POPULATION STUDIES IN PREDOMINANTLY SELF-POLLINATED SPECIES, I. EVIDENCE FOR HETEROZYGOTE ADVANTAGE IN A CLOSED POPULATION OF BARLEY

Abstract

The present study was undertaken to investigate the level of heterozygosity in an experimental population of barley, a predominantly self-fertilizing organism, and the adaptive significance of heterozygotes in the storage and release of genetic variability. The experimental material was Composite Cross V, a population developed by the late H. V. Harlan and associates from intercrosses among 31 varieties of barley. In 1937 the varieties were first crossed in 15 pairs. During the next three years the F1 hybrids (plus the uncrossed parent) of the preceding cycle were again crossed to produce ultimately a single hybrid stock. The first selfed generation, designated F2, was grown in 1941, the F3 in 1942, and so on, from random samples of seeds grown without conscious selection according to usual agricultural practice. The plan of the experiment was, first, to determine changes in frequency of homozygotes and heterozygotes at the several marker loci over generations, and then to identify the forces responsible for the observed changes by comparison with a genetic model taking into account the major factors that are recognized as affecting genotypic frequencies.