Population Size Of The Genetic Diversity Of Common Wild Rice (oryza Rufipogon Griff.),

The endangered species, common wild rice (Oryza rufipogon Griff.), known as the ancestor of Asian cultivated rice (O. sativa L.), is the most important germplasm for rice improvement. The continuous human activities has led to habitat fragmentation and population decline in O. rufipogon. In order to understand the genetic consequences of population decline, effective population size, genetic diversity, and mating systems of O. rufipogon populations with different sizes were analyzed using SSR markers in the present study. The possibility of inbreeding depression was also assessed based on comparing fitness performance between inbred and out-crossed progenies under common garden conditions. The results are shown as follows.1) Field investigating indicated that the population size of O. rufipogon ranged from small to large but most were small and suffering human disturbance, implying that human disturbance was the major factor causing population decline.2) A total of 1279 individuals from ten populations in Guangxi Zhuang Autonomous Region, Guangdong and Hainan Province were examined by 12 pairs of SSR markers. Autocorrelation analyses revealed that a total of 7 populations presented significant spatial genetic structure at fine-scale (SGS). The SGS might be derived from the restrict gene dispersal, inbreeding, and clonal growth. DL, PG, and SB populations did not show significant SGS, which might be due to random gene dispersal or extremely strong clone growth. The estimations of gene dispersal and neighborhood size within population indicated that the big population WX had the lowest distance of gene dispersal (4m) and neighborhood size (22-24), whereas the smallest population NB held the relatively long distance of gene dispersal (48-100m) and biggest neighborhood size (266-449). The findings implied that the distance of gene dispersal and the neighborhood size declined with the increasing of population density. The effective size (Ne) was found to be significantly correlated with the census population size (N) (r=0.917, P=0.010) and distribution area (S) (r=0.969, P=0.001) respectively.3) Relatively rich genetic diversity was detected in O. rufipogon populations (He=0.308). We found that Wuxuan population (WX) showed the highest level of genetic diversity (Ae=3.411, He=0.635) and clonal diversity (D=1, E=0.993), while Wanning Dongao population (DA) possessed the lowest level of genetic diversity (Ae=1.413, He=0.271) and clonal diversity (D=0.668, E=0.052), suggesting the influence of clonal growth on population genetic diversity. Significant genetic differentiation were found among O. rufipogon populations (FST= 0.491). There was no significant correlation between population size and genetic diversity.4) The analyses of mating systems for 9 populations with 1389 progenies using 7 SSR markers showed that O. rufipogon populations had relatively high level of outcrossing rate (mean tm=0.544) but considerable inbreeding within population. No significant relationship between outcrossing rate and population size were revealed.5) Common garden experiment showed that no significant difference occurred between inbred and outcrossed progenies, suggesting there seems to be no inbreeding depression existing in O. rufipogon populations. However, indirect estimation of inbreeding depression based on parameters of mating systems indicated that these O. rufipogon populations held considerable inbreeding depression. This result suggested that inbreeding depression might mainly occur at the stage of seed germination and seedling growth.The above results together demonstrate that, although the habitat fragmentation has resulted in population decline and eventually, the decline of the effective population size, there is no significant correlation between the census population size and the levels of genetic variability, outcrossing rate, and inbreeding depression. It therefore suggests that all the populations with different sizes have the same conservation values for genetic diversity. What we should take into action immediately is to avoid habitat disappearance that lead to the extinction of O. rufipogon populations.