Refinements to Hevea rubber breeding

Abstract

Hevea brasiliensis (Willd. Ex. A. de. Juss. Muell-Arg.) is the prime source of natural rubber. Domestication of rubber began since 1876 with Wickham collecting 70,000 seeds from Upper Amazon and transported them to Kew Botanic Gardens. Somehow, rubber trees covering millions of hectares are believed to be derived from “22 seedlings” of Wickham’s original stock. Improving dry rubber yield is the exclusive and ultimate objective of Hevea breeding with consistent yield of 70 to 80 g/tree/tapping. Ultimately, in a small holding, a planter must gain an average yield of around 2200 to 2400 kg/ha from his stand (under optimal conditions), after accommodating tree-to-tree variations due to stock-scion interactions and soil heterogeneity. This is arduous, but achievable. Initial production of high-yielding clones gave 1600 kg/ha against 496 kg/ha of unselected seedlings. Adaptation and yielding potential of clones to specific environments are optimized through localized experimentation. Studies on adaptation of clones to new environments, especially to sub-optimal or marginal areas, are gaining momentum. As this extension happens, demand for new clones is on the rise. Possibilities of using rubber trees for reforestation, carbon sequestration and application of genomics in deriving climate resilient clones may come up in future, which breeders may have to take up with required priority. Five major methodologies followed are (a) primary clones and seed gardens, (b) derivation of recombinants and clone selection, (c) genetic analysis and variability management (d) early selection and estimation of genetic value and (e) application of genomics. Primary clones have immensely contributed in exploiting heterosis and production of new clones. For evaluation of recombinants, families are to be raised in closer spacing (2 or 3 m) and allowed to attain tappable girth for evaluation. While a normal breeding cycle takes 35 to 40 years, through skipping SSCTs and LSCTs, the scheme proposed can derive a clone in 17 years. Recent advances like transcriptome sequencing of bark and EST sequences generated from suppression subtractive hybridization-cDNA libraries could facilitate marker-assisted selection that could very well be used for selecting high-yielding genotypes at juvenile stage. Paternity identification can be done through breeding without breeding (BwB) in half-sibs and poly-cross (open pollinated) seedlings. Transcriptome studies have to come a long way to yield meaningful results to tag vivid genes responsible for QTLs, resistance and other quality traits, especially markers for cold/drought stress. It is opined that instead of subjecting plants for artificial cold/drought conditions, plants continuously exposed to such stress conditions must be used for analyses that can give a comprehensive indication of stress tolerance. The application of genotyping-by-sequencing (GBS) technique to simultaneously discover and delineate single nucleotide polymorphism (SNP) markers is a robust and cost-effective approach for generating a common set of genome-wide SNP data suitable for constructing integrated linkage maps from multiple populations. Studies on mitochondrial and chloroplast DNAs are welcome steps towards understanding ATP efficiency of accessions that need to be augmented further, so that clones with higher ATP efficiency can be used for breeding. Such innovative techniques shall govern breeding Hevea rubber in the future, only when breeders and genomic specialists are working in tandem.