Effects of inbreeding and estimates of additive genetic variance within seven summer oilseed rape cultivars

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Abstract
Seven groups of inbred lines derived from the summer oilseed rape (Brassica napus ssp. oleifera) cultivars 'Westar', 'Regent', 'Lergo', 'Marnoo', 'Ariel', 'Karat', and 'R83-11' were evaluated at three sites over 2 years, for agronomic characters and seed oil and protein concentrations. Estimates of additive genetic variance and heritability were calculated. Comparison of the mean of inbred lines with the mean of their respective source cultivars indicated little, if any, inbreeding depression in all cultivars except 'Ariel'. It was concluded that dominance is not a major factor in the genetic determination of the characters studied. For every trait in every cultivar, individual inbred lines were identified that exceeded or equaled their respective source cultivar means, indicating that neither heterozygosity per se nor genetic heterogeneity is required to maintain maximum performance in summer oilseed rape. Therefore, the oilseed rape cultivars used in this study were considered to be genetically heterogeneous populations, consisting of some number of generally homozygous lines. Significant additive genetic variance was detected for most traits in most cultivars, indicating that superior inbred lines could be extracted from within cultivars.Key words: inbreeding depression, heritability, Brassica napus L.