Undercondensation and localized euchromatinization of the X chromosome in the grasshopper Melanoplus femur-rubrum

Abstract

In most animals, including grasshoppers, the X chromosome is heterochromatic (heteropycnotic) during prophase I and metaphase I of spermatogenesis. This report describes one grasshopper male in which at these states some of the X chromosomes contained an euchromatic (E) segment. In grasshoppers, the heteropycnotic state of the X is apparently established prior to the formation of the cysts. The spermatocytes containing the E segments, however, did not comprise whole cysts. It was concluded, therefore, that the E segments resulted from a localized euchromatinization rather than a failure to become heteropycnotic. The cytology of this male was unusual in two other respects. In most of the spermatocytes the chromosomes were longer and thinner than those of other males. In addition, in some of the cells undergoing meiosis, the cytoplasm failed to divide during both meiotic divisions and the resulting spermatids failed to differentiate into sperm. Because in this species both the presence of Xs with E segments and undercondensation are very rare and both involve condensation, it is likely that they are in some way related. Evidence for and against the possibility that the E segments were genetically active and that this activity led to the arrest of some of the spermatids is discussed.