Karyotypic evolution in an originally XY cell line of Drosophila melanogaster: a case of heterochromatin increase in vitro.

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.