The association of flowering time quantitative trait loci with duplicated regions and candidate loci in Brassica oleracea.

A population of 150 doubled haploid lines of rapid cycling Brassica oleracea, derived from an F1 from a var. alboglabra x var. italica cross, was scored for flowering time in two trials. Using information on 82 mapped molecular markers, spread evenly across the nine linkage groups, QTL were identified at six locations; one each on linkage groups O2 and O3 and two each on linkage groups O5 and O9. In total, these QTL explained 58 and 93% of the genetical variation in the two trials. Three of these QTL, on linkage groups O2, O3, and O9, were situated in regions showing considerable homology both with each other and with chromosome regions of B. nigra that have been shown to affect flowering time. These same regions are all homologous to a single tract of Arabidopsis chromosome 5, which contains a number of the flowering-related genes, one or more of which may be candidates for the QTL found in Brassica.

Higher recombination frequencies in female compared to male meisoses in Brassica oleracea.

Linkage maps of the nine chromosomes of Brassica oleracea, based on 75 informative molecular markers, have been compared in first and second backcross progeny from a cross between two doubled haploid lines. The second backcross progeny showed greater recombination frequencies for 75% of the pairs of adjacent markers, but there was no obvious indication that this effect was localised to particular regions of the chromosomes. Four chromosomes increased in genetic length more than twofold, while overall, the total map was 66% longer. The possible causes of this discrepancy are analysed. A sex difference in chiasma distribution and/or frequency at meiosis is thought to be the most likely explanation. The implications of this finding for mapping and map-based applications are discussed.

Isolation, characterisation and mapping of simple sequence repeat loci in potato.

Solanum tuberosum L. DNA sequences containing simple sequence repeat (SSR) motifs were extracted from the EMBL database, cDNA and selectively enriched small-insert DNA libraries. Enrichment was achieved using either triplex affinity capture or single-strand hybridisation selection. One hundred and twelve primer pairs which successfully amplified products of the correct size from potato DNA were ultimately designed and synthesised. Ninety-eight of these revealed length polymorphisms in a panel of four diploid and two tetraploid clones, in agreement with the high information content of this class of markers which has been found in other species. All of the markers were assigned a quality score of 1-5 based on their potential usefulness. Eighty-nine loci from 65 of the primer pairs were located on two genetic linkage maps of potato by segregation analysis of the amplified alleles. Fifty-two of the SSRs were clearly single locus. The maps were aligned using 23 SSR primer pairs and 13 RFLP loci mapped in both populations. The markers described constitute a class which should replace Restriction Fragment Length Polymorphisms (RFLP) as the markers of choice for future genetic studies in potato. The sequences of the primers, together with other information on these markers are provided.

The construction of a substitution library of recombinant backcross lines in Brassica oleracea for the precision mapping of quantitative trait loci.

The currently available methods for locating quantitative trait loci (QTLs) and measuring their effects in segregating populations lack precision unless individual QTLs have very high heritabilities. The use of recombinant backcross lines containing short regions of donor chromosome introgressed into a constant recipient background permits QTLs to be located with greater precision. The present paper describes the use of molecular markers to introgress defined short regions of chromosome from a donor doubled haploid calabrese line of Brassica oleracea (var. italica) into a recipient short generation variety (Brassica oleracea var. alboglabra). We demonstrate that in just two or three generations of backcrossing, combined with selection for mapped molecular markers, the generation of a library of recombinant backcross lines is feasible. The possible use and refinement of these lines are discussed. Key words : backcrossing, Brassica oleracea, introgression, molecular markers, near-isogenic lines, QTL mapping, recombinant backcross lines, substitution lines.