The structure,meiotic behaviour and effects of B chromosomes in Briza humilis Bieb. (Gramineae)

A single population of Briza humilis contained two types of B chromosome, one a large (B^L) and the other a small (B^S) acrocentric. DNA measurements show that the B^L chromosome contains approximately twice as much DNA per unit length as the members of the regular complement. The meiotic pairing behaviour of the Bs is variable and B^L and B^S are seen to pair in some cells. The presence of B^L depresses the chiasma frequency of the regular complement but the chiasma frequency of A and B chromosomes does not appear to be related. The transmission rate of the B chromosomes is variable and the B^L shows a non-disjunction mechanism during microsporogenesis that is absent during megasporogenesis. For the B^S chromosome the transmission rate is very low and there is no evidence of a non-disjunction mechanism. In general seeds containing B^L chromosomes germinate more slowly than those without B chromosomes.