B chromosomes in Stylidium crossocephalum (Angiospermae:Stylidiaceae) |
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Authors: | David J Coates |
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Institution: | (1) Botany Department, The Australian National University, 2600 Canberra, A.C.T. |
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Abstract: | Populations of Stylidium crossocephalum contain two common types of B chromosomes, macro B chromosomes and micro B chromosomes. The macro B chromosomes are telocentric, slightly smaller than the smallest A chromosomes and mitotically unstable. They have so far been found associated with 6 of the 16 stable genomes known in S. crossocephalum, occurring in populations covering a substantial portion of the species range. Micro B chromosomes are about one third the length of the smallest A chromosome, acrocentric and show some mitotic instability. They occur associated with 3 stable genomes in populations found in the more medial regions of the species range. Both types of B chromosomes generally show regular behaviour during meiosis, although when two or more are present their pairing efficiency is reduced when compared to the A chromosomes. In addition a single very large mega chromosome was found in a single cell of one heterokaryotypic plant. Its origin although conjectural at this stage may be of relevance in understanding the origin of macro and micro B chromosomes in this species. |
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