Chromosome numbers in the Boronieae (Rutaceae) and their bearing on the evolutionary development of the tribe in the Australian flora

Abstract

Chromosome numbers are reported for 69 species, belonging to 11 genera of the Boronieae. In comparison with the other divisions of the Rutaceae, chromosome numbers are remarkably variable, and base numbers range from 7 to 19. Seven genera and 10 species belonging to other tribes and subfamilies of the Rutaceae are also reported. In common with the Aurantoideae, Xanthoxyleae, and Ruteae, the primitive number in the Boronieae is almost certainly 9. Chromosomal evolution has involved both aneuploidy and polyploidy. In both diploid and polyploid species, chromosome pairing is essentially regular. All the polyploids, even within recognized taxonomic species, are essentially of allopolyploid constitution. Two stages in the occurrence of polyploidy can be recognized — the first giving rise to groups of generic status, and the second to polyploid species within genera. Many genera possess chromosome numbers which could be derived by aneuploid reduction, followed by polyploidy. Some fundamental change in chromosonle structure in the primitive Boronieae stock is suggested as a cause of its early chromosomal instability. From a correlation of geographical and cytological data and inference, a hypothesis of the historical development of the Boronieae in Australia is presented. Following their isolation in the continent, and the occurrence of a fundamental reorganization in chromosome structure, a period of generic radiation, with changes in basic number, occurred before the Miocene. Particular chromosome numbers are characteristic of genera. Differences in base number rank as generic rather than as specific criteria. On this basis the accepted taxonomy is essentially sound, and only minor alterations are suggested. Crowea is established, and Eriostemon lanceolatzis is separated from its congeners. Some sections of Boronia are deserving of generic rank, and B. serrulata is wrongly placed in the section Terminales.