Genetic mosaicism in plants and clonal animals

T he genetic mosaicism hypothesis (GMH) proposed that arborescent plants accumulate spontaneous mutations and become genetically mosaic as they grow. GMH predicted that the intraplant heterogeneity influences plant-pest interactions ecologically and provided a partial solution to the problem of how long-lived trees evolve resistance to short-lived pests. Theoretical models predict that genetic mosaics should be rare (about 5%) and that genetic vari ation within a clonal unit should be difficult to detect. Somatic mutations can contribute more to standing genetic variation in populations than do gametic mutations and thereby can increase plant evolutionary rates. If population size is small, somatic mutations can increase heterozygosity by two or more orders of magnitude. Reported frequencies of somatic mutants match the values expected in theory: The average value of mutant frequencies per locus is 10-6; the observed frequency for polygenic traits (such as chlorophyll-less tissues) is 6.3 x 10--4 per genome; and spontaneous mutants occur 0.1-19% in asexual plants. Like plants, many clonal animals violate Weismann's doctrine (sepa ration of germlines from soma), and GMH should apply, but no estimates of mutant frequencies or mutation rates within colonies of clonal invertebrate animals are yet available. Pests respond to intraplant heterogeneity and can impose selective differentials on modules, but the significance of clumped patterns of galling-aphids on witch-hazels previously reported by Gill (40) as supporting GMH is refuted here.