The cytology of Brachycome. II. The subgenus Metabrachycome: a general survey

Abstract

Chromosome counts for 12 of the 17 known species of Metabrachycome are reported. All species have x = 9. Nine species are diploid (2n = 18), and the other three species show intraspecific polyploidy on base x = 9. Two of these three have a sexual diploid within the complex. The polyploids are probably apomictic. G. L. Davis recognized morphological 'varieties' within each of the three polyploid species complexes, but the relationship of these varieties to the various ploid levels is unclear. Metabrachycome, which is primarily Western Australian, exhibits a wide range of morphological and ecological diversity, yet it is chromosomally very conservative, with no change in chromosome base number, and no obvious karyotype differentiation. This contrasts sharply with the primarily eastern Eubrachycome which is extremely labile chromosomally, with base numbers ranging from n = 2 ton = 15. It is suggested that the more stable environment in Western Australia since the mid Tertiary may have been less conducive to the establishment and maintenance of chromosomal rearrangements.