Increase of genetic variability through recombination in Drosophila melanogaster.

genetic variability concealed in natural populations is the evolutionary Tg material of Mendelian populations. How the genetic variability increases through recombination is important in obtaining an understanding of the genetic structure of Drosophila populations. In fact, this problem has been studied in natural and artificial populations of several species of Drosophila (D. pseudoobscura: SPASSKY et al. 1958; D. persimilis: SPIES 1959; D. prosaltans: DOBZHANSKY et al. 1959; D. willistoni: KRIMBAS 1961; D. melanogaster: SPIES and ALLEN 1961; ALLEN 1966). It may be concluded from the series of studies referred to above that recombination between linked genes and polygenes controlling viability yields an extensive array of variability. These studies have also shown that the amount of variability released through recombination seems to be different for different species and in different chromosomes of the same species. DOBZHANSKY (1946) first presented experimental evidence that even lethal chromosomes could be synthesized as the most extreme products of recombination within the second chromosome of Drosophila pseudoobscura. Thereafter, such “synthetic lethals” or “recombinational lethals” were reported in D. prosaltans and D. willistoni (DOBZHANSKY et al. 1959; KRIMBAS 1961). In Drosophila melanogaster, WALLACE et al. (1953) , GIBSON and THODAY (1962), BATTEN and THODAY (1969), and others reported “synthetic lethals” in the second chromosome; but GANTNER (1958), SPIES and ALLEN (1961), and ALLEN (1966) could not obtain decisive evidence for their production in the same chromosome. HILDRETH (1956), SPIES and ALLEN (1961 ) , and ALLEN (1966) were unable to detect such lethals in the third chromosome and HILDRETH (1956) could not exhibit them in the sex chromosome. Therefore, it is interesting to know whether “synthetic lethals” really exist in D. melanogaster. The purposes of the present study are to clarify how genetic variability is released through recombination and to determine the circumstances under which synthetic lethals (if they exist) are produced. Some preliminary results have been reported in KOSUDA and MORIWAKI (1968).