Ontogenes and Heterochromatin of D. Melanogaster

Chadov BF

Abstract

Mutations in Drosophila melanogaster ontogenes sharply increase the rate of X-chromosome nondisjunction in meiosis, suggesting that ontogenes are active in meiosis. The activity of homologous ontogenes drives homologs together (to pair); however, ontogenes interact at a distance without any contacts or chemical mediators. The meiotic chromosomes involved in pairing have the appearance of densely compacted chromatin, suggesting that the active ontogenes themselves have a heterochromatin nature.

Incomplete penetrance of the mutations in ontogenes generated in experiment also favors a heterochromatin nature of ontogenes. Incomplete penetrance can be regarded as a consequence of position-effect variegation (mosaicism), characteristic of heterochromatin but taking place in the germline cells of the mutants for ontogenes rather than in somatic cells. Thus, both the meiotic effect of mutations in ontogenes (chromosome nondisjunction) and their incomplete penetrance are two consequences stemming from the same root, namely, the fact that they belong to heterochromatin.

The cytogenetic association of ontogenes with heterochromatin supplements and concretizes the earlier proposed concept of elementary event in development.