Marianne K. Hughes

Natural selection on the peptide-binding regions of major histocompatibility complex molecules

The major histocompatibility complex (MHC) of vertebrates is a multigene family whose products encode cell surface glycoproteins that function to present peptides to T cells (Klein 1986). There are two major subfamilies, class I and class II, the members of which differ both structurally and functionally. Class I molecules have a nearly universal pattern of expression and present peptides to cytotoxic T cells, while class II molecules are expressed on antigen-presenting cells of the immune system. In mammals, the class I and class II loci are linked together in a single gene complex, which also includes some genes that have neither an evolutionary nor a functional relationship to the MHC genes, and others which, although not evolutionarily related to the MHC genes, encode molecules that play roles in peptide presentation. Both class I and class II molecules are heterodimers with four extracellular domains, but they achieve analogous structures in different ways. The class [ heterodimer includes an ct chain consisting of three extracellular domains (cq, a~, and ct3), a transmembrane portion, and a cytoplasmic domain. The o~3 domain associates noncovalently with ~2-microglobulin ([32m). ~32m is encoded outside the MHC gene complex but shows evidence of an evolutionary relationship with the t~3 domain, which like it is a C ltype domain belonging to the immunoglobulin (Ig) superfamily (Williams and Barclay 1988). The class II heterodimer consists of an o~ chain and a ~3 chain, each of which includes two extracellular domains (~1 and c~2 and 131 and ~3% respectively), a transmembrane region, and a cytoplasmic tail; both chains are encoded by genes within the MHC gene complex. Like the membrane-proximal domains of the

Adaptive evolution in the rat olfactory receptor gene family

SummaryComparison of DNA sequences of the rat (Rattus norvegicus) olfactory receptor gene family revealed an unusual pattern of nucleotide substitution in the gene region encoding the second extracellular domain (E2) of the protein. In this domain, nonsynonymous nucleotide differences between members of this subfamily that caused a change in amino acid residue polarity were over four times more frequent than nonsynonymous differences that did not cause a polarity change. This nonrandom pattern of nucleotide substitution is evidence of past directional selection favoring diversification of the E2 domain among members of this subfamily. This in turn suggests that E2 may play some important role in the functions unique to each member of the olfactory receptor family, and that it may perhaps be an odorant binding domain.

Self peptides bound by HLA class I molecules are derived from highly conserved regions of a set of evolutionarily conserved proteins.

An evolutionary analysis of self peptides reported to be bound by HLA class I molecules showed that these peptides are largely derived from proteins that have been highly conserved in the history of mammals. These proteins also often have universal tissue expression and have a higher than average frequency of highly hydrophilic residues. The peptides themselves are generally still more highly conserved than the source proteins and have a higher frequency of highly hydrophobic residues, evidently often derived from conserved hydrophobic cores of the source proteins. These results suggest that the mechanism by which peptides are derived for MHC presentation may preferentially select peptides from conserved protein regions. In the case of parasite-derived peptides, such a mechanism would be adaptive in that it would reduce the likelihood of escape mutants.

Female choice of mates in a polygynous insect, the whitespotted sawyer Monochamus scutellatus

SummaryAdult female whitespotted sawyers Monochamus scutellatus (Coleoptera: Cerambycidae) were placed on succeeding days with two males differing visibly in size. Females showed a significant preference for the larger of two males. The rate of oviposition was significantly higher when the female was paired with the larger male. Both the rate of movement by the paired female and her rate of indicating non-receptivity were significantly lower when the female was paired with the larger male. This species exhibits resource defense polygyny, and females appear to choose mates mainly on the basis of the quality of resources defended. This study suggests, however, that when resource quality is constant, females choose mates on the basis of their size.