THE RELATION OF GROWTH TO HETEROZYGOSITY IN PITCH PINE.

Abstract

The connection between fitness and heterozygosity has eluded geneticists for decades. The classic form of the Neo-Darwinian argument hypothesizes that heterozygosity confers genetic homeostasis (Lerner, 1954); i.e., multiple, molecular forms of the same enzyme endow the organism with a broader range of tolerance to environmental variation because different forms may differ in their optima for temperature, pH, and other factors (Johnson, 1976). In maize (Zea mays L.), inbred lines and their hybrids are similar in growth rate when raised under constant conditions at optimal temperatures, but hybrid superiority becomes progressively more obvious as conditions deviate from the optimum (McWilliam and Griffing, 1965). Apparently, homozygotes in maize can only deal successfully with a narrow range of conditions as compared to heterozygotes. In a variety of organisms, including butterflies, fish, and oysters, variance for morphological traits decreased with increasing heterozygosity at enzyme loci (Eanes, 1978; Mitton, 1978; Zouros et al., 1980), and bilateral symmetry, a measure of developmental homeostasis, increased with heterozygosity in lizards and bivalves (Soule, 1979; Kat, 1982). Growth rate increased with heterozygosity in oysters and salamanders (Singh and Zouros, 1978; Zouros et al., 1980; Pierce and Mitton, 1982). The perennial herb, cylindric blazing star (Liatris cylindracea Michx.), can be roughly aged by counting rings in the corm, the subterranean perennating organ. In natural populations, heterozygosity was higher among older than among younger individuals, suggesting poor survival for homozygotes (Schaal and Levin, 1976). Grown in a uniform environment, biomass production and reproductive potential of the herb were weakly, but positively, related to the level of enzyme heterozygosity. However, Mitton and colleagues (Mitton and Grant, 1980; Knowles and Grant, 1981; Mitton et al., 1981) noted conflicting results in forest trees, which are among the most polymorphic of organisms (Hamrick, 1979; Hamrick et al., 1979). In only one of three tree species was there a positive association between mean width of the annual ring, taken as a measure of growth and fitness, and heterozygosity. In two of the species, variance in ring width increased with increasing heterozygosity and in the third, it decreased. The relationship between growth and heterozygosity in trees is confusing, and more data are needed to draw firm conclusions. Our objective was to relate growth to heterozygosity in pitch pine (Pinus rigida Mill.), for which we had information on 21 enzyme loci in several populations. We were also interested in whether heterozygotes had greater longevity and whether annual growth was more or less well-buffered in heterozygotes than in homozygotes. A positive association be-