Northern Crop Science Laboratory, USDA-ARS, 1605 Albrecht Blvd N, Fargo, ND 58102-2765, USA.
Chromosome Res. 2013 Aug;21(5):507-21. doi: 10.1007/s10577-013-9378-4. Epub 2013 Aug 17.
Brachypodium distachyon is a wild annual grass belonging to the Pooideae, more closely related to wheat, barley, and forage grasses than rice and maize. As an experimental model, the completed genome sequence of B. distachyon provides a unique opportunity to study centromere evolution during the speciation of grasses. Centromeric satellite sequences have been identified in B. distachyon, but little is known about centromeric retrotransposons in this species. In the present study, bacterial artificial chromosome (BAC)-fluorescence in situ hybridization was conducted in maize, rice, barley, wheat, and rye using B. distachyon (Bd) centromere-specific BAC clones. Eight Bd centromeric BAC clones gave no detectable fluorescence in situ hybridization (FISH) signals on the chromosomes of rice and maize, and three of them also did not yield any FISH signals in barley, wheat, and rye. In addition, four of five Triticeae centromeric BAC clones did not hybridize to the B. distachyon centromeres, implying certain unique features of Brachypodium centromeres. Analysis of Brachypodium centromeric BAC sequences identified a long terminal repeat (LTR)-centromere retrotransposon of B. distachyon (CRBd1). This element was found in high copy number accounting for 1.6 % of the B. distachyon genome, and is enriched in Brachypodium centromeric regions. CRBd1 accumulated in active centromeres, but was lost from inactive ones. The LTR of CRBd1 appears to be specific to B. distachyon centromeres. These results reveal different evolutionary events of this retrotransposon family across grass species.
短柄草是一种野生一年生草本植物，属于禾本科 Poaceae，与小麦、大麦和饲料草的亲缘关系比水稻和玉米更为密切。作为一个实验模型，短柄草的全基因组序列为研究禾本科物种形成过程中的着丝粒进化提供了一个独特的机会。在短柄草中已经鉴定出着丝粒卫星序列，但对该物种中的着丝粒反转录转座子知之甚少。本研究利用短柄草（Bd）着丝粒特异性 BAC 克隆，对玉米、水稻、大麦、小麦和黑麦进行了细菌人工染色体（BAC）-荧光原位杂交（FISH）。8 个 Bd 着丝粒 BAC 克隆在水稻和玉米的染色体上没有检测到可检测的 FISH 信号，其中 3 个 BAC 克隆在大麦、小麦和黑麦中也没有产生任何 FISH 信号。此外，5 个三裂叶属 centromeric BAC 克隆中的 4 个没有与短柄草 centromeres 杂交，这表明短柄草 centromeres 具有某些独特的特征。对短柄草 centromeric BAC 序列的分析鉴定了一个短柄草的长末端重复（LTR）-着丝粒反转座子（CRBd1）。该元件以高拷贝数存在，占短柄草基因组的 1.6%，并且在短柄草 centromeric 区域富集。CRBd1 积累在活性着丝粒中，但从非活性着丝粒中丢失。CRBd1 的 LTR 似乎是短柄草 centromeres 所特有的。这些结果揭示了这个反转座子家族在禾本科物种中的不同进化事件。
