玉兰减数分裂观察及染色体构型分析

Meiosis observation and chromosome configuration analysis of Magnolia denudata

采用去壁低渗方法 ,观察研究了玉兰Magnoliadenudata有丝分裂和减数分裂的细胞学特征。实验结果证实玉兰存在两种染色体倍性 ,即 2n =4x=76和 2n =6x =1 1 4。通常 ,在木兰属甚至整个木兰科每个物种只具有一种染色体数目。玉兰有丝分裂间期核为复杂染色中心型 ,其中期染色体较小。玉兰在减数分裂中期I的构型表现出多样性 ,其中最主要的特点是比同源多倍体预期的二价体出现的频率更高些 ,其次是在减数分裂中期I可以观察到 1或 2个环状和 (或 )链状六价体。这些特征与同源异源六倍体或部分的异源六倍体种北美红杉Sequoiasempervirens和部分的异源四倍体种紫蓝大麦Hordeumvio laceum相似 ,由此推断玉兰为一个部分的异源六倍体种。整个减数分裂过程中未发现染色体桥和落后染色体 ,仅发现少量的细胞中存在染色体断片。在减数分裂后期I观察了 75个细胞 ,其中 9个细胞 (约1 2 % )出现少量断片 ;在后期II观察了 1 5个细胞 ,其中 1个细胞 (约 6.7% )出现少量断片。在检查的 1 5 2粒花粉中 ,有 1 0粒空花粉 ( 6.6% )。玉兰减数分裂过程基本正常。

Cytological characters of Magnolia denudata in mitosis and meiosis were investigated using the method of wall degradation and hypotonic treatment of making chromosomal preparations. It was confirmed that two chromosome numbers, tetraploid (2n = 4x = 76) and hexaploid (2n = 6x = 114), were present in M. denudata. This was unusual in the genus Magnolia and even in the family Magnoliaceae in which usually only one chromosome number was observed in one species. The resting nuclei belonged to the complex chromocenter type. The chromosomes at mitotic metaphase are small in size. The configurations at meiotic metaphase I were various in different cells, being characterized mainly by the higher frequency of bivalents than expected for an autopolyploid, and also by the occurrence of one or two circle and/or chain hexavalents at metaphase I. The features of configuration at meiotic metaphase I in M. denudata are similar to those in Sequoia sempervirens and Hordeum violaceum. The former is a segmental allohexaploid or an autoallohexaploid and the later is a segmental allotetraploid. M. denudata is possibly a segmental allohexaploid. No chromatin bridges or lagging chromosomes were observed and chromatin fragments only appeared in a few cells. At anaphase I, nine of 75 cells and at anaphase II, one of 15 cells had fragments which accounted for 12% and 6.7% respectively. The rate of sterile pollen grains was 6.6%. The process in meiosis of pollen mother cell in M. denudata might be basically normal.