水杉孑遗居群AFLP遗传变异的空间分布

Spatial structure of AFLP genetic diversity of remnant populations of Metasequoia glyptostroboides (Taxodiaceae)

本研究采用空间自相关分析方法对水杉 (Metasequoiaglyptostroboides)孑遗居群AFLP遗传变异的空间结构进行了研究 ,以探讨水杉孑遗居群遗传变异的分布特征及其形成机制。根据 6对AFLP选择性引物扩增的 46个多态性位点 ,选择了其表型频率在 2 5 %～ 75 %的 2 7个AFLP标记 ,运用等样本频率方法和等地理距离间隔方法分别对 3 9株和 3 7株原生母树进行了空间自相关系数Moran’sI值计算。结果表明 :水杉孑遗居群缺乏空间结构 ,绝大多数AFLP位点变异为随机分布的空间模式 ,但也有少数位点存在显著性随机相关 ,在 4～ 8km地理距离间隔显示负相关 ,说明该间隔可能是水杉孑遗居群的部分基因交流的有效屏障。水杉原生母树分布存在 12～ 2 8km的明显距离间隔空挡 ,说明人类从迁入该区域起就影响着水杉孑遗居群的原始生境 ,导致其生境片断化、景观破碎 ,进而形成岛屿状分布格局 ,并引起了水杉残留居群的随机遗传漂变。根据本研究结果 ,结合水杉孑遗居群较低的遗传多样性 ,分析探讨了水杉孑遗居群濒危的机理 ,并提出了相应的保育策略 。

The Tertiary relic species, Metasequoia glyptostroboides, is naturally distributed in a narrowly restricted area bordering Hubei, Hunan Provinces and Chongqing City in central China. The genetic background of its overall genetic diversity and population structure is prerequisite for formulating an efficient conservation program and effective practice in management of remnant populations. The spatial structure of gene frequencies of M. glyptostroboides remnant populations in its original habitat was investigated using AFLP markers followed by spatial autocorrelation analysis. A total of 39 individuals were sampled from remnant populations and subjected to spatial autocorrelation analysis in two approaches of equal gene frequency correlograms with five classes and equal distance interval correlograms with ten classes, respectively. Of 46 polymorphic bands generated by six selective primer-pairs, 27 polymorphic bands with frequencies ranging from 25% to 75% were then used to calculate Moran's I spatial autocorrelation coefficient. The result revealed a lack of spatial structure of genetic variation existing in the remnant population of M. glyptostroboides, indicating that genetic variations of the most polymorphic loci were randomly distributed, strongly supported by insignificant Moran's I coefficients. However, lump, depression or double cline structures of the genetic variation were found in a few AFLP markers with significant Moran's I in several distance classes. The distance interval 4-8 km is probably a sufficient barrier to gene flow, as evidenced by Moran's I coefficients showing negative correlations within this distance class. In addition, remnant individuals were not absent in distance classes from 12 to 28 km, suggesting that the habitat of the remnant populations had been disturbed since humans moved in and subsequent agricultural activities occurred. Apparently, habitat fragmentation resulted in an island distribution and genetic drift had occurred even before M. glyptostroboides was rediscovered in 1941. Based on the results of the present study combined with the lower genetic diversity previously detected in the remnant populations, possible mechanisms causing the endangerment of M. glyptostroboides are discussed and a conservation strategy for M. glyptostroboides remnant populations is proposed. The results provide valuable information for formulating future in situ and ex situ conservation programs of this relic species.