Chromosomal inactivation and reactivation in mealy bugs.

UNDERSTANDING genetic inactivation and reactivation in a developing organism is the key to understanding cytodifferentiation. One phenomenon which may help us formulate the proper questions about differentiation is the inactivation and reactivation of whole chromosomes or large segments of chromosomes during development. In this paper I am going to describe one such system of chromosomal modification, relate what is known about the underlying molecular events, and discuss the relationship between chromosomal activation and reactivation and development. FRANZ SCHRADER (1929) and later SALLY HUGHES-SCHRADER (for review see HUGHES-SCHRADER 1948) analyzed the system of chromosome behavior in coccids which is characterized by condensation of a haploid set of chromosomes during early embryogeny of the male. The condensed set is of paternal origin and is eliminated from the gene pool during spermiogenesis. The process of chromatin condensation during interphase is called heterochromatization in analogy to the staining phenomenon described by HEITZ in 1928 (for review see BROWN 1966). BROWN (1966) has suggested that such heterochromatin along with the sex chromatin of mammals be called facultative heterochromatin, delimiting it irom constituitive heterochromatin (for review see YUNIS and YASMINEH 1971 ) . The main distinguishing criterion is that facultative heterochromatin occurs during development in one homolog or set of homologs only, while constituitive heterochromatin occurs in both homologs. Facultative heterochromatization is a developmental phenomenon. In male mealy bugs the condensation of one haploid chromosome set occurs early in embryogeny when the nuclei migrate to the cortical surface of the egg (NUR 1967; SABOUR 1969). Nuclear RNA synthesis begins simultaneously. By inducing chromosome breakage in sperm prior to copulation, BROWN and NELSON-REES (1961) determined that the heterochromatic set is always the paternal chromosome complement. Since these chromosomes are later eliminated from the germ line during spermiogenesis it is apparent that the paternal grandparent makes no genetic contribution to the F, generation and that only the maternal grandparent’s chromosomes are passed on. Having passed through males they are probably pre-programmed to become heterochromatic in males of the F, generation (BROWN and NELSON-REES, 1961; BROWN and CHANDRA 1973). In mealy