Senuvr, J. L. eNo SrsraNssoN, B. R. 1983. Heterosis in summer rape (Brassica napus L.) Can. J. Plant The degree of heterosis for seed yield in F, hybrids of summer rape (Brassica napus L.) was examined in replicated yield trials during 2 yr (1980 and 1981) using intervarietal hybrids produced by manual crossing.

Development of cultivars of both tetraploid (Brassica napus L.) and diploid (B. rapa L.) rapeseed with reduced levels of polyunsaturated fatty acids and increased levels of oleic acid could increase both the utility and value of the oil. The objective of this experiment was to use chemcical mutagenesis to induce mutations for these traits in rapeseed. Imbibed seed of ‘R‐500’ (B. rapa) and ‘Cascade’ (B. napus) were treated with 5% v/v of ethyl methanesulfonate. The M2 generations of these populations were screened to identify mutants with low levels of polyunsaturated fatty acids (PUFA) using a modified thiobarbituric acid procedure. Putative mutants and their derived progeny were increased in the greenhouse and the seed analyzed for fatty acid composition using gas chromatography. The most promising mutant identified from 4 734 M2 seeds of B. rapa, was M‐30, which had 2.1% linoleic acid and 3.0% linolenic acid, vs. 11.9% linoleic and 8.6% linolenic acid in the original cultivar. This mutant was crossed with ‘Tobin’ to derive F4 lines having <6% total PUFA and oleic acid concentrations >87%. The most promising mutant identified from 39 504 M2 seeds of B. napus was X‐82, which had 6.6% PUFA, vs. 27.4% PUFA in Cascade. Several M3 and M4 lines derived from X‐82 had <6% total PUFA and >88% oleic acid. The development of commercial cultivars with reduced levels of polunsaturated fatty acid in both B. rapa and B. napus could develop new markets for both industrial rapeseed and edible canola oil.

A genetic linkage map has been constructed based on restriction fragment length polymorphism DNA markers for Brassica rapa L. using a segregating F2 progeny from a cross between the yellow sarson type 'R500' and the canola-type 'Horizon'. The map contains 360 marker loci detected by 269 genomic clones derived from a PstI library of 'Westar' (Brassica napus L.). The map consists of 10 linkage groups, covering a total of 1876 recombination units. The occurrence of substantial genome rearrangement during the evolution of B. rapa is evident from (i) a large number of homologous duplicate sequences either at genetically adjacent locations or in different linkage groups and (ii) a number of missing sequences in one or the other parental genome, detected as null alleles. Divergence of the parental genotypes is further supported by the frequency of loci with skewed allele segregation ratios. Comparison of molecular and cytogenetic data on genome structure is discussed. The probe set and map are being used to facilitate rapeseed breeding.Key words: Brassica campestris, Brassica rapa, genome structure, linkage map, restriction fragment length polymorphism.

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Bannerot et al. (191'7) into this cytoplasm using intergeneric crossing followed by backcrossing. Shiga and Baba (19'71, 1973) and ThomPson (19'72) Heyn (1979) and Rouselle (1979) found that all rape cultivars tested in their studies were maintainers for the ogu CMS. Bonnet (1975) China and Japan were winter types. The seedlings from these strains were vernalized at 4 C"

More than 100 different Brassica nucleo-cytoplasmic combinations were analysed for the presence or absence of the 11.3 kb mitochondrial plasmid. Contrary to some previous reports, no close association exists between the presence of the plasmid and cytoplasmic male sterility. Some novel abundant RNAs which copurified with Brassica mitochondria are described.

The inheritance of flower color was investigated in progeny from crosses between a normal, yellow flowering rape strain (Brassica napus L.) and a white flowering strain developed at the University of Manitoba by backcrossing the white flower color from Raphanobrassica into B. napus. The chromosome number of all yellow flowering plants examined was normal (2n = 38), whereas all white flowering plants examined contained a single or a pair of small radish (Raphanus sativus L.) chromosomes each carrying a dominant gene(s) for white flower color. The homozygous white flowering rape strain was in fact an alien addition line (2n = 40) disomic for a pair of radish chromosomes. The transmission and meiotic behavior of this alien chromosome as a univalent was investigated in the progeny of crosses involving the white flowering F1 hybrid. Cytological observations of meiotic metaphase 1 indicated that the univalent should be transmitted with 24.6% of the gametes. This was in agreement with the observed transmission of 24.3% through the female gametes. Transmission through the pollen ranged from 22.1% to 0.7%, the variation in certation depending on the genetic constitution of the pistil and on the time of pollination. The vigor, fertility and true breeding nature of the white flowering strain suggested that the white flower color factor from the radish had been introgressed into the rape genome. Cytological observations were necessary to demonstrate that introgression had not taken place and to aid in the interpretation of the genetic ratios observed.

A dwarf early Brassica napus line (D-001), derived from crosses with Diplotaxis muralis, was crossed to two B. napus cultivars, 'Regent' and 'Pivot' to study the inheritance of earliness (days to bud, days to first flower, and days to maturity), height, and leaf number. Spaced plants of P1, P2, F1, F2, BC1, and BC2 generations for both crosses were grown in a completely randomized design at two locations in Manitoba in 1984. Results of means analyses indicated that additive gene action predominated for all traits for both crosses and both locations. In addition, dominance gene action was found to influence days to first flower, days to maturity, and leaf number for the D-001 × 'Regent' cross and all traits for the D-001 × 'Pivot' cross. Nonallelic interactions were nonsignificant in all cases. Genotype × environment interactions were not significant for all traits except height. Variance analyses indicated a predominance of additive gene action. Broad sense heritabilities for phenological traits ranged from 55 to 90%, while narrow sense heritabilities ranged from 0 to 81%. For height, broad and narrow sense heritability estimates were equal and ranged from 51 to 84%. Heritability for leaf number ranged from 73 to 82% for broad sense and from 49 to 77% for narrow sense estimates. The predominance of additive gene action and the high heritabilities associated with these traits suggest that D-001 may be useful as a source of earliness in B. napus breeding programs. Key words: Brassica napus, earliness, inheritance, Diplotaxis muralis.