Somatic Crossing over in Relationship to Heterochromatin in Drosophila Melanogaster.

OMATIC crossing over, was first demonstrated by STERN (1936) in Drosophila srneZarzogaster, and has since been shown to occur in fungi. From the observation of single and twin mosaic spots in the hypoderm, STERN concluded that the exchanges occur at the four-strand stage, and that the chromatids segregate equationally proximal to the point of exchange and reductionally distal to the point of exchange. He also observed that somatic crossing over occurred most frequently adjacent to the centromere. KAPLAN (1953) and BROSSEAU (1957) showed that the preferential location for somatic crossing over is the proximal heterochromatic region. BROSSEAU also concluded that somatic crossing over early in embryogeny occurs only in the proximal heterochromatin of the X chromosomes, but that later occurrences involve both heteroand euchromatic exchanges, but with a higher frequency in the proximal heterochromatic region. The preferential location of the exchange in the proximal heterochromatic region led to the following two hypotheses to explain the observed higher somatic instability of ring X chromosomes as compared to rod X chromosomes (BROWN, WALEN and BROSSEAU 1962) : ( 1 ) since somatic crossing over largely takes place in heterochromatin then the greater amount of proximal heterochromatin, usually present in ring chromosomes as compared to rod chromosomes, increases the probability of somatic exchange. (2) The inherent shape of ring chromosomes may, for mechanical reasons, cause more stress and, therefore, more opportunities for breaks resulting in increased frequency of somatic crossing over. The present study was undertaken in an attempt to discriminate between these two hypotheses.