A CYTOGENETIC STUDY OF THE X CHROMOSOME OF Drosophila Busckii AND ITS RELATION TO PHYLOGENY

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Abstract
The normal mitotic chromosome complement of D. busckii, so far as is now known, is of one sort only. It contains three pairs of chromosomes a pair of rod-shaped sex chromosomes and two pairs of nearly equal-sized V-shaped autosomes. The X-chromosome has a prominent achromatic constriction that corresponds with the nucleolus organizer and which lies approximately one-third (or one-fourth) the length of the metaphase chromosome from the proximal end. The centromere of the mitotic X is very nearly at the tip of the proximal end, and perhaps this chromosome is telocentric. The Y chromosome has a small knoblike thickening at the centromeric end. The primary constriction can be observed between this knoblike thickening and the long distal part of the Y. Thus this chromosome is acrocentric, or J-shaped. The short euchromatic element of the polytene X-chromosome represents the proximal part of the X, and it is now proved that this element is homologous with the euchromatic portion of the polytene Y-chromosome, that corresponds to the proximal knoblike thickening, or right arm, of Y. The homologous euchromatic portions of X and Y of D. busckii are, in turn, homologous with the microchromosomes of other species of Drosophila. The data given here concerning these homologies provide the first cytogenetic evidence for the supposition that the microchromosomes of other dotless species may have become parts of macrochromosomal elements during the process of species differentiation.

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