Overdominance in the bid for fitness
Overdominance occurs when individuals that are heterozygous have a higher Darwinian fitness level than both homozygotes. Genetic diversity in populations can be maintained when there is overdominance even if there is a fitness cost, one classic example being sickle cell anaemia. The most serious form of the disease is caused by a double dose of the recessive gene. The OMUBAS (Overdominance: From spontaneous mutations to balancing selection in natural populations) project has investigated the prevalence of overdominance that occurs with spontaneous mutation and segregation. The team used a wild population of yeast, Saccharomyces cerevisiae, over about 800 asexual generations. For each experimental population the researchers chose a single genotype with both invasive fitness and a shallow fitness-frequency function to test for overdominance. Fitness was measured by head-to-head competition against two strains with ancestor genotypes. Having uncovered at least one parental clone with heterozygous advantage, the team crossed descendants from the parent to test if the advantage is due to overdominance or linkage. The scientists also tested to see if the proportion of overdominant mutations is correlated to initial fitness of the genotype. They constructed homozygous descendant genotypes for testing and the work is ongoing after project close. Significance of OMUBAS research spans a wide range of areas. Biotechnology is an important sector – for example in industrial fermentation processes – where optimum fitness of a microbial population is important,. The results also apply to hybrid vigour in crops as this can be achieved in inbred lines using artificial selection.
Keywords
Overdominance, fitness, evolution, heterozygous, OMUBAS, hybrid vigour