Polysomy in Omocestus bolivari: endophenotypic effects and suppression of nucleolar organizing region activity in the extra autosomes

Abstract

The grasshopper Omocestus bolivari shows polysomy for the M4 autosome in the male germ line but not in the male and female soma or the female germ line. All of the polysomic males identified were mosaic for this character, so that there was interfollicular, but not intrafollicular, variation for the number of extra copies of M4. All M4 chromosomes in excess of two are heterochromatinized (h-M4) as can be deduced from their evident heteropycnosis during the first meiotic division. Some of these h-M4 chromosomes lag during meiosis and therefore form microspermatids. This occurs mainly in follicles with a single h-M4. The heritable factors which are assumed to determine mosaicism for male germ line polysomy may also play a role in reducing mean cell chiasma frequency. The h-M4 chromosomes themselves, however, raise chiasma frequency in an additive manner. The silver impregnation technique identifies the primary nucleolar organizing regions (NORs) on both the X and the M4 chromosomes, together with a series of secondary micro NORs on L2, M5, M6, and S8. The h-M4 autosomes of polysomic males, however, have no detectable nucleolar activity, which appears to be causally related to the facultative heterochromatinization of the M4 and a loss of the positive C-banding character of their nucleolar associated heterochromatin.Key words: polysomy, mosaic, heterochromatin, chiasmata.