Genetic parameters and quantitative trait loci mapping in tea, Camellia sinensis (l.) O. Kuntze

Abstract

Efficient breeding and selection of elite tea (Camellia sinensis (L.) O. Kuntze) clones require sound knowledge and understanding of genetics associated with yield, quality and tolerance to biotic and abiotic stresses. A series of experiments were carried out to elucidate the genetic parameters and map quantitative trait loci (QTL) for agronomic and quality attributes in tea. The combining abilities and heterosis for yield, drought tolerance (DT), and quality traits (percent total polyphenols (%TP), fermentability (FERM), theaflavins (TF), thearubigins (TR) and pubescence (PUB)) in tea were estimated in a 4 x 4 full diallel analysis. Generally, parents with good combining ability produced progeny with above average performance for all the evaluated traits. The general combining ability (GCA) effects were significant for yield, %TP, FERM, DT, TF, TR, PUB and bud weight, while specific combining ability (SCA) effects were significant for %TP, FERM, PUB and bud weight. Strong maternal effect for all traits was evident except for thearubigins and bud weight signifying the importance of female parents in breeding for yield, DT, and diversified tea products such as silvery tips. The heterosis analysis revealed that the mid-parent heterosis (MPH) and the betterparent heterosis (BPH) averaged across the families, were high for fermentability (MPH: 108.5%; BPH: -12.82) and DT (MPH: 16.44 %; BPH: 2.30%) but not so for yield (MPH: 2.17%; BPH: -5.09%) and total polyphenols (MPH: -2.81%; BPH: -5.71%). The heritability estimates for yield (h2 = 0. 44 ± 0.16; H2 = 0.56 ± 0.15), DT (h2 = 0.61 ± 0.06; H2 = 0.96 ± 0.01) and fermentability (h2 = 0.45 ± 0.04; H2 = 0.98 ± 0.01) were high indicating that the three traits are highly heritable and could be improved through hybridization and judicious clonal selection. Estimates of genetic gains indicated that moderate gains are achievable on clonal selection for characters such as yield and fermentability at 14.1% and 8.9%, respectively, while DT and PUB registered higher gains at 20.6% and 30%, respectively, based on h2. Mapping of QTL linked to yield, %TP, DT and shoot traits was assessed on a pseudotest cross comprising 42 clonal progeny between clones TRFCA SFS150 and AHP S15/10. QTL associated with root knot nematode resistance were investigated on a different cross consisting of 41 clonal progeny arising from TRFCA SFS150 and TRFK 303/577. Bulk segregant analysis was performed followed by complete genotyping. Out of 260 informative markers, 100 markers that showed 1:1 segregation were used to construct a linkage map. The map contained 30 (19 maternal and 11 paternal) linkage groups that spanned 1411.5 cM with mean interval of 14.7 cM between loci. A total of 64 QTLs controlling various traits across the two sites were detected. Of these, QTLs linked to YLD-T, YLD-K, DT-K and PUB were localised at 2 cM, 2.7 cM, 3 cM and 1.4 cM from markers OPG-07-2800, E-AGC/M-CAG- 725, OPT-18-2500 and OPO-02-650, respectively. No QTL was detected at both sites, which showed strong genotype x site interaction (G x E). Marker OPF-09-600 that co-segregated (P < 0.0000) with susceptibility to the root knot nematode was mapped at 0 cM from locus OPF- 09-600 on linkage group 8. Considering the long time expended in developing improved tea varieties, the identification of putative QTLs tightly linked to agronomic traits augmented by known genetic parameters provide room for marker-assisted selection thereby hastening tea improvement efforts.