Fig. 1. Geographical distribution of intra-populational gene diversity. Size of the circle stands for the amount of average gene diversity, H, in each population.
1) Faculty of Agriculture, Hokkaido University, Sapporo, 060 Japan
2) National Institute of Genetics, Mishima, 411 Japan
In 1992 and 1993, we made a survey of wild rice species in the basin of Central Amazon in collaboration with the staff of College of Agriculture, Sao Paulo University. Outline of this survey was reported in our trip report (Morishima and Martins 1994). Two wild rice species, O. glumaepatula (with AA genome, synonymous to the American form of O. perennis) and O. gradiglumis (with CCDD genome) were found during our trip. Here we report the results of our preliminary study on the genetic structure of natural populations of O. glumaepatula.
Thirty-seven population samples of O. glumaepatula collected along Rio Solim6es (the Amazon) and its largest tributary Rio Negro were examined for their genetic diversity in isozymes. In general, degree of polymorphism was much less than that observed in Asian common wild rice. Among 26 loci examined, 14 were polymorphic. Average gene diversities within population computed for the polymorphic loci ranged from 0 to 0.215 (0.074 on the average, 0.158 for 29 Asian populations). Mean frequency of heterozygote was 0.4%. Geographical distribution of intrapopulational gene diversity was shown on the map (Fig. 1). As shown in the figure, populations distributed in the lower Solimoes near Manaus seemed to contain larger genetic diversity than those distributed upstream.
Fig. 1. Geographical distribution of intra-populational gene diversity.
Size of the circle stands for the amount of average gene diversity, H, in
each population.
To elucidate geographical differentiation in Amazonian wild rice, genetic distances were computed between populations grouped into ten regions (see Fig. 1). Genetic distances were 0.006-0.099, indicating that differences among regions were very small. Yet, a cluster analysis revealed a trend to differentiation among different regions. As shown in Fig. 2, populations distributed along Rio Negro and Rio Solimoes, including those of their respective tributaries, tended to be differentiated. Those collected in Rio Japura and Rio Purus (two tributaries to the upper and the lower Rio Solim6es, respectively) formed an isolated cluster.
O. glumaepatula growing in the Amazon basin seemed to have a unique life history. In contrast to the same species distributed in Central America which have the perennial habit, they propagate mostly by seeds, germinating on the parched land during a short terrestrial phase. During aquatic phase, the plants elongate their internodes in response to the rapid rise of river water, and then their brittle culms are easily separated at the nodes forming free-floating vegetations. We observed such floating patches with many reproductive tillers flowing down the river. Another factor responsible for shaping genetic structure of the Amazonian wild rice might be a high rate of extinction caused by unstable water conditions, particularly water depth which varies markedly during the terrestrial phase from year to year. In 1993, no wild-rice seedlings came out in one of the
Fig. 2. Differentiation among ten regions in O. glumaepatula populations
distributed along the Amazon basin. Genetic distances were computed based on
allozyme data for 26 loci.
study-sites along the Rio Negro because of high water at the germination stage (personal communication from Ms. A. Lubim, Nat. Inst. for Res. in Amazonia). Most probably, Amazonian wild-rice populations have been frequently threatened by bottlenecks for survival. Gene flow over a long distance and frequent extinction might explain a low level of polymorphism observed in O. glumaepatula in the Amazon basin.
References
Morishima, H. and P. S. Martins (ed.), 1994. Investigations of plant genetic resources in the Amazon basin with the emphasis on the genus Oryza. Special Rep. from Nat. Inst. Genet., Japan pp. 100.