Experiments on the Effects in Heterozygous Condition of Second Chromosomes from Natural Populations of Drosophila Willistoni

Abstract

Samples of wild populations of D. willistoni were collected at 4 locations in Brazil. 52 second chromosomes which were lethal when homozygous, and 60 second chromosomes which gave homozygotes close to normal viability, were selected and maintained in separated balanced stocks containing also the dominant mutant Star. 4 series of crosses were made as follows Star/lethal x Normal/Normal, Star/Normal x Normal/Normal, Star/lethal x Star/lethal (non-allelic lethals), and Star/Normal x Star/Normal. The wild type offspring from the first cross, which were heterozygous for a lethal chromosome, were less frequent (55.20 %) than the wild type offspring from the second cross (57.78 %), which did not carry a lethal chromosome. Similarly the frequency of wild type progeny from the third cross, carrying two lethals, was less (41.47%) than that of wild type progeny from the fourth cross (46.32%), carrying one lethal. When the crosses of Star x Normal and Star x Star were considered separately at the different locations, in 6 out of 7 possible combinations the lethal-containing wild offspring were less numerous than the lethal-free offspring. An analysis of variance gave significantly greater heterogeneity between combinations of chromosomes than between cultures of the same combination of chromosomes for the Star x Normal crosses, but not for the Star x Star crosses. For all crosses combined, the mean frequencies of wild-type flies were significantly lower in the lethal-bearing than in the lethal-free progenies. The average lowering of the viability produced by heterozygosis for the lethals was 7.2% (coefficient of inviability).