Chromosomal diversity in the Australian Phasmatodea

Abstract

A cytological survey of the Australian Phasmatodea, involving 24 species from the five major subfamilies present in Australia, has confirmed earlier indications of the karyotypic diversity of this Order of insects. Male diploid chromosome numbers range from 26 to 45, and XO and XY sex-chromosome mechanisms occur. Whilst being within the overall range of cytological variation already established for the Order, with respect to level of variability, the diversity shown by Australian species exceeds that of previous observations. Numerical variation is present within some species, as well as within subfamilies and genera. The five Australian species which show a geographic pattern of chromosomal differentiation are the first such examples known amongst phasmatids. Didymuria violescens, the most variable, has at least 10 chromosome races, a range in chromosome number from 26 to 39 (2n), and three forms of the sex-chromosome system. Ctenomovpha chronus has at least three chromosome races. It is suggested that the cytological and biological characteristics of this group of insects, including features of their population structure and dynamics, predispose phasmatids to high levels of variability for otherwise conservative chromosome characters. Newly arisen structural rearrangements have been fixed repeatedly in the evolutionary history of the Order. Many of these have resulted in a change in chromosome number; some few have been responsible for XY types of sex mechanism, by incorporation of autosomal material into the primitive XO system. In view of the amount of karyotypic variation present, cytological characters are effectively useless as indicators of broad phylogenetic relationships within the Phasmatodea. Only chromosome size shows some possible correlation with established interrelationships at the subfamily level. The incidence of intraspecific chromosomal differentiation, together with the usual concurrence of chromosomal differences with intrageneric species differences, further suggests that karyotypic differentiation at a racial level may be involved as a normal stage in the speciation pattern of many phasmatids.