The cytology of Brachycome lineariloba

The frequencies and behaviour of two types of accessory chromosome in Brachycome lineariloba species A (n=2) are reported. These include a larger (ca. 4.0 macro-B chromosome, and a minute (<1.0 ) micro-B. Twelve per-cent of the 452 plants studied were carriers of one, two or more rarely three macro-B chromosomes. These were usually mitotically stable, but showed some meiotic irregularity when present in odd number. An equilibrium situation is inferred, and loss must be balanced against mechanisms tending to increase B frequency. A surplus of plants with two B chromosomes also suggests that mechanisms of doubling may operate. A possible source of the macro-B's is considered to be hybridisation between the several species of the B. lineariloba complex. — The micro-B chromosomes were found in 3.8% of two only of the three known cytological races of the species. They showed extreme irregularity in both mitotic and meiotic behaviour. Their relationship to the normal (A) chromosomes and to the macro-B's is obscure.