G. R. Stringam

Identification and evaluation of root maggot (Delia spp.) (Diptera: Anthomyiidae) resistance within Brassicaceae

Abstract Current varieties of canola/oilseed rape, Brassica napus L. and Brassica rapa L., are susceptible to infestation by the root maggots Delia radicum (L.) and Delia floralis (Fallen) (Diptera: Anthomyiidae) in western Canada. Crop losses from root maggot infestations can be very significant, and infestation levels have increased substantially from 1983 to present. Although cultural and chemical strategies exist to reduce crop losses caused by root maggots, these methods are either not sufficiently effective or pose unacceptable environmental risk. This paper reports progress achieved to develop canola cultivars resistant to attack by root maggots as the most environmentally sustainable strategy for management of these pests. Twelve species of Brassicaceae, and many genotypes within species, were evaluated for levels of root maggot resistance, in addition to many intergeneric hybrids produced by crosses of Sinapis alba × B. napus . Sinapis alba had greatest resistance to infestation by root maggots among the Brassicaceae species evaluated, and five intergeneric hybrids had levels of resistance to root maggots that were similar to S . alba and were maintained from year to year. Low amounts of S . alba DNA were detected in the hybrids using genomic slot-blot analysis. These results indicate that a genetic basis for resistance to root maggot infestation exists in the Brassicaceae, and our research represents the first step toward introgression of genetic sources of root maggot resistance from related species into commercial canola varieties.

Molecular markers for yield components in Brassica juncea – do these assist in breeding for high seed yield?

A population of 112 F1-derived doubled haploid lines was produced from a reciprocal cross of Brassica juncea. The parents differed for seed quality, seed color and many agronomic traits. A detailed RFLP linkage map of this population, comprising 316 loci, had been constructed, and was used to map quantitative trait loci (QTL) for seed yield and yield components, viz. siliqua length, number of seeds per siliqua, number of siliques per main raceme and 1000-seed weight. Stable and significant QTLs were identified for all these yield components except seed yield. For yield components, a selection index based on combined phenotypic and molecular data (QTL effects) could double up the efficiency of selection compared to the expected genetic advance by phenotypic selection. Selection indices for high seed yield, based on the phenotypic data of yield and yield components, could only improve the efficiency of selection by 4% of the genetic advance that can be expected from direct phenotypic selection for yield alone. Inclusion of molecular data together with the phenotypic data of yield components in the selection indices did not improve the efficiency of selection for higher seed yield. This is probably due to often negative relationships among the yield components. Most of the QTLs for yield components were compensating each other, probably due to linkage, pleiotropy or developmentally induced relationships among them. The breeding strategy for B. juncea and challenges to marker assisted selection are discussed.

Quantitative trait loci for early maturity and their potential in breeding for earliness in Brassica juncea

A doubled haploid population of Brassica juncea, developed from a cross between two parental lines differing for days to maturity, was used to study the efficiency of indirect selection for a primary trait through selection of secondary trait(s) over direct selection for the primary trait when quantitative trait loci information is available for both primary and secondary traits, and applied. Days to maturity was considered as primary trait, while days to first flowering, days to end of flowering, flowering period and plant height were considered as secondary traits. An RFLP linkage map was employed for QTL analysis of maturity and maturity-determinant traits, and a stable QTL B6 simultaneously affecting these two types of traits was identified. This linked QTL explained 11.7% phenotypic variation for days to maturity, 20.7% variation for days to first flowering, 24.3% variation for days to end of flowering and 14.4% variation for plant height. Phenotypic evaluation of maturity and/or maturity-determinant traits, viz. days to first flowering, days to end of flowering and plant height revealed that limited genetic advance for early maturity can be achieved through phenotypic selection of the primary and/or the secondary trait(s). However, the estimates of genetic advance for early maturity based on combined phenotypic evaluation and linked QTL data was found to be, at least, three times higher compared to genetic advance based on phenotypic evaluation only, demonstrating the potential of marker-assisted selection in breeding for early maturity in B. juncea.

Screening of Brassica napus against blackleg caused by Leptosphaeria maculans: effects of inoculum concentration, subculturing of the pathogen, and time of disease screening

The effects of inoculum concentration, subculturing of the pathogen, and time of disease screening on the development of blackleg disease caused by Leptosphaeria maculans were studied using the Brassica napus canola cultivars Quantum (resistant) and Profit (susceptible). Disease development was not affected by inoculum concentration in the range of 5 × 105 - 4 × 106 pycnidiospores/mL or subculturing of stock cultures of L. maculans up to 71 times on V8 juice rose Bengal agar. Disease development, however, showed seasonal variation in the greenhouse. While the disease severity values for the resistant cultivar Quantum increased during the summer, those for the susceptible cultivar Profit remained consistent throughout the year.

Detecting and estimating segregation distortion and linkage between glufosinate tolerance and blackleg resistance in Brassica napus L.

SummaryThe segregation and linkage between glufosinate (transgenes ‘Rf3’ and ‘T177’) and blackleg resistance genes in canola (Brassica napus L.) were assessed using F1 microspore-derived doubled haploid (DH) populations from four crosses including reciprocals, two involving the transgene ‘Rf3’ and the other two involving the transgene ‘T177’. To relax the assumption of no segregation distortion required for the conventional analysis of segregation and linkage, we employed Baileys analysis that allows detecting segregation distortion at linked loci. The significant departures from the 1:1 segregation were detected in the crosses involving the transgene ‘T177’ but not in the crosses involving the transgene ‘Rf3’. The apparent deficit of the herbicide tolerant DH lines in the crosses with the transgene ‘T177’ is likely due to differential selection against the gametes carrying ‘T177’ during microspore culture. The linkage was strong between blackleg resistance and the transgene ‘Rf3’ but weak or absent between blackleg resistance and the transgene ‘T177’, suggesting that the two transgenes are probably inserted into distant regions of the genome. The observed linkage offers an opportunity to develop new canola cultivars with both glufosinate tolerance conferred by transgene ‘Rf3’ and blackleg resistance.