The Cytology of Brachycome lineariloba 7. Meiosis in Natural Hybrids and Race Relationships

A comparison of the karyotypes of races D (2n=8), E (2n= 10), B (2n= 12) and C (2n= 16) of B. lineariloba suggests that these races have in common a basic set of four chromosome pairs, and that the higher chromosome number races are related to race D by successive chromosome addition. - A study of meiosis in B x C and A1 x B hybrids supports this contention and elucidates the homologies of the additional chromosomes. - Meiotic pairing in hybrids between A and C is very complex. At present it can only be stated that there are extensive interchromosomal homologies between the two races. - Two phyletic schemes of the relationships of the races are considered. The second, which is favoured, involves successive chromosome addition, with the quasidiploid race E (2n= 10) giving rise to race B by diploidisation of the univalent chromosomes. This scheme is supported by features of univalent behaviour in the various races and their hybrids. - The ecogeographic distribution pattern of the races shows replacement of D by E by B by C as the species extends into more arid and more harsh environments. This replacement is also associated with in- creasing vigour. - It seems most likely that the addition chromosomes are derived from a race A (2n=4) source since they are added always by twos, and each addition increases both vigour and drought tolerance. Race A is the most vigorous and one of the most drought tolerant of the five races. It is suggested that the evolution of the races can be related to the increasing aridity of the Late Pleistocene and Recent geological epochs.