Aust. J. BioI. Sci., 1977, 30, 319-28 Population Cytology of the Genus Phaulacridium V. * Ph. marginale - The Omarama Population M. Westerman Department of Genetics and Human Variation, La Trobe University, Bundoora, Vic. 3083. Abstract During the 1975 meiotic season a sample of 105 Ph. marginale males was collected from Omarama (South Island, New Zealand). This population is polymorphic for a small metacentric B chromosome (BM) and for extra segments on the three smallest autosomes. Only 10% of males have the basic karyotype. Previous collections from this population had similar frequencies of the BM chromosomes and of the extra segments. One individual carried a small telocentric B chromosome (B'), unusual at Omarama, as well as the BM typical of this population. This may be significant for the proposed evolution of B chromo- somes in the genus Phaulacridium. Both types of supernumerary heterochromatin (B chromosomes and segments) produce a similar increase in mean cell chiasma frequency, and maximal increase is produced by one B chromosome or one segment. There is no evidence for any additive or synergistic action of B chromosomes and segments. Introduction In addition to occasional chromosome rearrangements the New Zealand grass- hopper Phaulacridium marginale is extensively polymorphic for B chromosomes and extra heterochromatic segments (Martin 1970a, 1970b; Westerman and Fontana 1973; Westerman 1974, 1975b). The pattern of distribution of these polymorphisms in North and South Islands of New Zealand suggests that the extra heterochromatin may be better tolerated in ecologically 'central' rather than in 'marginal' populations (Westerman 1975b). Three morphologically distinct types of B chromosomes occur, only one type being present in anyone population. These are: a large telocentric (BT) approximately equal in size to the X chromosome, and morphologically similar to the BT chromo- somes in Ph. vittatum and Ph. nanum; a smaller telocentric (Bt); and a small meta- centric isochromosome (B M ). All three types of B chromosomes are mitotically stable in the male germ line. The frequency of B-carrying males in a population may vary from 4 to 50 %. These adult frequencies are stable from year to year with no evidence for either meiotic 'drive' or 'drag'. At Omarama (South Island) there is a population with an abundance of BM-con- taining males (41-50 %) and which also has the polymorphisms for heterochromatic segments on the three smallest autosomes. In each of three collections (1972, 1973, 1975) only 10% of males had the basic karyotype. Although analysis of the limited data had not revealed any significant effect of the extra heterochromatin on either * Part IV, Heredity, 1975, 35, 165-72.