Parallel polymorphism for supernumerary segments inChorthippus parallelus (zetterstedt)

A complex and parallel pattern of polymorphism for heterochromatic supernumerary segments in the M₇ and S₈ chromosomes has been found in 14 populations of the meadow grasshopperChorthippus parallelus. Nine distinct karyotype classes for these two chromosome pairs occur though they are not equally represented in different populations. Populations differ also with respect to the frequency of supernumerary segments they contain. In all populations the presence of supernumerary segments leads to a significant elevation of mean cell chiasma frequency compared to individuals from the same population lacking such segments. The extent of the effect appears to differ in different populations. The observed frequencies of S₈ karyotypes conform to the expectations of a Hardy-Weinberg distribution. Those of the M₇, however, do not, and in all but one of the 14 populations there is a significant excess of homokaryotypes. In the Ashurst population 26% of the individuals sampled were characterised by germ-line polysomy for the M₄ chromosome, either in the form of entire tetrasomics or more usually as mosaics ranging from tri- to hepta-somics. In all these polysomics the M₄ chromosomes in excess of two were regularly heteropycnotic at first meiotic prophase from zygotene to diakinesis. As a consequence of this multivalents are rare. Extra M₄ chromosomes do not modify the chiasma characteristics of the other chromosomes in the complement. Nor do they modify the action of the supernumerary segments in any way.