Heterosis at Allozyme Loci under Inbreeding and Crossbreeding in PINUS ATTENUATA.

The dependence of heterosis at isozyme loci on inbreeding and crossbreeding was studied in 10-yr-old trees of knobcone pine (Pinus attenuata Lemm.). Heterozygosity was determined at 24 polymorphic isozyme loci and related to the rate of vegetative growth and cone production. The inbreds, created by selfpollination, had 46% of the heterozygosity of their mothers; the crossbreds, created by interpopulation crossing, had 155% of the heterozygosity of their mothers. Within the crossbreds, heterozygosity was positively correlated with trunk growth, but negatively correlated with cone production. Results in the crossbreds, however, were strongly influenced by a few individuals that showed unusually slow growth, high reproduction and low heterozygosity. Without those individuals, there was no relationship of heterozygosity to either growth or reproduction.-Within the inbreds, heterozygosity was positively correlated with both trunk growth and cone production. Each locus that was heterozygous in the mothers was calculated to mark about 3% of the genome for identity by descent in the inbred progeny; the total proportion of the genome marked was between 10 and 11%. Using these estimates to relate heterozygosity to the inbreeding coefficient (F) gave estimates of inbreeding depression per unit of F that fell within the range of published values for conifers. The strength of heterosis found among the inbreds suggests that single-locus or multilocus overdominance should be exceedingly difficult to detect in natural populations of predominantly outcrossing species.