Marcon Adriana Buarque, Barros Iva Carneiro Leão, Guerra Marcelo
Universidade Federal de Pernambuco, Centro de Ciências Biológicas, Departamento de Botânica, Rua Nelson Chaves s/n, Cidade Universitária, 50.670-420, Recife, Pernambuco, Brasil.
Ann Bot. 2005 Jan;95(2):271-6. doi: 10.1093/aob/mci022. Epub 2004 Nov 26.
Selaginella is the largest genus of heterosporous pteridophytes, but karyologically the genus is known only by the occurrence of a dysploid series of n=7-12, and a low frequency of polyploids. Aiming to contribute to a better understanding of the structural chromosomal variability of this genus, different staining methods were applied in species with different chromosome numbers.
The chromosome complements of seven species of Selaginella were analysed and, in four of them, the distribution of 45S rDNA sites was determined by fluorescent in situ hybridization. Additionally, CMA/DA/DAPI and silver nitrate staining were performed to investigate the correlation between the 45S rDNA sites, the heterochromatic bands and the number of active rDNA sites.
The chromosome numbers observed were 2n=18, 20 and 24. The species with 2n=20 exhibited chromosome complement sizes smaller and less variable than those with 2n=18. The only species with 2n=24, S. convoluta, had relatively large and asymmetrical chromosomes. The interphase nuclei in all species were of the chromocentric type. CMA/DA/DAPI staining showed only a weak chromosomal differentiation of heterochromatic bands. In S. willdenowii and S. convoluta eight and six CMA+ bands were observed, respectively, but no DAPI+ bands. The CMA+ bands corresponded in number, size and location to the rDNA sites. In general, the number of rDNA sites correlated with the maximum number of nucleoli per nucleus. Ten rDNA sites were found in S. plana (2n=20), eight in S. willdenowii (2n=18), six in S. convoluta (2n=24) and two in S. producta (2n=20).
The remarkable variation in chromosome size and number and rDNA sites shows that dramatic karyological changes have occurred during the evolution of the genus at the diploid level. These data further suggest that the two putative basic numbers of the genus, x=9 and x=10, may have arisen two or more times independently.
卷柏属是异型孢子蕨类植物中最大的属，但从核型角度来看，该属仅因出现了n = 7 - 12的非整倍体系列以及低频率的多倍体而为人所知。为了有助于更好地理解该属的染色体结构变异性，对不同染色体数目的物种应用了不同的染色方法。
分析了七种卷柏属植物的染色体组，并对其中四种植物通过荧光原位杂交确定了45S rDNA位点的分布。此外，进行了CMA/DA/DAPI和硝酸银染色，以研究45S rDNA位点、异染色质带和活性rDNA位点数量之间的相关性。
观察到的染色体数目为2n = 18、20和24。2n = 20的物种其染色体组大小比2n = 18的物种更小且变异性更低。唯一2n = 24的物种，即内卷卷柏，具有相对较大且不对称的染色体。所有物种的间期核均为染色中心型。CMA/DA/DAPI染色仅显示出异染色质带的微弱染色体分化。在威氏卷柏和内卷卷柏中分别观察到八条和六条CMA +带，但未观察到DAPI +带。CMA +带在数量、大小和位置上与rDNA位点相对应。一般而言，rDNA位点的数量与每个细胞核中核仁的最大数量相关。在平叶卷柏（2n = 20）中发现了十个rDNA位点，在威氏卷柏（2n = 18）中发现了八个，在内卷卷柏（2n = 24）中发现了六个，在拟大叶卷柏（2n = 20）中发现了两个。
染色体大小、数目和rDNA位点的显著变异表明，在该属的二倍体水平进化过程中发生了剧烈的核型变化。这些数据进一步表明，该属假定的两个基数，即x = 9和x = 10，可能已经独立出现了两次或更多次。
