内蒙古典型草原羊草种群遗传分化的RAPD分析

Genetic differentiation in eight populations of Leymus chinensis in Inner Mongolia Steppe

运用 RAPD技术对内蒙古典型草原不同生境 8个羊草种群进行分析。采用 2 4个随机引物 (10 nt)在 8个种群中共检测到2 2 4个扩增片断 ,其中多态性片断 173个 ,总的多态位点百分率达 77.2 % ,特异性片断 2 2个 ,占 9.82 % ,平均每个引物扩增的DNA带数为 9.3 3条。利用 Nei指数和 Shannon指数估算了 8个种群的遗传多样性 ,并计算种群相似系数和遗传距离 ,运用UPGMA法进行聚类分析。结果表明 :羊草大部分的遗传变异存在于种群内 ,只有少部分的遗传变异存在于种群间 ,Nei指数和Shannon指数计算结果分别为 85.4%和 66.8% ;羊草不同种群的遗传多样性存在差异 ;8个羊草种群平均遗传距离为 0 .2 3 16,变异范围为 0 .1587～ 0 .2 70 0 ,说明 8个羊草种群间的遗传变异不大 ,即 :在较小地理范围内羊草的遗传分化程度较小 ;8个种群可聚为 3个类群 ,聚类结果显示生境相似的种群能够聚在一起 ,而地理距离最近的种群不一定归为一类 ,说明小范围内羊草种群间的遗传分化与地理距离不存在相关性 ,而与其生境间的相似度相关。影响遗传相似性的不是单一因子而是各种因子的综合作用 。

To study the magnitude and nature of genetic variation in Leymus chinensis,the level and pattern of RAPD variation were investigated for eight populations located in middle and east part of Inner Mongolia. Ten plants from each of eight populations were screened for variation using 24 RAPD primers. The mean number of DNA bands amplified per primer was 9.33. The number of polymorphic bands generated was primer dependent, ranging from 4 to 12. The RAPD analysis showed that 173 out of 224 amplified fragments (77.2%) were polymorphic and 22 loci were unique (9.82%). Genetic diversity at species level was relatively high (H_(sp)=0.586), and the mean population genetic diversity was also high (H_(pop)=0.501). The largest proportion of the total RAPD diversity was found within, rather than among the populations (G_(ST)=0.146). In terms of proportion of polymorphic loci and genetic diversity of populations, the population in artificial grassland had the highest level of polymorphic loci (67.9% on average) and Nei index (0.555), while the population in woodland and that in sand land had the lowest percentage of polymorphic loci and Nei index. There seemed to be a positive correlation (r=0.58) between the genetic diversity and the soil water content in the habitats where the population was located. The results indicated that smaller populations were lower in polymorphism, due to the effect of small population size and relative isolation. The relatively low level of polymorphism might also reflect the extent of clonal growth within a small population, reducing the chances of gene flow and outcrossing. Strong gene flow occurred among the populations in Inner Mongolia steppe, with the number of migrants per generation (N_m) being 1.463. Similarity and genetic distance matrices were established from the RAPD data on the basis of Nei's estimates of similarity indices and genetic distance,and dendrograms were constructed based on UPGMA clustering. All the populations were found to be in the range of 0.7300~0.8413 in similarity, and the average genetic distance among the eight populations was 0.2316, varying from 0.1587 to 0.2700, which indicated that there were relatively low genetic variations among the eight populations and low genetic differentiation of Leymus chinensis within a geographical region, which was in turn indicative of a narrow genetic base among the various populations with a medium level of polymorphism. The observed low level of genetic variation was attributed to interplay of sexual and vegetative modes of reproduction and similarity of local environments of L.chinensis. Bigger difference between the populations in soil moisture of the habitats resulted in greater genetic distances. The dendrogram divided eight populations of Leymus chinensis into three groups. The populations from similar habitats, but not from locations within a short geographic distance,aggregated into one group, which showed that there was no obvious correlation between genetic differentiation and geographic distance among populations of Leymus chinensis in this region. The present study implied that the level of genetic differentiation was most likely affected by more than one ecological factors, and the genetic variation among populations of Leymus chinensis in a small geographic region was determined by the environmental heterogeneity. These results have profound implications for genetic conservation and breeding of Leymus chinensis.