Chromosome elimination in Heteropeza pygmaea

Abstract

Chromosome elimination in the 3rd cleavage division of the gall midge Heteropeza pygmaea was observed with the Differential Interference Contrast method and recorded with photomicrography and time-lapse cinémicrography. The chromosomes which move all the way to the poles (S-chromosomes) are included in the presumptive somatic nuclei while the lagging chromosomes are eliminated (E-chromosomes). In early prometaphase of an elimination division the nuclear envelope is replaced by the spindle envelope which persists until late telophase and separates nucleoplasm and cytoplasm. In prometaphase the volume of the spindle decreases considerably. Until mid-anaphase the E and the S-chromosomes cannot be distinguished from each other either morphologically or topologically and they both behave like chromosomes in a normal cleavage division. In early anaphase the velocity of the E-chromosomes is usually less than that of the S-chromosomes. After variable amounts of anaphase movement the E-chromosomes return towards the equator with a velocity which is less than their velocity in early anaphase. Their kinetochores are still oriented towards the poles. The two chromatids of an E-chromosome usually move symmetrically towards the poles and back to the equator. At the time when the E-chromosomes stop moving towards the poles the S-chromosomes sometimes accelerate.