Intraspecific hybridisation and the release of mutator activity

Abstract

IT has been suggested by Grant^1–2 that mutation and hybridisation may not be independent of one another as sources of genetic variation in natural populations. Indeed, it has been shown in both plants and animals^3–8 that hybridisation can stimulate the production of new mutations and chromosome breakage events. The possible role of intraspecific hydridisation in inducing genetic change has recently been discussed in various contexts, especially in relation to the action of mutator genes in natural populations^1,2,9–11. We have previously suggested that mutator factors, which are known to induce both visible and lethal mutations and chromosome breakage, are commonly present in natural populations of Drosophila melanogaster. Within sub-populations, however, these mutators are genetically suppressed. On hybridisation between subpopulations, genetic suppression breaks down, resulting in the release of mutator activity and, hence, in an explosive increase in genetic variation. Clearly, these explosions of mutation, which have been documented in some natural populations^12,13, might have an important role in evolution and speciation. Here, we test this hypothesis of ‘hybrid release’ directly by determining whether intraspecific hybridisation increases the frequency of mutation above that found in the sampled natural population. Our study differs significantly from other assays of mutator gene effect^14–16 in that mutation frequencies before and after hybridisation are compared directly. The results show that hybrid release of mutator activity does occur as the result of hybridisation, producing a severalfold increase in the frequency of mutation. Thus, hybrid release may be a major mechanism in the induction of genetic variation in natural populations and so may be a driving force in evolution.