Puertas M J
Departamento de Genética, Facultad de Biología Universidad Complutense, Madrid, Spain.
Cytogenet Genome Res. 2002;96(1-4):198-205. doi: 10.1159/000063047.
This work reviews recent advances providing insights on the origin and evolution of B chromosomes (Bs) in representative plant species. Brachyome dichromosomatica has large and micro Bs. Both carry an inactive ribosomal gene cluster. The large Bs contain the B-specific Bd49 family, mainly located at the centromere. Multiple copies are present in the A chromosomes (As) of related species, whereas only a few copies exist in B. dichromosomatica As. The micro Bs share sequences with the As, the large Bs and have the B-specific repeats Bdm29 and Bdm54. It seems that the large and micro Bs are related in origin. It is very unlikely that the Bs originated by simple excision from the As. Rye Bs are composed of sequences predominantly shared with the As. B-specific sequences are located at the heterochromatic end of the long arm. Probably, they originated from the As after many rearrangements, with a tendency for duplication. The E3900 family derives from a Ty3 gypsy retrotransposon, but the D1100 family shows no evidence of genic origin. The overall composition of maize As and Bs is similar suggesting a common origin. Several B-specific sequences have been found, the most studied being pZmBs, which is located at the B centromere. It shows partial homology to the centromere of chromosome 4 and to the knobs. It is not known whether the B centromere derives from centromere 4, or whether both have a more distant common origin. The dynamics of Bs in populations depends on their non-Mendelian mechanisms of transmission, their effects on carrier fitness and on A genes modulating their parasitic properties. Three representative examples are reviewed. The Bs of Allium schoenoprassum are transmitted at a mean lower than Mendelian and adversely affect vigour and fertility. However, there is a differential selection operating in favour of B-containing seedlings. Rye Bs undergo strong drive, which is counteracted by harmful effects on fertility and instabilities at meiosis. Both nondisjunction and meiotic behaviour, and consequently the establishment of B polymorphisms, mainly depend on the Bs themselves. B nondisjunction in maize is controlled by the B, but the As control preferential fertilisation. Considering the non-equilibrium model, the Bs of Allium seem to have been neutralised by the A genome, the As of maize provide defence against B attack, whereas the Bs of rye are only slightly neutralized.
这项工作回顾了近期的进展，这些进展为代表性植物物种中B染色体（Bs）的起源和进化提供了见解。二色短柄草具有大B染色体和小B染色体。两者都携带一个无活性的核糖体基因簇。大B染色体包含B特异性的Bd49家族，主要位于着丝粒处。相关物种的A染色体（As）中存在多个拷贝，而在二色短柄草的A染色体中只有少数拷贝。小B染色体与A染色体、大B染色体共享序列，并具有B特异性重复序列Bdm29和Bdm54。似乎大B染色体和小B染色体在起源上是相关的。Bs不太可能通过从As简单切除而起源。黑麦Bs由主要与As共享的序列组成。B特异性序列位于长臂的异染色质末端。它们可能在经过多次重排后从As起源，并且有重复的趋势。E3900家族源自Ty3逆转座子，但D1100家族没有基因起源的证据。玉米A染色体和B染色体的总体组成相似，表明有共同的起源。已经发现了几个B特异性序列，研究最多的是pZmBs，它位于B染色体的着丝粒处。它与4号染色体的着丝粒和结节显示出部分同源性。尚不清楚B染色体的着丝粒是否源自4号染色体的着丝粒，或者两者是否有更遥远的共同起源。群体中Bs的动态取决于它们的非孟德尔遗传传递机制、它们对携带者适合度的影响以及对调节其寄生特性的A基因的影响。文中回顾了三个代表性例子。葱的Bs以低于孟德尔遗传的平均比例进行传递，并对活力和育性产生不利影响。然而，存在有利于含B幼苗的差异选择。黑麦Bs经历强烈的驱动，但受到对育性的有害影响和减数分裂不稳定性的抵消。不分离和减数分裂行为，以及因此B多态性的建立，主要取决于Bs本身。玉米中的B不分离由B染色体控制，但A染色体控制优先受精。考虑到非平衡模型，葱的Bs似乎已被A基因组中和，玉米的A染色体提供了抵御B攻击的能力，而黑麦的Bs仅被轻微中和。
