Laboratory of Plant Cytogenetics, Department of Botany, Federal University of Pernambuco, 50.670-420, Recife, PE, Brazil.
Chromosome Res. 2010 Jun;18(4):503-14. doi: 10.1007/s10577-010-9130-2. Epub 2010 May 21.
Most species of Citrus and related genera display a similar karyotype with 2n = 18 and a variable number of terminal heterochromatic blocks positively stained with chromomycin A(3) (CMA(+) bands). Some of these blocks are 45S rDNA sites, whereas others may correspond to the main GC-rich satellite DNA found in several Citrus species. In the present work, the distribution of the 45S rDNA and the main satellite DNA isolated from C. sinensis (CsSat) were investigated by in situ hybridization in seven species of Citrus, two species of closely related genera (Fortunella obovata and Poncirus trifoliata) and four species of the subfamily Aurantioideae, which were less related to Citrus (Atalantia monophylla, Murraya paniculata, Severinia buxifolia, and Triphasia trifolia). In Citrus, Fortunella, and Poncirus, most CMA(+) bands colocalized only with CsSat sites, whereas others colocalized only with rDNA sites. However, some of these species displayed a few CMA(+) bands that colocalized with sites of both probes and other CMA(+) bands that did not colocalized with any of the probes. On the other hand, in the four species less related to Citrus, no CsSat signal was found on chromosomes. On Southern blot, the CsSat probe hybridized with genomic DNA from Citrus, Fortunella, and Poncirus at high stringency only, while under the less stringent conditions, it also hybridized with distantly related species. Therefore, CsSat sequences are the principal component of the heterochromatic blocks of Citrus, Poncirus, and Fortunella, whereas CsSat-like sequences seem to be widespread in the subfamily Aurantioideae. These data further suggest that the variable number of terminal CMA(+) bands observed on chromosomes of Citrus and related genera are probably the consequence of amplification or reduction in the number of CsSat-like sequences distributed on chromosome termini, paralleled by mutation and homogenization events, as proposed by the library hypothesis.
大多数柑橘属及其相关属的物种显示出相似的核型，具有 2n = 18 和可变数量的端粒异染色质块，这些块用染色质 A3（CMA(+) 带）阳性染色。其中一些块是 45S rDNA 位点，而其他块可能对应于几种柑橘物种中发现的主要富含 GC 的卫星 DNA。在本工作中，通过原位杂交研究了来自甜橙（Citrus sinensis）的 45S rDNA 和主要卫星 DNA（CsSat）在 7 种柑橘属、2 种亲缘关系密切的属（Fortunella obovata 和 Poncirus trifoliata）以及 4 种与柑橘属关系较不密切的副科（Atalantia monophylla、Murraya paniculata、Severinia buxifolia 和 Triphasia trifolia）中的分布。在柑橘属、 Fortunella 和 Poncirus 中，大多数 CMA(+) 带仅与 CsSat 位点共定位，而其他带仅与 rDNA 位点共定位。然而，这些物种中的一些显示出少数 CMA(+) 带与两种探针的位点共定位，而其他 CMA(+) 带与任何探针都不共定位。另一方面，在与柑橘属关系较不密切的 4 个物种中，染色体上未发现 CsSat 信号。在 Southern blot 上，CsSat 探针仅在高严格条件下与柑橘属、 Fortunella 和 Poncirus 的基因组 DNA 杂交，而在较不严格的条件下，它也与远缘物种杂交。因此，CsSat 序列是柑橘属、Poncirus 和 Fortunella 端粒异染色质块的主要成分，而 CsSat 样序列似乎广泛存在于副科 Aurantioideae 中。这些数据进一步表明，在柑橘属及其相关属的染色体上观察到的可变数量的端粒 CMA(+) 带可能是由于 CsSat 样序列在染色体末端的数量增加或减少所致，这与库假说所提出的突变和同质化事件平行。
