Introduction

Multigene families can be defined as sets of genes with significant nucleotide similarity encoding RNA or protein products with related functions. The ubiquitous nature of multigene families has made the study of their organization, expression, and evolution essential to understanding the eukaryotic genome. In some instances multiple, nearly identical, genes appear to be needed by an organism to produce the large quantity of products that are required at specific developmental periods. The classic example of such multigene families is the hundreds of ribosomal RNA (rRNA) genes needed to produce the high levels of rRNA found in all cells. In most instances, however, the genes within a multigene family encode slightly different gene products that are required by the cell for a series of related functions. Examples of such families are the genes encoding the actin proteins in most eukaryotes, cuticle proteins in insects, and the major his to compatibility proteins found in mammals.

When one compares the same multigene family in two species, characteristic sequence features can be found that are shared by all family members in one species but are not seen in the family members of the second species. Although selection pressures can be cited to explain the maintenance of key sequence features within a family, these pressures are not sufficient to explain why apparently neutral changes can also be shared by all family members, a process termed concerted evolution. The phenomenon of concerted evolution implies the operation of recombinational processes that spread and fix new sequence variants in a multigene family (reviewed by Ohta, 1980; Baltimore, 1981; Dover, 1982; Arnheim, 1983).