Genetic variation and inheritance of seed fibre content in winter oilseed rape (Brassica napus L.)

The meal from oilseed rape is a valuable product as substitution for soybean meal in animal diets due to its high protein content and its excellent balanced composition of essential amino acids. However, the presence of high fibre fraction in oilseed rape meal, which is mostly located in seed hull, is the major obstacle for its use in animal feeding. The fibre fraction is poorly digestible and essentially dilutes the available energy and protein. Consequently, oilseed rape meal has less metabolizable energy and reduces the value of the meal relatively to soybean meal. Fibre contents are related to plant cell walls components, namely cellulose, hemicellulose and lignin.  The proportion of hemicellulose, cellulose and lignin can be quantified as neutral detergent fibre (NDF), acid detergent fibre (ADF) and acid detergent lignin (ADL). The NDF fraction consists (predominantly) of hemicellulose, cellulose and lignin content, while the ADF fraction comprises cellulose and lignin. ADL represents the non-digestible lignin fraction.  To improve the quality of oilseed rape meal, breeding research has been undertaken to select genetic material with lower fibre content. Recently, selection for low fibre content has also been conducted in black-seeded B. napus material. Taking advantage of natural genetic variation for fibre content among black-seeded germplasm may complement ongoing approaches to reduce fibre content in oilseed rape meal as well as to increase oil and protein content in the seed.  Selection for low fibre content in canola quality winter oilseed rape cultivars is promising since the material is high yielding and adapted. The objectives of this study were: (i) to analyse genetic variation for fibre content and seed hull proportion in a set of European winter oilseed rape cultivars and to develop near-infrared reflectance spectroscopy (NIRS) calibrations for fibre content, (ii) to analyse genetic variation of fibre content and seed hull proportion, to investigate correlations between fibre content and other quality traits, and to identify quantitative trait loci (QTL) associated with fibre content by linkage analysis in three doubled haploid populations of black-seeded winter oilseed rape; (iii) to analyse genetic variation and genotype x environment interactions for fibre content and seed hull proportion and to conduct association analysis in a larger set of canola quality winter oilseed rape cultivars.  The results of the present study revealed that the effect of genotype on fibre content and seed hull proportion was significant in all populations as well as the effect of environment.  The heritabilities for NDF, ADF, ADL and seed hull proportion ranged from 0.78 to 0.93, 0.75 to 0.96, 0.41 to 0.98 and 0.56 to 0.91, respectively.Different types of relationships between seed oil, protein of defatted meal and fibre content of defatted meal have been observed in winter oilseed rape cultivars and DH populations. A set of 28 European winter oilseed rape cultivars and DH population SG DH14 x Express 617  showed positive correlation between seed oil and protein content of defatted meal and negative correlation between seed oil and fibre content of defatted meal. For breeding of improved meal quality, the type of relationship between oil, protein and fibre content found in a set of 28 European winter oilseed rape cultivars and DH population SG DH14 x Express 617 is preferred, because in these populations the increase of oil content occurred at the expense of fibre content instead of protein content. Negative correlations between protein content of the meal and fibre content found in all populations is expected because selection for low fibre content will lead to an increase of protein content in the meal.  Reducing seed hull proportion will give a benefit to increasing oil content because of its indirect effect in reducing the content of condensed tannins and increasing seed embryo proportion. In addition, it will simultaneously improve meal quality by reducing the lignin content. Thousand kernel weight is an important trait that should be considered in selection due to its indirect effect to lower fibre content by reducing seed hull proportion as shown by negative correlation between these two traits. Favorable QTL that reduced seed fibre content and seed hull proportion, and simultaneously increased seed oil content were detected in population SGDH 14 x Express 617 located on linkage group C05 at the interval 31.6 – 46.4 cM. QTL for fibre content in this interval showed overlapping confidence interval with the QTL for seed hull proportion with the same direction of additive effect, as well as with the QTL for seed oil content with an opposite direction of additive effect.  Association mapping performed in a set of 81 canola quality winter oilseed rape cultivars revealed 74 markers being associated with fibre-related traits representing 2 to 22 QTL for respective traits. QTL for NDF and hemicellulose content on linkage group C03 identified in association mapping were also mapped on the same location in DH population Express 617 x R53 suggesting that those might be the same QTL. The QTL located on linkage group C03 and C05 could be used in future breeding programs aimed at improving meal quality. Further study could be focused to verify the genes underlying QTL associated with fibre content detected in linkage group C03 and C05 in SG DH14 x Express 617 DH population.The NIRS calibrations developed specifically for predicting NDF, ADF and ADL content in black-seeded winter oilseed rape could be used for screening large numbers of genotypes in a fast, reliable, cost efficient and non-destructive way.  With relatively large genetic variation, medium to high heritabilities for fibre fractions, and the availability of NIRS calibration, selection for low fibre content in black-seeded winter oilseed rape could be performed in commercial breeding programs.