SUMMARYRhynchosciara angelae has 8 chromosomes in somatic cells in females (2A+XX) and 7 in males (2A+X0). As females have two Xs and males only one, and the expression of the genes is the same in both sexes, there is a mechanism of dosage compensation for the X-linked genes. The single X polytene chromosome of males is paler than those of females, but has almost the same width. We studied the DNA and RNA synthesis in the sex chromosomes and in the autosomes of males and females in order to investigate dosage compensation at the chromosomal level.We injected tritiated thymidine or uridine into 44-day old larvae of R. angelae. We also placed salivary glands in a medium with tritiated thymidine or uridine. The intensity of DNA and RNA metabolism was evaluated by the number of silver grains per unit of area in the autoradiographs. The results suggest that there is an activation of the X-chromosome in males in relation to RNA synthesis, and at the same time an inactivation of the autosomes in the males.
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