Pepper (Capsicum) production is constrained by heavy infestations of thrips, causing direct and indirect (by transmitting viruses) damage. Thrips control using chemical insecticides, biological agents, culture practices and integrated pest management has limited success. The availability of thrips-resistant varieties would increase the effectiveness of thrips control and may also delay and reduce the transmission of viruses. This thesis is aimed at obtaining more knowledge regarding thrips resistance in pepper, including the identification of new sources of resistance, the elucidation of resistance mechanisms, identification of factors contributing to resistance and a QTL analysis.
We developed several test methods to evaluate plant resistance to thrips and showed that in vitro tests correlate well with greenhouse tests. We used these methods to test a collection of Capsicum accessions of widely different origin and crop types. This resulted in the identification of a few accessions (mostly C. annuum) with high levels of resistance to two thrips species: Frankliniella occidentalis and Thrips parvispinus. Since C. annuum is the most widely cultivated species, the finding of resistance in C. annuum is means that the resistance can be easily introgressed through conventional crossing and selection.
The effect of resistance in pepper on thrips reproduction and development was studied using three highly resistant, three medium resistant and three susceptible accessions selected based on damage ratings. Adult and pre-adult survival, developmental time and reproduction rate were assessed in a detached leaf system. Resistance factors in leaves of resistant pepper accessions were shown to have significant effects on oviposition rate, larval mortality and life-cycle period, indicating that this resistance is based on antibiosis.
In order to map QTL for resistance we developed an F2 population from the cross between a susceptible C. chinense accession and the resistant C. annuum AC 1979. A genetic linkage map for this population was based on AFLP and SSR markers, where the SSR markers served to assign and orient most linkage groups to pepper chromosomes. As larval stages were highly affected by resistance in pepper leaves, damage caused by larvae and larval survival were used as parameters to detect QTLs conferring resistance to thrips. Interval mapping detected one QTL for each of these parameters, all co-localizing near the same marker on chromosome 6. This QTL explained about 50% of the genetic variation, and the resistance allele of this QTL was inherited from the resistant parent. No other resistance QTLs were detected in this population.
Since resistance to thrips was clearly expressed in pepper leaves we proceeded to study leaf traits that may contribute to resistance. Morphological leaf characters and metabolites have frequently been linked with resistance to thrips in other plant species. However, we found no convincing evidence that any of these traits played a role in thrips resistance in pepper. In the F2 mapping population we found no correlation and no QTL co-localization of resistance with leaf morphological characters previously linked to resistance in pepper against insect pest and in other plant species against thrips e.g. color, toughness, trichome density, and cuticula thickness. GC-MS (Gass Chromatography – Mass Spectrometry) analysis of the three resistant, three intermediate and three susceptible accessions mentioned above showed that seven metabolites were correlated with resistance to thrips and six compounds with susceptibility. However, when we applied GC-MS and LC-MS (Liquid Chromatography – Mass Spectrometry) to leaves of the F2 mapping population, we found no strong correlation between resistance and any detected metabolites. Two metabolite QTLs co-localized with the resistance QTL. However, these QTLs explained only a small proportion of the variance and the co-localization was not supported by strong correlations of the metabolites with resistance. This suggests that the major resistance factor(s) in pepper against thrips may not or only partially be determined by the presence or absence of specific metabolites.
This thesis provides a strong basis for the development of thrips resistant pepper varieties through introgression of the resistance QTL region on chromosome 6 originating from resistant C. annuum accessions. However, the effect of resistance QTL on chromosome 6 should be confirmed in another population such as a population of F3 lines. In vitro leaf assay can be used as evaluation methods in pepper breeding program. This has the advantages of minimizing the risk of contamination and of controlled environmental conditions. Elucidation of factors contributing to resistance should be continued by giving attention to other possibilities such as proteins, specifically proteinase inhibitors, or other leaf anatomical and morphological traits. Also other extraction and detection methods may be used to discover other metabolites that might be related to resistance. Finally, for practical applications it is necessary study how to use the antibiosis based mechanism against thrips found in this thesis in thrips control and/or management practices.
|Qualification||Doctor of Philosophy|
|Award date||18 Nov 2013|
|Place of Publication||Wageningen|
|Publication status||Published - 2013|
- capsicum annuum
- capsicum chinense
- sweet peppers
- insect pests
- disease vectors
- frankliniella occidentalis
- pest resistance
- quantitative trait loci
- interspecific hybridization
- plant breeding