Houben A, Verlin D, Leach C R, Timmis J N
Molecular Biosciences (Genetics), Adelaide University, SA, Australia 5005.
Chromosoma. 2001 Dec;110(7):451-9. doi: 10.1007/s00412-001-0173-1. Epub 2001 Nov 1.
A major sequence component of the micro B chromosome of Brachycome dichromosomatica (2 n=4) is the tandem repeat Bdm29, which was found by in situ hybridisation to be distributed along the entire length of the chromosome. A high copy number of this sequence does not occur as a regular feature of the A chromosomes in this species but it was found in infrequent individuals in two wild populations that were analysed. In these instances Bdm29 is localised within heterochromatic, polymorphic segments on the long arm of chromosome 1. The origin of the micro B chromosomes was investigated by determining whether they are related to this A chromosome polymorphism by simple excision and/or integration. Results obtained by using Bdm29, together with a newly isolated repeat sequence, Bdm54, and a number of other sequences known to occur on the micro B chromosome, as probes in in situ hybridisation and Southern analysis demonstrated that the formation of micro B chromosomes is a complex multistep process. The observation that the genomic organisation of the micro B chromosome is unlike anything found on the A chromosomes precludes their origin by simple excision and also indicates that micro Bs do not integrate directly into the A complement to form polymorphic heterochomatic segments.
二色短角牛（2n = 4）的微小B染色体的一个主要序列成分是串联重复序列Bdm29，通过原位杂交发现它分布于该染色体的整个长度上。该序列的高拷贝数并非该物种A染色体的常规特征，但在分析的两个野生种群中不常见的个体中发现了它。在这些情况下，Bdm29定位于1号染色体长臂上的异染色质多态性区段内。通过确定微小B染色体是否通过简单切除和/或整合与这种A染色体多态性相关来研究微小B染色体的起源。使用Bdm29以及新分离的重复序列Bdm54和已知存在于微小B染色体上的许多其他序列作为原位杂交和Southern分析的探针所获得的结果表明，微小B染色体的形成是一个复杂的多步骤过程。微小B染色体的基因组组织不同于在A染色体上发现的任何东西，这一观察结果排除了它们通过简单切除产生的可能性，也表明微小B染色体不会直接整合到A染色体组中形成多态性异染色质区段。
