Molecular markers to devise predictive models for juvenile selection in Hevea rubber

Abstract

AbstractNatural rubber is extracted from the Hevea tree, through shaving the bark. As a major raw material source for many industries especially the tyre sector, Hevea rubber clones are intensively cultivated in the tropics. The world production of Hevea rubber is almost 13.64 million metric tonnes, and the productivity is around 1200 kg ha−1. However, the field production achieved so far is still well below the theoretical yield estimated (7000–12,000 kg ha−1). Even though this is a calculated estimate, efforts can be kept live to achieve this target, because Hevea rubber is being increasingly cultivated under several suboptimal environments after the 1980s. In this era of climate change, newer biotic and abiotic stresses encounter Hevea rubber culture that demands new clones at definite intervals. Hence, like any other species, Hevea rubber breeding is a continuous process in quest of new clones achieved through exploiting new sources of genetic variation. Hevea rubber breeding is to produce high‐yielding clones with good timber value. First priority is to explore and exploit genetic variation in clones, half‐sib, full‐sib and illegitimate (open‐pollinated) seedlings, where dry rubber yield and wood volume are the target traits. Next is to maximize genetic gains per unit time, through understanding juvenile–mature correlations and to practice selection of high‐yielding genotypes at the juvenile stage. Hevea rubber had a long breeding cycle of 33 years but could be reduced to 17 years after culling non‐essential steps of deriving clones. However, this could be reduced to 10 years, if a DNA marker‐assisted selection system is applied to seedlings. This system must also ensure significant juvenile–mature correlations on yield. This is a formidable task because under given circumstances, such a correlation is almost absent. However, this can be turned into a reality through employing yield‐related DNA markers with genomic selection (GS). The impact of DNA markers linked with REF, SRPP, acetyl‐CoA acetyltransferase, HMGS, HMGR and JAZ need to be specifically evaluated. GS that aims to improve quantitative traits by using genome‐wide marker data without requiring identification of markers associated with quantitative trait loci (QTLs) of interest can be used in Hevea also. Subsequently, by a predictive model, genomic estimated breeding values (GEBVs) can be calculated for untested individuals from a ‘testing population’. GS captures the total additive genetic variance with genome‐wide marker coverage and effect estimates. Therefore, selection of an individual without phenotypic data can be performed based on the individual's predicted breeding value. This review makes an assessment of the molecular markers in Hevea rubber and aims to streamline an efficient selection system to be applied onto seedlings through GS that owes a significant correlation with the mature phase.