Genetic exchanges between partially homologous nucleotide sequences: possible implications for multigene families.

Many sequences, particularly in the genome of the higher eukaryotes, are reiterated several or many times, but are only partially homologous. It is commonly accepted that such sequences (for example, genes of a multigene family) may, on occasions, undergo reciprocal and non-reciprocal genetic exchanges by crossing-over or gene conversion. This relies on the implicit assumption that partially homologous sequences may pair to form heteroduplex structures. We discuss the possibility that correction of such heteroduplexes into homoduplexes may be a common genetic mechanism with significant evolutionary importance, particularly in the higher eukaryotes: it provides a basis for a novel source of point and non point mutation and a means of generating many variations in a gene sequence in a single step. We also emphasize the possible role of non coding nucleotides, particularly introns, in permitting (when conserved) or blocking (when divergent) such exchanges between members of a multigene family.