Population cytogenetics of Atractomorpha similis

Abstract

Atractomorpha similis (2 n= 19 c~, 20 ~) is a hy- grophilous, tropical to temperate, species ofpyrgomorphine grasshopper. We have sampled 70 populations covering the known distributional range of this species within Australia. All of them proved to be polymorphic for heterochromatin content as revealed by C-band analysis of embryonic neu- roblasts. This polymorphism affects all ten members of the basic haploid set and includes variants involving differences in either the presence or the amount of procentric, intersti- tial and terminal C-blocks, as well as variation in the occur- rence and nature of short arms on otherwise telocentric chromosomes. A majority of these variants appear to result from heterochromatin addition since the presumptive sib- ling, Atractomorpha australis, like other species of the genus that have been C-banded, is generally depauperate in het- erochromatin. The net result of this extraordinary polymor- phism is that each chromosome of A. similis exists in 10- 50 distinct morphs. Consequently, there is a high level of chromosomal heterozygosity in all populations in terms of the number of heterozygous pairs present within a comple- ment and an even higher level in terms of the total range of karyomorph patterns. There is also a wide range of total heterochromatin content, as measured by the percent of the total chromosome area occupied by C-band material, with values ranging from 13% to 44%. Specific marker chromosomes which predominate in particular geographi- cal areas serve to distinguish six major cytotypes within A. similis. The two most southerly of these cytotypes show a narrower range of heterochromatin content but with high- er values which reflect the more general occurrence of sub- stantial terminal C-blocks within them. Finally, the popula- tions from Fraser Island constitute a particularly distinctive cytotype characterised by the least number of morphs, the lowest level of chromosomal heterozygosity and a restricted range of heterochromatin content confined to the lower end of the known distributional spectrum.