The Effects of Recombination on Quasinormal Second and Third Chromosomes of DROSOPHILA MELANOGASTER from a Natural Population.

ENETIC recombination is a natural source of variation. Experiments with chromosomes of Drosophila pseudoobscura, D. prosaltans, D. persimilis, D. willistoni, and D. melanogaster with quasinormal viability show increases in the variation of viability values for homozygotes after recombination between strains ( SPASSKY, SPASSKY, LEVENE, and DOBZHANSKY 1958; DOBZHANSKY, LEVENE, SPASSKY, and SPASSKY 1959; SPIESS 1959; KRIMBAS 1961; WALLACE, KING, MADDEN, KAUFMANN, and MCGUNNINGLE 1953; SPIES and ALLEN 1961). If most of the chromosomes showing quasinormal viability in the homozygous condition had identical karyotypes, little variation would be expected after recombination between such chromosomes. Since variation is produced at a highly significant level, it may be concluded that few, if any, chromosomes from different origins with similar viability values have the same genetic constitution. I t seems that the effects of recombination are greater for some species of Drosophila than others, which may also apply for different chromosomes in the same species. It was observed that chromosome 3 of D. melanogaster was more sensitive to recombination than chromosome 2 (SPIES and ALLEN 1961 ) . Effectively. recombination breaks up linkage groups to form new ones. Linkage groups arising from chromosomes that show better than average viability when homozygous show a reduction in average viability or approach the natural condition after recombination. The most extreme expression of recombination is the production of lethals. Lethals thus produced were first termed synthetic by DOBZHANSKY (1 946) from work with D. pseudoobscura. Evidence for synthetic lethals has been reported for second chromosomes of D. pseudoobscura (DOBZHANSKY et al. 1959; SPASSKY ef al. 1958), D. prosaltans (DOBZHANSKY et al. 1959), D. willistoni (KRIMBAS 1961). and D. melanogaster (WALLACE et al. 1953; MISRO 1949; THODAY 1963). However, in recombination experiments with third and X chromosomes of D. melanogaster (HILDRETH 1955, 1956), with second chromosomes of D. persimilis (SPIESS 1959), and second and third chromosomes of D. melanogaster (SPIES and ALLEN 1961 ) there was little conclusive evidence for the production of synthetic lethals. There is little doubt that recombination can produce combinations of genes