Sunday, March 02, 2008

Basic Concepts of Selection in Plant Breeding


Generally breeders in two ways can change variability in their working population. Firstly by allowing individuals to produce next generation within limits of breeders preference or by natural choice in which which some of these individuals may be allowed to produce many offsprings and some of which may be allowed to produce which few or no offsprings. This non-random reproduction in a breeding population is called selection. The two agencies involved in the selection, one is by the natural choice is called natural selection and the other that is done by the breeder is the artificial selection. Secondly, breeders can decide how the selected individuals will be mated to each other either by inbreeding and cross-breeding.

Following basic principles operates on selection:

1. Selection works only on heritable variation
Variation that is governed by genetic makeup of the individual alone can be recovered in the progenies, not the variation that is environmentally dependent.

2. Selection works only in existing variability ie., it cannot create new variation. 
Hence, selection can only shuffle the variation and also can combine variations of different individuals of the population, which is however, pre-exist within the population itself.

3. Selection works by providing preference to certain individuals to reproduce, therefore certain set of genes get preferential inheritance at the expense of the unselected genes. Asa consequence gene and genotypic frequency  get altered. 

4. Continuous selection leads to loss of variability, because unselected genes get eliminated in the course of generation advancement.

5. When a new variability is created in a population either by mutation or by movement of genes from outside (from another unrelated population), new selection pressure is generated on the introduced genes.

6. Purpose and consequence of selection varies from type of selection, individual genes, mating methods etc.

Plant breeding employs artificial selection to generate plant populations that are cultivated for the benefit of the mankind. Artificial selection is fundamentally aimed at the following facts:

1. To change the phenotype of the selected individual in the most desired way to the man
2. To bring down the variability in the selected population
3. To increase uniformity in the selected population
4. To increase number of populations with different level of variability between them
5. To increase the range of uniform populations

Various artificial selection methods used in plant breeding are, 

In self pollinated crops,
1. Mass selection
2. Pureline selection
3. Pedigree selection

In cross pollinated crops,
1. Mass selection
2. Progeny selection (ear to row methods, modified ear to row methods etc)
3. Family selection (half sib, full sib family selection and their modifications)
4. Recurrent selection

Clonally propagated crops,  
1. Clonal selection.

Thursday, January 10, 2008

Crop improvement in Para rubber tree (Hevea brasiliensis)


Para rubber tree (Hevea brasiliensis) belong to Euphorbiaceae. It is the major source of natural rubber in the world. The genus Hevea occurs naturally through out the Amazon basin and in parts of Matto Grosso, Upper Orinoco and the Guianas (Schultes, 1970). Hevea brasiliensis (2n = 36) is a monoecious perennial tree belonging to Euphorbiaceae. There are ten species recognised under this genus, H. brasiliensis, H. benthamiana, H. camporum, H. camargoana, H. nitida, H. pauciflora, H. guianensis, H. microphylla, H. spruceana and H. rigidifolia. These species are highly cross-pollinated and putative hybrids of natural intercrossing between the species occur in the natural habitat. Experimental crosses between species show no hybridisation barriers (Webster and Paardekooper, 1989).

Genetic resources

Though there are few reports of introduction of rubber plants to other areas of the world, all of them lack evidence to substantiate such claims. Recorded history of introduction the rubber from its natural habitat starts with the collection of rubber seeds by H.A. Wickham near Santarem during 1872. In 1876, Wickham collected about 70,000 seeds of H. brasiliensis from near Boim on the Rio Tapajoz and from the well-drained undulating areas near Rio Madeira and despatched them to Royal Botanic Gardens at Kew, England. About 2800 seedlings raised at Kew, 2397 were despatched to Sri Lanka and few to Malaysia, Singapore and Indonesia. Virtually all the rubber trees cultivated in Asian countries originated from this collection.

After Wickham, there were several attempts to introduce H. brasiliensis to Asian countries, but these introductions were confined to very few in number did not contribute much to the introduced genetic base. Later wild species and few cultivated cloned were introduced to Rubber Research Institute of Malaysia from Brazil. Besides few other organised attempts by International Rubber Research And Development Board (IRRDB), a major collection of wild Hevea Germplasm was carried out in 1981 by IRRDB team of scientists from major member countries, in collaboration with Brazil collected 64736 seeds from the states of Acre, Rondonia and Mato Grosso and budwoods from 194 high yielding trees free of major diseases like abnormal leaf fall (Phytophthora sp.) and South American leaf blight (SALB) caused by Microcyclus ulei (Ong et al., 1983). Of these collections, India introduced about 9000 accessions from Malaysia, of which about 6000 are surviving and being conserved in ex situ gardens. Besides, India also introduced 127 commercial clones from other countries where Hevea is grown on commercial scale.

Apart from H. brasiliensis, SALB affects only three more species, H. benthamiana, H. guianensis and H. spruceana (Langford, 1945; Chee and Holliday, 1986). Others species are free from infection. Eleven physiological races are reported to be identified for this disease. Though H. brasiliensis is totally affected by this disease immune reactions are shown by some H. benthamiana, H. spruceana and H. pauciflora derivatives. Sporadic attempts to use these immunities, by crossing with H. brasiliensis had produced clones with transient and non-durable resistance (Simmonds, 1989). Evaluation of wild Hevea germplasm for sources for biotic and abiotic stresses are in progress at major rubber growing countries.

Crop improvement

The rubber tree is introduced into cultivation very recently. The genetic base of the cultivated germplasm is very narrow, converging to a very few seeds collected from upper Amazon basin near river Tapajos in Brazil. Though rubber tree is infected by many diseases, the plantation industry suffers mainly from only one really devastating pathogen causing South American leaf blight (SALB). This disease in still confined to American sub-continent and all the rubber growing areas of South and Southeast Asia are free from it. However, it is already proved that none of the cultivated varieties in Asia are resistant to this disease when tested at various locations of South America. However, in recent years, another disease Corynespora leaf fall (CLF) has gained importance for wiping out entire RRIC 103 plantations of Sri Lanka, which had almost gained its name SALB of Asia.

Clonal selection is the most important procedure followed in breeding rubber. Clones can be evolved at any stages of different breeding steps. Usually selective hybridisation of promising parents is done among themselves and also with wild germplasm lines. The progenies are directly selected from seedling nurseries and cloned for further evaluation. Also, natural seedling population or half-sib population are also screened for desirable characters including resistance. Susceptible and poor performing clones are generally discarded. Polycross gardens comprising of pre-potent clones are also utilised and the selection is generally exercised in the polyclonal seedling orchards, even at the stage of maturity. The selections are directly carried forward for clonal evaluation and selection. A general scheme of rubber improvement is provided in Fig.

A general scheme of Hevea breeding

The major thrust of quality improvement in Hevea does not orient to the quality of rubber, but on the quality of secondary products like Hevea wood. Rubber trees produce enormous quantity of semi-hardwood at every replanting cycle. The shrinking availability of natural timber from the forests has made this so valuable. The wood on appropriate chemical treatment could be used as a best substitute for timber for furniture making and for similar uses. The treated wood is now being used to produce very high quality furniture, panel boards, house-hold articles and for flooring purpose.

Owing to the growing importance of rubber wood, the improvement in the direction of developing timber – latex clones is in progress. The clones combining better yield, high vigorous growth, short life span, high quality strong wood, free from diseases and with good branching habits are preferred.
Rubber tree is a prolific producer of honey. Honey is produced on extrafloral nectaries located on the Hevea leaves. High honey production can give additional income to plantation sector, and population that yield more honey are preferred.

Though rubber products are being used for so many decades, recently, the problem of latex protein allergy has emerged mainly in America. The allergy is reported to be caused by the proteins present in the latex, which are found in traces in the finished products. However, the allergy reported is mainly of type IV allergy of cutaneous origin. There are several processing methods available for the deproteinization of the rubber products. However, it would be better to look out for genotypes, which do not accumulate harmful proteins in the latex.

Biotechnology rubber crop improvement

The one area of biotechnology with clear applications in rubber is that of molecular markers. Marker-assisted selection offers prospects of accelerating the process of long term breeding objectives offered by the conventional approaches. An important first step towards developing linked markers is the construction of a linkage map. Maps have been published for rubber (Seguin et al, 1996). A map allows the selection of markers which are evenly distributed over the genome, thus enhancing the probability of finding markers linked to quantitative trait loci (QTL). Molecular markers that are linked to these QTL will co-segregate with the genes involved in desirable traits and could be used efficiently to follow introgressions and accumulation of favourable traits during recombination cycles.

Besides, interfering with the biochemical pathway are also being contemplated, in producing many biomolecules including antibiotics. However, the lesson is that a single, apparently simple, change in a synthetic pathway may have unexpected side effects. Successful transformation programmes have been those where the transformation work is integrated into a conventional breeding programme, allowing individuals with the required phenotype to be developed from a range of transformants.


Chee, K.H. and Holliday, P. (1986) South American leaf blight of Hevea rubber. Malaysian Rubber Research and Development Board, Kuala Lumpur. 50p.
Langford, M.H. (1945) ) South American leaf blight of Hevea rubber trees. USDA Technical Bulletin, 882: 31.
Ong,S.H., Ghani, M.N.A. and Tan, H. 1983. New Hevea germplasm: Its introduction and potential. Proceedings of the RRIM Planters Conference, Kuala Lumpur. pp. 3-17.
Seguin, M., Besse, P., Lespinasse, D., Lebrun, P., Rodier-Goud, M., and Nicolas, D., 1996. Hevea molecular genetics. Plantations, Recherche, Développement 3(2): 77-87 
Shultes RE (1970) The history of taxonomic studies in Hevea. Bot. Rev., 36: 197-276.
Simmonds, N.W. 1989. Rubber Breeding. In: Rubber (C.C. Webster and W.J. Baulkwill, eds). Longman Scientific and Technical, U.K. pp. 85-124.
Webster,C.C and Paardekooper. 1989. Botany of the rubber tree In: Rubber (C.C. Webster and W.J. Baulkwill, eds). Longman Scientific and Technical, U.K.