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.
Shift and adapt: the costs and benefits of karyotype variations
