Karyotype evolution in the genus Boronia (Rutaceae)

New chromosome counts are reported for Boronia clavata 2n = 14, B. heterophylla 'Near White' 2n = 15, B. 'Carousel' 2n = 16, B. deanei 2n = 22, B. chartacea 2n = 32, B. keysii 2n = 32, B. pilosa 2n = 44, B. anethifolia 2n = 36 and B. citriodora 2n = 108. Studies in 20 genotypes of 18 species and one interspecific hybrid revealed that they are highly complex in terms of chromosome number, ploidy level, chromosomal length, karyotype constitution and asymmetry. Karyotype analysis indicated that Boronia taxa with high chromosome numbers are primitive and those with lower numbers are derived. The basic chromosome number for this genus is suggested to be x = 18. Analysis of chromosome number, variations of total chromosome length (TCL) and average chromosome length (ACL), Nombre Fondamental (NF) and karyotype asymmetry suggest that dysploid reduction is the major mechanism in Boronia karyotype evolution. Chromosomal rearrangements might also have been involved. Origin, chromosome number changes and spread of Boronia are discussed in relation to the species divergence and the geological and climatic changes of the Australian continent. (C) 2003 The Linnean Society of London, Botanical Journal of the Linnean Society, 2003, 142, 309-320.