ALLOZYME DIVERSITY IN NATURAL POPULATIONS OF BETULA ALNOIDES FROM GUANGXI, CHINA
广西区西南桦天然居群遗传多样性的研究

Betula alnoides is a valuable tree species in tropical and warm sub_tropical areas in South_east Asia and China. Its natural populations have become increasingly fragmented due to over_harvesting for local economic development in Guangxi, China during the last decade. Thus conservation of its genetic resources is of urgent need for future genetic improvement programs and sustainable management. Here, the allozyme diversity of Betula alnoides was analyzed by starch gel electrophoresis with improved Tris_maleate extraction buffer. Young leaves were collected from seedlings grown by seeds from 11 natural populations in Guangxi, China. Of the 21 enzymes tested, 10 (AMP, FBA, GDH, G6PD, IDH, MDH, PGD, PGI, PGM and SKD) could be consistently resolved and scored. Among 15 interpretable loci there were 9 polymorphic loci (0.95 criterion); the remaining 6 loci were monomorphic. High genetic variability was quantified by three indices: percentage of polymorphic loci ( P ), mean number of alleles per locus ( A ) and mean expected heterozygosity ( He ). The observed values of these indices were 55.2%, 2.00 and 0.204 respectively, all of which were higher than the averages of outcrossing wind_pollinated woody species (53.0%, 1.84 and 0.154, summarized by Hamrick et al., 1989). Ho was always higher than He in the 11 populations, indicating an excess of heterozygotes, which perhaps resulted from a higher probability of survival for heterozygous individuals. No significant correlation was detected between the genetic diversity and geographic variables of these populations. It is worth noting that populations 2 (Dizhou, Jingxi County), 5 (Haicheng, Pingguo County) and 9 (Zhemiao, Tianlin County) should be selected and managed as in_situ conservation localities, because they contain a majority of alleles and high levels of genetic diversity. The method and findings in this study could be applied in further studies on the genetic structure, diversity and genetic improvement of this species and other species of the genus Betula.