Variation in populations of Tribulus terrestris (Zygophyllaceae). 2. Chromosome numbers.

Variation in chromosome number was investigated as part of a study to identify the origins of the widespread weed and potential biological control target Tribulus terrestris L. s.1. (Zygophyllaceae). Three ploidy levels, tetraploid 2n = 24, hexaploid 2n = 36 and octoploid 2n = 48, were detected in 24 Australian and 24 overseas collections. Northern Territory and Queensland collections, possibly representing a native species of Tribulus, had counts of 2n = c. 24. The majority of collections of T. terrestris in southern and north-western regions of Australia had counts of 2n = c. 36, but two collections had counts of 2n = c. 24. Collections from South Africa and Namibia also had a mixture of tetraploid and hexaploid forms. The USA, Mediterranean and West Asian samples (except for three Kuwait samples) were all hexaploid. All Indian and three Kuwait samples were the only octoploid collections observed. These ploidy levels agree with those previously reported for T. terrestris. It was concluded that the polyploid complex has an allopolyploid origin. The predominantly hexaploid collections in Australia indicate an overseas origin, and the presence of two tetraploid collections points to two or more separate introductions of T. terrestris into Australia. Counts for T. micrococcus and T. occidentalis were 2n = c. 48, which provide further evidence for a base number of x = 6 or 12 for the genus.