Chromosome Races and Structural Heterozygosity in Calycadenia Ciliosa Greene (Asteraceae)

A biosystematic study of Calycadenia ciliosa resulted in the recognition of five homoploid ( n = 6) chromosome races. The cytogenetic evidence indicates that each of these races is differentiated from its nearest relative by a single reciprocal chromosome translocation, although at one point in the evolutionary history a pericentric inversion may have been a concurrent cytological event. The data also show that the chromosome phylogeny includes two instances of redundant translocation. Mean pollen stainabilities of interracial hybrids range from 16–80%. In a survey of four natural populations 30–60% of the individuals sampled were found to be structurally heterozygous for reciprocal chromosome translocations. Pericentric inversion heterozygosity was also detected in one population. Another population contained morphologically indistinguishable individuals separated by as little as 120 m that were differentiated by a minimum of four chromosome translocations. These observations were compared with similar instances in other species in an effort to determine their significance in the process of plant evolution.