The Genetic Population Structure of Brazilian Drosophila willistoni

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Abstract
Samples of natural populations of Drosophila willistoni were collected in various parts of Brazil. Among the 2,414 second chromosomes from these samples which were tested for viability effects in homozygous condition, 41.5 ± 0.1 per cent proved lethal or semilethal to homozygotes. Strains containing 321 of these lethal or extreme semilethal chromosomes from 13 different localities were intercrossed in order to test which of these lethals were allelic. In 10,185 intercrosses the lethal chromosomes were derived from populations of remote localities; in 16 of these intercrosses the lethals were allelic. The rate of allelism between lethals of independent origin is, therefore, 0.00157. In 2,956 intercrosses the pairs of lethal chromosomes were derived from the same locality; in 5 of them the lethals were alleles. The rate of allelism between geographically related lethals is, then, 0.00169. Only the lethals found in the populations of small isolated islands of Angra dos Reis were alleles somewhat more frequently: 7 alleles in 1,363 intercrosses, or a rate of 0.00514. From the rate of allelism of independently arisen lethals, the number of gene loci in the second chromosome of D. willistoni can be estimated to be at least 1,063. This is more than double the estimate of the corresponding number in the second chromosome of D. melanogaster made by Wallace. And yet, the second chromosomes of these two species have been shown by Spassky and Dobzhansky to carry mostly the same loci. One of the possible reasons of the discrepancy may be that some of the lethals in the chromosomes of natural populations of D. willistoni are 'synthetic,' i.e., due to linkage of two or more loci neither of which is lethal by itself. Although the possibility of the occurrence of synthetic lethals introduces an element of ambiguity in calculation of certain genetic population parameters, the fact that the rates of allelism of lethals are independent of the geographic origin of the latter (except in the islands of Angra dos Reis) is very significant. It shows that the genetic structure of Brazilian populations of D. willistoni approaches that of ideal populations of infinite effective size. In this respect D. willistoni differs greatly from the California populations of D. pseudoobscura which have been shown by Wright and Dobzhansky to be of limited genetically effective sizes.

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