**1. Introduction**

432 Genetic Diversity in Plants

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Germplasm collections have provided original materials for plant breeding program and crop improvement. Because of their genetic diversity and possible occurrence of particular desirable genes, germplasm collections are useful targets for plant breeders as well as other biologists. Recently, many of germplasm are being lost worldwide due to habitat destruction, invasion of foreign species, and reliance on fewer high yielding strains. Therefore, maintaining germplasm of agricultural crops is very important. Normally, a germplasm collection will be utilized based on its characters of immediate perceived value or its potential variation. Furthermore, it can also be used to better understand the properties and performances of the plants, particularly at the genomic level. Nowadays, countries all over the world have set up facilities for conservation, characterization and utilization of germplasm collections of various crops either directly or indirectly.

Rubber tree (*Hevea brasiliensis* Willd. ex A. de Juss. Müell. Arg) which produces natural rubber is a tall deciduous perennial tree belonging to the Euphorbiaceae family. *Hevea brasiliensis* is a native of the Amazon basin and was introduced to countries in the tropical belts of Asia and Africa during late 19th century. It can be termed as one of the most far reaching and successful introductions in plant history, resulting in plantations with about 10.6 million hectares in the world for providing the industry with natural rubber of 9.62 million tonnes in the year 2009 (IRSG, 2009). Approximately 78% of the cultivation has taken place in Southeast Asia, 15% in other Asia countries, 5% in Africa, and 2% in Latin America (IRSG, 2009). The main producing countries were Thailand, Indonesia, Malaysia, India and Vietnam (IRSG, 2010). As the fifth natural rubber producer in the world, Vietnam produced 723,700 tonnes that shared about 7.5% of the world's natural rubber production and total area under rubber trees were 674,200 hectares that shared 6.1% of the world's rubber area (Hoa, 2010). In Vietnam, areas under rubber trees are mainly in the South-eastern region (65.2%), followed by the Highlands (23.4%), central coastal area (9.7%) and the new areas developed in the North-western region (1.6%) (Hoa, 2010).

*Hevea brasiliensis* was introduced into Vietnam in 1897 from Bogor (Indonesia) by Alexandre Yersin. Since then, rubber tree has been considered as one of the most important crops and widely cultivated throughout the country, particularly in the Southeast and Highlands regions of Vietnam. Currently, about 3,500 accessions of *Hevea brasiliensis* have been collected and conserved in Vietnam. The majority of this germplasm were derived from the IRRDB'81 germplasm collected in the Amazonian habitats of the genus. This collection has been considered as the key factor contributing to the improvement of rubber tree through breeding programs. The characterization and evaluation of the germplasm are considered to be important aspects of *Hevea* germplasm conservation. Without proper characterization and evaluation, valuable genetic variation in the collections cannot be used for rubber improvement effectively. Understanding the genetic diversity of different genetic resources of *H. brasiliensis* would be important in `order to optimize their management and to utilize *Hevea* germplasm in breeding programs.

#### **2.** *Hevea* **genetic resources**

The genus *Hevea* is basically composed of 10 species: *H. brasiliensis*, *H. guianensis, H. benthamiana*, *H. pauciflora*, *H. spruceana*, *H. microphylla*, *H. rigidifolia*, *H. nitida*, *H. camporum*, and *H. camargoana* (Schultes, 1990). According to Clement-Demange et al. (2000), it is generally considered that there is no biological barrier between them, and some species proved to be inter-crossable by hand-pollination; therefore, the *Hevea* species can be considered as a species complex. *H. paludosa* has been identified in Brazil by Ule in 1905 as the eleventh species (Gonçalves et al., 1990; Priyadarshan and Gonçaalves, 2002). A review on the elaborate description of taxonomical and botanical aspects of *Hevea* has been reported by Schultes (1977, 1987, 1990) and Wycherley (1992). As its natural habitat, *Hevea* species have presented in Brazil, Bolivia, Peru, Ecuador, Colombia, Venezuela, Surinam and French Guiana as shown in Figure 1. These *Hevea* species have 2n = 36 chromosomes, with the possible exception of one triploid clone of *H. guianensis* (2n = 54) and the possible existence of one genotype of *H. pauciflora* with 2n = 18 (Baldwin, 1947; Majumder, 1964), and *H. brasiliensis* behaves as an amphidiploid (Ramaer, 1935; Ong, 1975; Wycherley, 1976).

It is known that all high-yielding cultivars of rubber tree (*Hevea brasiliensis* Muell. Arg.) in the world originated from breeding programs initially developed in Southeast Asia with a very narrow genetic base. Historically, the introduction of the rubber trees into Asia began with the transfer of 70,000 seeds to England by Henry Wickham in 1876 (Wycherly, 1968). During the years of 1876-1877, a total of 2,397 *Hevea* seedlings were sent to several Asian countries such as Sri Lanka (1,919 seedlings), Bogor (Java) (18 seedlings) and Singapore (22 seedlings) (Wycherly, 1968; Dean, 1987; Baulkwill, 1989). Because of the centrally geographical position of Singapore in Asia and the influences of the British to the development of the rubber industry during these years, the collection of 22 seedlings planted in the Singaporean botanical garden became the main source of the rubber trees which was distributed to and planted in Asian countries later. Since this introduction, the rubber tree has become an important perennial crop as the major source of commercial rubber in the world (Fig. 2) and this collection was mentioned as Wickham collection (W). Since the current *Hevea* varieties all came from such a single population, it is necessary to enlarge the genetic basis for *Hevea* breeding program.

In order to enlarge the genetic basis for *Hevea*, a large collection of *H. brasiliensis* accessions from various areas in Colombia was gathered by Schultes after 1945 and then duplicated in

widely cultivated throughout the country, particularly in the Southeast and Highlands regions of Vietnam. Currently, about 3,500 accessions of *Hevea brasiliensis* have been collected and conserved in Vietnam. The majority of this germplasm were derived from the IRRDB'81 germplasm collected in the Amazonian habitats of the genus. This collection has been considered as the key factor contributing to the improvement of rubber tree through breeding programs. The characterization and evaluation of the germplasm are considered to be important aspects of *Hevea* germplasm conservation. Without proper characterization and evaluation, valuable genetic variation in the collections cannot be used for rubber improvement effectively. Understanding the genetic diversity of different genetic resources of *H. brasiliensis* would be important in `order to optimize their management and to utilize

The genus *Hevea* is basically composed of 10 species: *H. brasiliensis*, *H. guianensis, H. benthamiana*, *H. pauciflora*, *H. spruceana*, *H. microphylla*, *H. rigidifolia*, *H. nitida*, *H. camporum*, and *H. camargoana* (Schultes, 1990). According to Clement-Demange et al. (2000), it is generally considered that there is no biological barrier between them, and some species proved to be inter-crossable by hand-pollination; therefore, the *Hevea* species can be considered as a species complex. *H. paludosa* has been identified in Brazil by Ule in 1905 as the eleventh species (Gonçalves et al., 1990; Priyadarshan and Gonçaalves, 2002). A review on the elaborate description of taxonomical and botanical aspects of *Hevea* has been reported by Schultes (1977, 1987, 1990) and Wycherley (1992). As its natural habitat, *Hevea* species have presented in Brazil, Bolivia, Peru, Ecuador, Colombia, Venezuela, Surinam and French Guiana as shown in Figure 1. These *Hevea* species have 2n = 36 chromosomes, with the possible exception of one triploid clone of *H. guianensis* (2n = 54) and the possible existence of one genotype of *H. pauciflora* with 2n = 18 (Baldwin, 1947; Majumder, 1964), and *H. brasiliensis* behaves as an amphidiploid (Ramaer, 1935; Ong,

It is known that all high-yielding cultivars of rubber tree (*Hevea brasiliensis* Muell. Arg.) in the world originated from breeding programs initially developed in Southeast Asia with a very narrow genetic base. Historically, the introduction of the rubber trees into Asia began with the transfer of 70,000 seeds to England by Henry Wickham in 1876 (Wycherly, 1968). During the years of 1876-1877, a total of 2,397 *Hevea* seedlings were sent to several Asian countries such as Sri Lanka (1,919 seedlings), Bogor (Java) (18 seedlings) and Singapore (22 seedlings) (Wycherly, 1968; Dean, 1987; Baulkwill, 1989). Because of the centrally geographical position of Singapore in Asia and the influences of the British to the development of the rubber industry during these years, the collection of 22 seedlings planted in the Singaporean botanical garden became the main source of the rubber trees which was distributed to and planted in Asian countries later. Since this introduction, the rubber tree has become an important perennial crop as the major source of commercial rubber in the world (Fig. 2) and this collection was mentioned as Wickham collection (W). Since the current *Hevea* varieties all came from such a single population, it is necessary to

In order to enlarge the genetic basis for *Hevea*, a large collection of *H. brasiliensis* accessions from various areas in Colombia was gathered by Schultes after 1945 and then duplicated in

*Hevea* germplasm in breeding programs.

**2.** *Hevea* **genetic resources** 

1975; Wycherley, 1976).

enlarge the genetic basis for *Hevea* breeding program.

Ivory Coast (Nicolas, 1985). In addition, a collecting survey in the Madre de Dios basin in Peru was organized in 1948 by the Peruvian Ministry of Agriculture (Rands and Polhamus, 1955); the resulting seedlings were multiplied by grafting, and then introduced and studied in Liberia, Guatemala and Brazil under the name of MDF accessions (Bos and McIndoe, 1965). Moreover, another collecting expedition was carried out in the Brazilian states of Acre and Rondonia by both the Brazilian Agricultural Research Corporation (EMBRAPA) and French Institute for Rubber Research (IRCA) in 1974 (Hallé and Combe, 1974; Seguin et al., 2003); 42 resulting wild elite-tapped trees were collected and propagated by grafting under the name as EMBRAPA/IRCA accessions. Remarkably, in 1981, the International Rubber Research and Development Board (IRRDB) conducted an expedition covering three western states of Brazil, namely Acre (AC), Rondonia (RO), and Mato Grosso (MT), in 16 different districts and in 60 different locations overall to collect wild *Hevea* germplasm (refered as IRRDB'81 collection). As a result, a total of 63,768 seeds, 1,413 meters of budwood from 194 high yielding trees and 1,160 seedlings were collected (Tan, 1987; Simmonds, 1989; Onokpise, 2004). Of these, 12.5% and 37.5% of the seeds were sent to Malaysia and Ivory Coast, respectively, and the remaining 50% of the collections were maintained in Brazil (Clément-Demange et al., 2007). The genotypes issued from budwood collection were also then brought to Malaysia and Ivory Coast. The collection planted in Malaysia and Ivory Coast has then been distributed to all IRRDB members as clones since 1984.

Fig. 2. Distribution of rubber tree (*H. brasiliensis*) in the world where this species has been planted (adapted from Orwa et al. (2009)). Native range: Bolivia, Brazil, Colombia, Peru and Venezuela; Exotic range: Brunei, Cambodia, China, Ethiopia, India, Indonesia, Laos, Liberia, Malaysia, Myanmar, Philippines, Singapore, Sri Lanka, Thailand, Uganda, Vietnam, Guatemala, Cameroon, Ivory Coast, Ghana, Gabon, Guinea, Liberia, Nigeria, Congo, Bangladesh, Papua New Guinea, and Mexico.
