我国原生植物假泽兰(菊科)的细胞学研究

Cytological study on Mikania cordata(Asteraceae), a native plant in China

关键词:  菊科, 染色体数目, 核型, 假泽兰

Key words:  Asteraceae, chromosome number, karyotype, Mikania cordata

摘要:

该文研究了我国原生植物假泽兰(Mikania cordata)台湾花莲居群、苗栗居群、宜兰居群以及台北居群的染色体数目和染色体形态。结果表明:所有居群的染色体数目为2n=36,第一对染色体为近中部着丝粒染色体,其长臂中部具有次缢痕,显著大于其余染色体。各居群的核型公式皆为2n=18m+18sm,核型均为2B型,染色体内不对称性指数(A₁)的变化范围为0.38～0.39,染色体之间不对称性指数(A₂)的变化范围为0.30～0.32。此为我国假泽兰居群染色体数目的唯一报道,也是对该种核型的首次报道。结合前人对假泽兰染色体数目的研究结果,认为假泽兰存在种内非整倍性现象,但在中国台湾的居群中目前仅发现基于x=18的二倍体(2n=36)。假泽兰的第一对染色体的长臂中部具次缢痕,与假泽兰属已报道的核型相似,这一次缢痕可作为假泽兰属的细胞学标记。核型资料、野外观察以及ISSR数据显示薇甘菊(M. micrantha)在我国的成功入侵与入侵种和本土种之间的杂交渐渗无关。根据标本记录和野外考察结果,我国假泽兰现在的分布区与过去相比有了很大的缩减,推测生境的破坏和薇甘菊的侵入可能是导致假泽兰在中国台湾地区逐渐消失的主要原因。

Abstract:

This study aimed to confirm the chromosome number of Mikania cordata(Asteraceae), a native plant in China, and to test the possible hybridization scenario between M. cordata and its invasive congener M. micrantha from a cytological perspective. Chromosome number and chromosome morphology of four populations in M. cordata were investigated. These included Hualian, Miaoli, Yilan and Taipei populations from Taiwan, China. The chromosome number 2n=36 was determined for all four populations. Their karyotypes, all formulated as 2n=18m+18sm, were characterized by having a remarkably larger, submedian centromeric chromosome pair with a secondary constriction in the middle of the long arms. All the karyotypes were Stebbins's 2B type. The intrachromosomal asymmetry index(A₁)varied from 0.38 to 0.39, and the interchromosomal asymmetry index(A₂)varied from 0.30 to 0.32. This is the only report of the chromosome number for M. cordata from China, and also the first karyotype report for this species. Our results, together with previous reports, indicate that there is aneuploid variation within this species, but 2n=36(diploid based on x=18)is the only number currently known from the populations in Taiwan, China. The karyotypes of M. cordata resemble those reported for other Mikania species in the secondary constriction of the first chromosome pair, which could be considered a cytological marker for this genus. Mikania cordata and M. micrantha have the same chromosome number and ploidy but different karyotypic characteristics. However, intermediate karyotype was not found within populations where the two came into close contact. No intermediates between the two species were observed during field work, and ISSR analyses also failed to detected hybrid individuals. Therefore, the above evidence suggest that the successful invasion of M. micrantha in China is not associated with hybridization and introgression between this invasive species M. cordata and its only indigenous congener in China. After comparing past herbarium records with results of recent field surveys, we found that the distribution range of M. cordata in China has undergone a great reduction. We infer that habitat destruction and invasion of M. micrantha may be the main causes for the gradual disappearance of M. cordata in Taiwan, China.