Neuroblast chromosomes of 16 Drosophila melanogaster laboratory stocks (15 wild type and 1 carrying the mutant vermillion) were carefully analyzed for Q-banding patterns and morphological characteristics, in all the mitotic phases. Two forms of intraspecific heterochromatin variations, involving three types of chromosomes, are described: 1) differences in the fluorescence pattern with regard to the Y chromosome and the centromeric heterochromatin of the pair II; 2) differences in the size of the heterochromatic segment of the X chromosome. An unambiguous evidence of such variants was obtained by comparing homologous chromosomes in the F1 hybrids, as well as in the F2 offspring, where differences in appearance of the heteromorphic chromosomes was readily identified as to the parental origin. The possible evolutionary significance and the usefulness of such cytologically detectable genetic differences between various strains, are considered.
The cell line Ca of Drosophila melanogaster, characterized initially by a nearly diploid and normal male karyotype (XY), was used to study chromosomal variation over a period of 5 years of cultivation in vitro. Some general aspects of cell population dynamics which are in accordance with previous findings are pointed out. Various phenomena regarding chromosomal changes leading to karyotype polymorphism are outlined, with a particular emphasis being given to the sex chromosomes. Accordingly, with the aid of fluorescence analysis, some features of the Y and the X chromosomes providing evidence of an enlargement of the heterochromatin (due to addition and to saltatory replication) are described. Moreover, a case of variation in cell morphology accompanied by karyotypic changes was observed, as well as the emergence of a new cell subline of XX type derived from the original of XY type.
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