Pollen dispersal and gene flow in the grass meadow fescue (Festuca pratensis Huds.) were studied using two populations which were homozygous for different allozymes at the Gpi-2 locus. The populations were established in a concentric donor-acceptor field experiment. Gene flow was found mainly to be affected by the distance between the donor and acceptor plants. Analysing 21 132 acceptor plant progenies, gene flow was shown to decrease rapidly with distance to the donor field up to 75 m, and beyond this distance much more slowly. The ability of donor pollen to fertilize acceptor plants depended very much on the density of the acceptor plants. Pairs of acceptor plants produced more compatible pollen locally, and captured significantly less donor pollen than single-plants. Despite the higher seed production of acceptor plants planted in pairs, the absolute number of heterozygous seeds carrying the donor allele was always lower than for single-plants. Wind direction had only a slight effect upon the type of pollen captured. Because of pollen production within the two plant populations being continuous and overlapping, the time when anthesis occurred had little effect on gene flow between the populations. Vigorous and tall acceptor plants with many panicles, high seed yield and high 1000-seed weight were able to capture more donor pollen than shorter plants. The results may be used to assess the risk of gene flow and to develop strategies for monitoring the spread of transgenes from genetically modified grasses.

RFLP and RAPD markers were evaluated and compared for their ability to determine genetic relationships in a set of three B. napus breeding lines. Using a total of 50 RFLP and 92 RAPD markers, the relatedness between the lines was determined. In total, the RFLP and the RAPD analysis revealed more than 500 and 400 bands, respectively. The relative frequencies of loci with allele differences were estimated from the band data. The RFLP and RAPD marker sets detected very similar relationships among the three lines, consistent with known pedigree data. Bootstrap analyses showed that the use of approximately 30 probes or primers would have been sufficient to achieve these relationships. This indicates that RAPD markers have the same resolving power as RFLP markers when used on exactly the same set of B. napus genotypes. Since RAPD markers are easier and quicker to use, these markers may be preferred in applications where the relationships between closely-related breeding lines are of interest. The use of RAPD markers in fingerprinting applications may, however, not be warranted, and this is discussed in relation to the reliability of RAPD markers.

We have used artificial 1∶1 DNA mixtures of all pairwise combinations of four doubled haploid Brassica napus lines to test the ability of RAPDs to function as reliable dominant genetic markers. In situations where a specific RAPD band is present in one homozygous line but absent in the other, the band is expected in the artificial heterozygote, i.e. in the 1∶1 DNA mixture. In 84 of all 613 heterozygous situations analysed, the expected band failed to amplify in the RAPD reaction. Thus, RAPD markers will lead to an erroneous genetic interpretation in 14% of all cases. In contrast, the formation of non-parental heteroduplex bands was found at a frequency of only 0.2%. Analysis of 1∶ 1 mixtures using (1) a different set of optimized reaction conditions and (2) a material with low genomic complexity (Bacillus cereus) gave identical results. Serial dilutions of one genome into another, in steps of 10%, showed that all of the polymorphic bands decreased in intensity as a linear function of their respective proportion in the mixture. In dilutions with water no differences in band intensity were detected. Thus, competition occurs in the amplification of all RAPD fragments and is a major source of genotyping errors in RAPD analysis.

The most important rhizomania-resistance gene in sugar beet is the Rz1 gene from the Holly Sugar Company in California, the source widely used to breed partially resistant varieties. Other important gene sources are WB41 and WB42, which both originate from Beta vulgaris subsp. maritima collected in Denmark, and which have been reported to be similar. The major resistance gene in WB42 is known as Rz2. We studied the resistance in WB41 and used markers to map the major resistance gene in this source, which we call Rz3. It was identified on chromosome III. This is the chromosome that Rz1 and Rz2 have been mapped to. Data from greenhouse tests and ELISA showed that Rz3 had incomplete penetrance, with heterozygotes varying widely in resistance levels. The involvement of additional minor genes in the strong resistance of the original WB41 source cannot be excluded.

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