Genetic structure, outcrossing rate and heterosis in Astrocaryum mexicanum (tropical palm): implications for evolution and conservation

Abstract

The population genetics of the understory tropical rain forest palm Astrocaryum mexicanum were studied in Los Tuxtlas, Veracruz, Mexico, using enzyme electrophoresis. The percentage of polymorphic loci was 31.8 and the mean expected heterozygosity was 0.153. Segregation patterns for five polymorphic loci met Mendelian expectations. Outcrossing rates were estimated using single and multilocus methods, and in most cases were not statistically different from 1.0. An excess of heterozygotes, both for seeds and adults, was found, as shown by the fixation indices estimated in 1987 (mean F for adults = −0.41, mean F for seeds = −0.19). Low but significant levels of genetic differentiation were found, especially for adults (mean Fst for adults = 0.040, mean Fst for seeds = 0.009). There was a positive relationship for adults between trunk growth and an individual's heterozygosity. No significant correlation was found between heterozygosity and fecundity. The genetic structure of A. mexicanum appears to be the result of a balance between cross-pollination and long distance movement of pollen by pollinators (beetles) that reduce genetic differentiation among plots, and natural selection that could operate during the long life cycle of this palm, and may increase the genetic differentiation among sites and the proportion of heterozygotes. The relatively high level of genetic variation, low genetic spatial differentiation, excess of heterozygotes, high outcrossing rate and heterosis found in A. mexicanum seem to be common in tropical trees. These population genetics characteristics appear not to satisfy the conditions necessary for non-adaptive evolution, a hypothesis commonly invoked to explain high tropical tree diversity. Management and conservation strategies aimed at preserving tropical tree's high intrapopulation genetic variation will probably require the maintenance of large tree populations.