Breeding Hevea rubber : Formal and molecular genetics

From the beginning of its domestication, rubber cropping was developed in the industrial sector, consisting of large estates with monospecific plantations. But the small-holding sector progressively increased its share and now owns more than 80% of the world's natural rubber production. Small-holders' plantations sometimes look similar to estates, but most are rubber-based agroforests (in Indonesia) or multicropping systems, especially in the young age (Gouyon, 1996; Viswanathan and Rajasekharan, 2001). The aim of rubber breeding is to provide small-holders as well as agro-industrial estates with an easily reproducible and genetically superior planting material in the form of a varietal type, currently represented by grafted clones, for latex production and, more recently, wood production. Although breeding is a long-term activity, introducing superior clones in plantation and replantation projects proved to be a convenient and cheap way to improve or maintain the profitability of rubber cropping. Now, biotechnologies hold new promise for deriving improved planting material and increasing the efficiency of rubber breeding. This review is an attempt to take stock of the progress made in breeding Hevea rubber worldwide. Hevea (Para rubber) is a tall, deciduous, perennial ligneous tree, with rhythmic growth and orthotropic ramification, belonging to the Euphorbiaceae family. It can reach 30 m high and a girth of over 3 m in its region of origin, the Amazon basin. With a history spanning more than 450 years, it attained prominence only during later half of the nineteenth century as a consequence of the discovery of vulcanization by Goodyear in 1839, which gave to natural rubber the required level of quality and durability for wide industrial utilization. Introduced from the Amazon basin to southeast Asia, it staked almost 40% of the export revenue of Brazil, nearly equaling coffee in importance until 1940 (Dean, 1987). Rubber statistics during 2002 kept Thailand (2.3 million tons) at the helm of rubber producers, with Indonesia, India, Malaysia, China, Vietnam, Côte d'Ivoire, Liberia, Sri-Lanka, Brazil, Philippines, Cameroon, Nigeria, Cambodia, Guatemala, Myanmar, Ghana, Democratic Republic of the Congo, Gabon, and Papua New Guinea in the descending order of production. The southeast Asian countries continue to enjoy dominance in rubber production and trade by maneuvering more than 90% of the 7.97 million tons of rubber produced worldwide in 2003 (Sekhar, 2004). Thailand, Indonesia, India, and Malaysia together contributed 74% of the rubber produced. The center of origin, Latin America, shares only 2%, mainly due to the occurrence of South American leaf blight (SALB), caused by Microcyclus ulei P. Henn. Von. Arx (Chee, 1976). Confronted with the competition of synthetic elastomers, 75% of the natural rubber produced is consumed by the automotive industry (mainly in tires). More than 50% of natural rubber is consumed in Asia. (Résumé d'auteur