Combining Ability of Certain Chromosomes in Drosophila Willistoni and Invalidation of the "Wild-Type" Concept

Five second chromosomes which were known to be lethal in double dose, and 23 chromosomes producing about normally viable homozygotes, were chosen from samples of chromosomes from two South-Brazilian populations of Drosophila willistoni. The lethal chromosomes were selected among a much larger collection of lethals because they seemed to produce exceptionally vigorous heterozygotes with other chromosomes from the same populations. The quasi-normal chromosomes were picked out at random. Heterozygotes with various combinations of lethal and quasi-normal chromosomes were produced by appropriate crosses. The viability of these heterozygotes was measured by observing the ratios of individuals which carried the chromosomes to be tested and individuals carrying chromosomes with suitable marking mutant genes. Some heterozygotes for lethal and quasi-normal chromosomes proved more viable than heterozygotes which did not carry chromosomes known to be lethal when homozygous. A chromosome which is lethal in double dose may, therefore, be heterotic when present in a heterozygote. However, two chromosomes which give a highly viable heterozygote may produce either a high or a low viability in heterozygotes with a third chromosome. This is true for chromosomes which are lethal when homozygous, as well as for chromosomes which give quasinormal homozygotes. The combining ability of chromosomes may be measured empirically, but the rules which might permit prediction are unknown at present. Natural populations of Drosophila, and presumably of other sexual species, contain great stores of chromosomes with different complexes of genes. An individual carries, then, two chromosomes of each pair chosen at random from the gene pool of the population. No one of the resulting variety of genotypes and phenotypes can be regarded as "normal," or representing the "wild-type" of the species or population. The "wild-type" is an abstraction the usefulness of which is limited to serving as a point of reference in describing genetic and environmentally induced aberrations.