A complete arthropod Hox gene family existed in the ancestor of the onychophoran/arthropod clade. No new Hox genes were therefore required to catalyze the arthropod radiation; instead, arthropod body-plan diversity arose through changes in the regulation of Hox genes and their downstream targets.

Evidence suggests that the New World was colonized only 11,000-40,000 years ago by Palaeo-Indians. The descendants of these Palaeo-Indians therefore provide a unique opportunity to study the effects of selection on major histocompatibility complex class I genes over a short period. Here we analyse the class I alleles of the Waorani of South America and the Zuni of North America. Four of the Waorani HLA-B alleles were new functional variants which could be accounted for by intralocus recombination. In contrast, all of the Zuni HLA-A and -B molecules were present in caucasians and orientals. This suggests that the new Waorani HLA-B variants arose in South America. The description of four new HLA-B alleles in the Waorani and another five new HLA-B alleles from two other tribes of South American Amerindians indicates that the HLA-B locus can evolve rapidly in isolated populations. These studies underline the importance of gathering genetic data on endangered native human populations.

Major histocompatibilit complex (MHC) loci are some of the most polymorphic genes in the animal kingdom. Recently, it has been suggested that although most of the human MHC loci are relatively stable, the HLA-B locus can undergo rapid changes, especiafly in isolated populations. To investigate the mechas of HLA-B evolution we have compared the sequences of 19 lILA-B homologues from chimpan zees and bonobos to 65 HLA-B sequences. Analysis of the chimpanzee and bonobo HLA-B homologues revealed that despite obvious similarties between chimpanzee and human alleles in exon 2, there was little conservation ofexon 3 between humans and the two chimpanzee species. This finding suggests that, unlike all other HLA loci, recombination has characterized the HLA-B locus and its homologues for over 5 million years.

Although major histocompatibility complex (MHC) class I molecules are, as a rule, highly polymorphic in mammalian species, those of the New World primate Saguinus oedipus (cotton-top tamarin) exhibit limited polymorphism. We have cloned and sequenced twelve MHC class I cDNAs from this species. Since cloned cotton-top tamarin cell lines express three to six MHC class I molecules, this species must have at least three functional MHC class I loci. There was, however, no evidence of locus-specific substitutions in the tamarin cDNAs. Unlike all other species studied, tamarin MHC class I cDNAs displayed limited nucleotide sequence variation. The sequence similarity between the two most divergent tamarin cDNAs was 95%. To ensure that the polymerase chain reaction (PCR) primers employed in these studies had amplified all of the tamarins' expressed MHC class I genes, we used another set of primers to amplify only exons 2 and 3 from RNA and DNA. PCR of genomic DNA resulted in the amplification of six distinct clones, of which only three were well expressed. Two of these nonexpressed genes were pseudogenes and the other was a nonclassical gene. Southern blot analysis demonstrated that the tamarin has 8-11 MHC class I genes, suggesting we had indeed cloned the majority of these genes. Cotton-top tamarins are, therefore, unique among mammalian species studied to date in that they express MHC class I molecules with limited nucleotide sequence variation.

Classical human major histocompatibility complex (MHC) class I molecules are the products of highly diverse gene loci. It has been suggested that segmental exchange may play a role in the generation of diversity at the antigen recognition site of MHC class I molecules. Here we present the cloning, sequencing and expression of two gorilla A locus cDNAs. One of these cDNAs shows remarkable similarity to the non-functional HLA-AR locus gene (5.4-LBF) only in exon 2. The remainder of the cDNA, however, is most closely related to other classical higher primate A locus genes. This suggests that a segmental exchange may have occurred between the ancestor of the non-functional HLA-AR gene and a classical gorilla A locus gene. Furthermore, the recombination event resulting in Gogo-A3 has affected its antigen recognition site. These data, therefore, demonstrate that segmental exchange can generate diversity at the antigen recognition sites of primate MHC class I molecules and suggest that non-functional genes can contribute to the generation of diversity of classical MHC class I genes.