Mestriner C A, Galetti P M, Valentini S R, Ruiz I R, Abel L D, Moreira-Filho O, Camacho J P
Departamento de Genética e Evolução, Universidade Federal de São Carlos, via Washington Luiz, Km 235 Caixa Postal 676, 13565-905 São Carlos, SP, Brazil.
Heredity (Edinb). 2000 Jul;85 ( Pt 1):1-9. doi: 10.1046/j.1365-2540.2000.00702.x.
Astyanax scabripinnis possesses a widespread polymorphism for metacentric B chromosomes as large as the largest chromosome pair in the A complement. On the basis of C-banding pattern, it was hypothesized that these B chromosomes are isochromosomes that have arisen by means of centromere misdivision and chromatid nondisjunction. In the present paper we test this hypothesis by analysing (i) the localization of a repetitive DNA sequence on both B chromosome arms, and (ii) synaptonemal complex formation, in order to test the functional homology of both arms. Genomic DNA digested with KpnI and analysed by gel electrophoresis showed fragments in a ladder-like pattern typical of tandemly repetitive DNA. These fragments were cloned and their tandem organization in the genome was confirmed. A 51-bp long consensus sequence, which was AT-rich (59%) and contained a variable region and two imperfect reverse sequences, was obtained. Fluorescence in situ hybridization (FISH) localized this repetitive DNA into noncentromeric constitutive heterochromatin which encompasses the terminal region of some acrocentric chromosomes, the NOR region, and interstitial polymorphic heterochromatin in chromosome 24. Most remarkably, tandem repeats were almost symmetrically placed in the two arms of the B chromosome, with the exception of two additional small clusters proximally located on the slightly longer arm. Synaptonemal complex (SC) analysis showed 26 completely paired SCs in males with 1B. The ring configuration of the B univalent persisting until metaphase I suggests that the two arms formed chiasmata. All these data provided strong support for the hypothesis that the B chromosome is an isochromosome.
斯氏丽脂鲤（Astyanax scabripinnis）拥有广泛的多态性，其B染色体为中着丝粒染色体，与A染色体组中最大的染色体对一样大。基于C带模式，推测这些B染色体是通过着丝粒错分和染色单体不分离产生的等臂染色体。在本文中，我们通过分析（i）重复DNA序列在B染色体两条臂上的定位，以及（ii）联会复合体的形成，来检验这一假设，以测试两条臂的功能同源性。用KpnI消化并通过凝胶电泳分析的基因组DNA显示出呈梯状模式的片段，这是串联重复DNA的典型特征。这些片段被克隆，并证实了它们在基因组中的串联组织。获得了一个51bp长的共有序列，该序列富含AT（59%），包含一个可变区和两个不完全反向序列。荧光原位杂交（FISH）将这种重复DNA定位到非着丝粒组成型异染色质中，该异染色质包括一些近端着丝粒染色体的末端区域、核仁组织区（NOR）以及24号染色体的间质多态性异染色质。最值得注意的是，串联重复序列几乎对称地分布在B染色体的两条臂上，只是在稍长的臂近端还有另外两个小簇除外。联会复合体（SC）分析显示，具有1条B染色体的雄性中有26个完全配对的SC。B单价体的环状结构一直持续到中期I，这表明两条臂形成了交叉。所有这些数据为B染色体是等臂染色体这一假设提供了有力支持。
