were obtained and assigned to 12 loci o n the basis of intron 2 length differences. Some of the loci appear to be related to class I loci identified previously in cichlid fish. The intron 2 sequences and insertions/deletions in exon 2 group the loci into three families (with one family divided further into two subfamil ies) derived from different ancestral genes. The ancestors presumably diverged from one another before the divergence of Gasterosteiformes f rom Perciformes. The 12 distinct class II B sequences may be derived f rom six loci, which are, however, closely related to one another in bo th exonic and intronic parts and may have diverged from a single commo n ancestor after the divergence of Gasterosteiformes from Perciformes. The intron 2 of some of the class I genes contains two microsatellite s that can be used as markers, in addition to the polymorphism of the Mhc genes in their exonic regions.