Micro- and nucleolar-organizing b-chromosomes in Calycadenia (Asteraceae).

Cytological studies of seven taxa in four species of the genus Calycadenia revealed the existence of several types of B-chromosomes. Of special interest is the occurrence of at least three forms of nucleolar-organizing B's in a total of six taxa in three species of the genus. Also of note is the occurrence in one species of an unusual structure less than 1μ in length that is interpreted as a micro-B chromosome. Its morphology is reminiscent of that of a “naked” centromere. The B-chromosomes of Calycadenia are variable in size, morphology, and meiotic behavior. They have probably had multiple origins, perhaps as abnormal meiotic products in structural heterozygotes produced by hybridizing populations or species or as by-products of aneuploid reduction.