Identification of simple sequence repeat markers for sweetpotato weevil resistance

The development of sweetpotato [Ipomoea batatas (L.) Lam] germplasm with resistance to sweetpotato weevil (SPW) requires an understanding of the biochemical and genetic mechanisms of resistance to optimize crop resistance. The African sweetpotato landrace, ‘New Kawogo’, was reported to be moderately resistant to two species of SPW, Cylas puncticollis and Cylas brunneus. Resistance has been associated with the presence of hydroxycinnamic acids esters (HCAs), but the underlying genetic basis remains unknown. To determine the genetic basis of this resistance, a bi-parental sweetpotato population from a cross between the moderately resistant, white-fleshed ‘New Kawogo’ and the highly susceptible, orange-fleshed North American variety ‘Beauregard’ was evaluated for SPW resistance and genotyped with simple sequence repeat (SSR) markers to identify weevil resistance loci. SPW resistance was measured on the basis of field storage root SPW damage severity and total HCA ester concentrations. Moderate broad sense heritability (H2 = 0.49) was observed for weevil resistance in the population. Mean genotype SPW severity scores ranged from 1.0 to 9.0 and 25 progeny exhibited transgressive segregation for SPW resistance. Mean genotype total HCA ester concentrations were significantly different (P < 0.0001). A weak but significant correlation (r = 0.103, P = 0.015) was observed between total HCA ester concentration and SPW severity. A total of five and seven SSR markers were associated with field SPW severity and total HCA ester concentration, respectively. Markers IBS11, IbE5 and IbJ544b showed significant association with both field and HCA-based resistance, representing potential markers for the development of SPW resistant sweetpotato cultivars

Phytochemical mediated resistance in sweetpotato to sweetpotato weevils

This study evaluated sweetpotato resistance to sweet potato weevils (Cylas spp.) and investigated the phytochemicals that mediated this defence. New Kawogo, LIR302 and ARA228 were shown to be the most resistant based on stem and root damage. Significant differences were observed on the number of faecal droppings, feeding holes and eggs laid on the root in a choice and no-choice bioassays confirming that New Kawogo, LIR302 and ARA228 affected development and emergence of adult weevils. Six hydroxycinnamic acid esters, including hexadecylcaffeic, hexadecylcoumaric, octadecylcaffeic, octadecylcoumaric, heptadecylcaffeic and 5-O-caffeoylquinic acid esters were identified on the root surface and amounts of these differed significantly between resistant and susceptible varieties. The mean number of C. puncticollis and C. brunneus feeding holes, faecal droppings and egg laid on the root core were significantly different among the root cores treated with synthetic hydroxycinnamic acid esters. The study also showed that there were significant differences in the root volatiles of resistant and susceptible variety both before and after infestation. The larval survival of sweetpotato weevil was significantly affected by hydroxycinnamic acid esters treatment and Bt-toxin applied on the diet. There was also significant differences in percentage sweetpotato weevil root infestation among genotypes of the segregating population. The genotype by environment (GxE) interaction effect was also significant on the sweetpotato weevil damage on the stem portion of the sweetpotato vine indicating that weevil stem damage is dependent on the season. The mean number of sweetpotato weevil feeding holes differed significantly on the root of the genotypes of the segregating population in the feeding and oviposition bioassay. There was significant difference in total hydroxycinnamic acid (HCA) esters among the genotypes of the segregating population. The distribution of genotype mean total HCA ester concentration was skewed to the left and only one progeny, NKB257, had higher total HCA ester concentration than New Kawogo, the resistant mother used in the crossing. A weak but significant correlation between total HCA ester concentration and sweetpotato weevil root damage was observed signifying that resistance to sweetpotato weevils depended on other factors as well. The results are discussed in terms of how they might be incorporated into integrated pest management of sweetpotato weevils