Genetic Diversity, Genetic Erosion, and Conservation of the Two Cultivated Rice Species (Oryza sativa and Oryza glaberrima) and Their Close Wild Relatives

Abstract

Rice cultivated gene pool includes two species. Asian rice, Oryza sativa, displays a very large phenotypic diversity resulting from a long history of domestication driven by human demographic expansion and sympatry with its wild relatives. African rice, Oryza  glaberrima, represents a typical case of domestication bottleneck. Recent sympatry of the two species in Africa has given birth to new diversity. Current rice in situ genetic diversity results from the succession of a number of long-standing evolutionary events and the contemporary reversal of the trend of increasing diversity, referred to as genetic erosion. Since the early twentieth century, human demographic growth, agricultural modernisation and the advent of formal breeding systems, have affected the in situ diversity of cultivated rice species and their wild relatives. The evolutionary processes had produced a very large number of Landraces (LV) of which some 500,000 are conserved ex situ. The contemporary changes have resulted in the replacement of a large proportion of LV by a small number of Modern varieties (MV) in more than 70 % of rice-growing areas in Asia and Latin America, 38 % in Africa. The most important feature of rice in situ diversity emerging from our case studies in China, South and Southeast Asian countries, West Africa and Madagascar, is the diversity of situations. Aggregated data suggest massive absolute genetic erosion and sharp reduction of diversity indexes, particularly in irrigated ecosystems. Detailed surveys indicate smoother genetic erosion in rainfed ecosystems. However, the perspectives of rice in situ genetic diversity are gloomy even in rainfed ecosystems. The most realistic and promising option for the future is a dynamic management in the framework of the emerging concept of ecological intensification.