X-ray induced visible alterations in the giant chromosomes of Phryne cincta (Nematocera, Diptera): relation of radiation sensitivity to pronuclear chromosome structure.

In order to induce chromosomal rearrangements, males were exposed to x-rays and then mated to non-irradiated females. The number of each type of structural alteration was determined by examination of the polytene chromosomes of the F1 progeny. -- A comparison of the results with similar studies made on Drosophila revealed a significantly greater sensitivity in Phryne. Parallel to that an extremely high frequency of small inversions was ascertained in Phryne, and the observed ratio of inversions to translocations was the inverse of that which would be expected from purely mathematical considerations based on the lengths of the different chromosomes. These facts allow the conclusion that the paternal pronuclear chromosomes in Phryne are highly spiralized. Besides, the kinetochore-to-translocation-breakpoint distance was measured in both of the chromosomes involved in each reciprocal translocation and the differences (kinetochore-break distance differences) were registered and from them the arrangement of the chromosomes in the pronucleus of Phryne deduced. The data obtained support the assumption of an ordered, polar-field type of orientation. In Drosophila, in contrast, the comparable data showed that the pronuclear chromosomes are not spiralized and are randomly arranged (Bauer, 1939). -- These results seem to indicate that a close correlation exists between the different radiation sensitivities of Drosophila and Phryne and the different states of spiralisation and arrangements of their chromosomes in the pronucleus stage. It is hypothesized that the influence of the maternal genome on the degree of spiralization of the paternal chromosomes could account for differences in the pronuclear chromosome structure of both species.