The cytogenetics of some New Zealand grasshoppers (Acrididae)

Abstract

Since there is no cytogenetical record of the New Zealand species of Acrididae the first aim of this study was to describe the meiotic chromosome complements plus any abnormal chromosome behaviour. From the data gathered, a more general investigation of the movement, frequency and distribution of chiasmata, and also the occurrence and effects of B-chromosomes was made. The species studied were Phaulacridium marginale (twelve populations), Brachaspis collinus (two populations), Alpinacris crassicauda (two populations) and Sigaus piliferus (three populations). A few individuals of Brachaspis nivalis were also examined. All species possess 23 autosomes plus one X-chromosome (in the male). Meiosis is normal but some abnormalities such as polyploid cells, heterochromatic fragments, non-homologous chromosome associations and heterochromatic "stickiness" were found. It was suggested that these abnormalities may be due to abnormal protein or DNA synthesis. Chiasma terminalisation occurs during meiosis I but it does not cause a reduction in chiasma frequency. Chiasma frequencies varied significantly between species and between the populations of P. marginale. Differences in habitat and breeding systems were suggested to explain these variations. Chiasma distribution was studied in two species, P. marginale and B. collinus. It was found that the distribution varied with chromosome length and chiasma frequency. It was concluded that chiasma formation probably begins distally. Interchromosomal effects in chiasma frequency were determined by calculating correlation coefficients between the chiasma frequencies of pairs of bivalents. From the results it was concluded that there are two systems of chiasma control. Firstly by the genetic control of the synthesis of the histones needed for synapsis. This is at the level of the cell as a whole. Second by the regulation of interference distance through the genetic control of chromosome coiling. This is at the level of the individual bivalents. Individuals possessing B-chromosomes were found in three populations and an individual which was heterozygous for a supernumerary, heterochromatic segment on the S11 bivalent was found in one population. The B-chromosomes did not appear to have any exo- or endophenotypic effects.