Functional significance of changes in the shape of the polytene X chromosome in Phryne

Abstract

In contrast to the autosomes, the salivary gland X chromosome of Phryne has distinctly different shapes in different cells (Wolf, 1957). These changes in shape are not correlated with alterations of the nucleic acid synthetic activities. The X in males and females always replicates in step with the autosomes, and the XB and the autosomes always have the same relative DNA contents (the Y is under-replicated). The transcriptive activity of the X is always about the same as that of the autosomes in the same cell. Breeding conditions which cause larvae to attain higher final polyteny levels enhance the stretching of the X, but the polyteny is not directly correlated with the degree of stretching; Xs with the same polyteny vary considerably in shape. — The DNA content of 2C female nuclei is 0.34×10−12 g.