The Fate of X-Ray Induced Chromosomal Rearrangements Introduced into Laboratory Populations of Drosophila melanogaster

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Abstract
Several lines of Drosophila melanogaster of various geographic origins were subjected to intensive inbreeding for over 60 generations then small samples of adult males were taken from some of the inbred lines and irradiated resulting in various types of chromosome structural rearrangements in the sperm: 30 chromosomal rearrangements were subsequently recovered and cytologically identified. Members of each culture that contained a newly-induced rearrangement one of either 15 paracentric, or 9 pericentric inversions or 6 translocations-were mated to nonirradiated members of each of the other inbred lines to initiate a series of populations that initially contained known frequencies of a specific rearrangement and that also possessed varying degrees of genie dissimilarity between the chromosomes of each pair of founding lines. After 8 or 9 generations, larval salivary gland chromosome samples taken from each population were examined in order to determine the frequency of structural heterozygotes within the populations after having been subjected to the effects of "natural" selection. Paracentric inversions were considerably more able to maintain structurally polymorphic populations than either the pericentric inversions or translocations. Observed frequencies of the paracentric inversions in populations showed that: (a) individuals structurally heterozygous for a 2-break paracentric inversion suffer from an average selective disadvantage of 0.35-relative to non-inversion homozygotes due to the breakage of the chromosomes; this disadvantage was seen to be counteracted to varying degrees by the amount of genie heterozygosity included within the inverted segment; (b) there is a curvilinear relationship between the length of theinverted segment and structural heterozygote fitness; this relationship likewise varies with the amount of genie heterozygosity included within the inverted segment; (c) individuals structurally heterozygous for chromosomes of unrelated origins maintain almost 50% more genie hybridity-per small chromosome segment[long dash]than do those whose chromosomes came from the same mass culture of flies; and (d) since there is relative success of the paracentric inversions in maintaining structurally polymorphic populations, the genes contributing to fitness heterosis are well-scattered among the chromosome arms of this species.