AN EXTREME CASE OF HETEROSIS IN A CENTRAL AMERICAN POPULATION OF DROSOPHILA TROPICALIS

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Abstract
P. tropicalis populations collected at Lancetilla and La Lima, Honduras, and at San Salvador, El Salvador, were found to consist of 4 types with respect to inversions in II L no heterozygous inversions, a single subbasal inversion, a single sub-median inversion, and both the subbasal and submedian inversions. In the Lancetilla population 70.3% out of 74 larvae were heterozygous for the subbasal inversion. In order to test the hypothesis that this significant deviation from 50% in a random breeding population is due to lowered viability of the homozygotes, an experimental Lancetilla population was allowed to breed freely for 4 mos. at 25[degree]C, after which egg samples were taken and the larvae raised under optimum conditions. Of 150 larvae 90.7% were heterozygous for the subbasal inversion, indicating differential mortality of the homozygotes. In order to determine whether any homozygotes survived to the adult stage, virgin males and females which had emerged from pupae developed under the crowded conditions of the population cages, were crossed with a Palma strain from Brazil, which was structurally homozygous. Of 200 adult flies tested, 8 were homozygous for the gene arrangement in the basal part of II L and 192 were heterozygous for the subbasal inversion. Mortality of the homozygotes occurred chiefly in the egg stage, but to some extent in the larval and pupal stages. The Lancetilla population thus approaches the status of a balanced lethal system. In comparison with other spp., D. tropicalis appeared with the highest frequencies at Lancetilla and La Lima, but less frequently at El Salvador, where the population contained more than 50% homozygotes for gene rearrangements in subbasal II L, and rarely in Brazil, where the population had no subbasal inversions.