DISTRIBUTION OF HETEROCHROMATIN ON THE MITOTIC CHROMOSOMES OF MUSCA-DOMESTICA L. IN RELATION TO THE ACTIVITY OF MALE-DETERMINING FACTORS

In the housefly, male sex is determined by a dominant factor, M, locat ed either on the Y, on the X, or on any of the five autosomes. M facto rs on autosome I and on fragments of the Y chromosome show incomplete expressivity, whereas M factors on the other autosomes are fully expre ssive. To test whether these differences might be caused by heterochro matin-dependent position effects, we studied the distribution of heter ochromatin on the mitotic chromosomes by C-banding and by fluorescence in situ hybridization of DNA fragments amplified from microdissected mitotic chromosomes. Our results show a correlation between the chromo somal position of M and the strength of its male-determining activity: weakly masculinizing M factors are exclusively located on chromosomes with extensive heterochromatic regions, i.e., on autosome I and on th e Y chromosome. The Y is known to contain at least two copies of the M factor, which ensures a strong masculinizing effect despite the heter ochromatic environment. The heterochromatic regions of the sex chromos omes consist of repetitive sequences that are unique to the X and the Y, whereas their euchromatic parts contain sequences that are ubiquito usly found in the euchromatin of all chromosomes of the complement.