No description available.

DNA markers tightly-linked to nuclear fertility restorer genes for cytoplasmic male sterility (CMS) are valuable tools for breeders and researchers working with these genes. Two different targeting approaches were used to identify markers linked to the Rfp1 restorer gene for the pol CMS of canola (Brassica napus L.): nearly isogenic line (NIL) comparison and bulked segregant analysis. These methods were equally efficient in identifying markers linked to Rfp1; combining them allowed a targeting efficiency of 100% to be achieved. The efficiency of bulked segregant analysis was found to be limited by the inadvertent occurrence of shared homozygosity at specific chromosomal regions in the bulks, in contrast with the efficiency of NIL comparison which was limited by the occurrence of residual DNA from the donor cultivar at scattered sites around the genome of the NILs. Eleven DNA markers linked to the Rfp1 gene were identified, one of which perfectly co-segregates with Rfp1. The linkage group on which Rfp1 is localized contains 17 DNA markers. Two restorer genes of the pol CMS, Rfp1 and Rfp2, and a Rfn restorer gene of the nap CMS were found to be at least tightly linked to one another and may all reside at the same locus. A fourth restorer gene, the Rfo restorer for the ogu CMS, was, however, found to be unlinked to the other restorer genes. Different restorer genes for the nap CMS were found in the lines 'Westar-Rf and 'Karat'. A linkage map of the B. napus genome containing 146 markers organized into 23 linkage groups covering a total length of 850.2 cM was constructed from a BC$ sb1$ population. This map contains 63 loci previously localized on the B. napus genome through analysis of an F$ sb2$ population. Comparative analysis indicates that the total length of the BC$ sb1$-derived map is smaller than that of the F$ sb2$-derived map, which suggests that a reduction in recombination frequency is occurring in male gametes. The preferential use of two or three probe-

No description available.

Arabidopsis expressed sequence tags (ESTs) and microsatellites of Brassica napus have been developed and used as PCR-based markers for both mapping and genetic diversity studies in B. napus . Out of 300 random Arabidopsis ESTs screened, 43 markers were mapped onto a genetic map of B. napus and then used in a diversity study involving 48 B. napus cultivars. A second set of EST markers were developed from chromosome 1 of Arabidopsis and used in genetic mapping studies of B. napus. From 192 primer pairs developed, 50 markers were added onto the B. napus reference map. Microsatellite markers were developed using a "GA" enriched genomic library from B. napus. From 152 designed primer pairs, 23 markers were added onto the B. napus reference map. Microsatellite markers were also used in genetic diversity studies of B. napus, where, from the 152 primer pairs, 40 revealed polymorphism between the 48 B. napus cultivars.

Cytoplasrnic male sterility (CMS) is a maternally inherited defect in the production of pollen, the male gamete of the flower. This sterility can be suppressed by nuclear Restorer of Fertility (Rf) genes that normally downregulate the expression of the CMS-associated novel mitochondrial gene. In Brassica napus, nap CMS and pol CMS are associated with related chimeric mitochondrial genes orf222 and orf224, respectively. CMS in both nap and pol is associated with a polar loss of locule development, loss of synchronous locule development and clumping of sporogenous tissue away from the tapetal cell layer, as well as secondary effects on petal and bud formation. In nap CMS, early accumulation of orf222 transcripts in the locule regions of developing anthers is associated with sterility, while the absence of orf222 transcripts from the locules is associated with fertility restoration. Accumulation of novel antisense transcripts of atp6 in a cell specific manner which matches that of sense transcripts of orf222 and atp6 in nap CMS anthers may be indicative of a post-transcriptional regulatory mechanism associated with CMS in flower buds. / Restoration of fertility in Brassica napus nap and pol CMS is associated with nuclearly encoded genes Rfn and Rfp, respectively. These restorers are very closely linked to one another, and may be allelic. Further efforts to isolate Rfp have narrowed the genomic region to approximately 105 kb of a syntenic region in Arabidopsis thaliana. Cosmid clones isolated from a library of Brassica rapa genomic DNA introgressed with Rfp have been successfully sorted into contigs through the application of the amplified fragment length polymorphism technique. The region to which Rfp is mapped is syntenic to a region of Arabidopsis DNA that is a duplication of a second location at the 23 megabase region of chromosome 1 of that genome. This region contains pentatricopeptide (PPR) motif-encoding genes that are highly related to other restorers of fertility of other species. By inference, Rfp from Brassica napus may encode PPR motifs. The PPR genes related to these previously characterized restorers of fertility are often found alongside the restorer genes existing as mini-clusters of several PPR-encoding genes. This is likely caused by selective pressure acting on PPR-encoding genes that resulted in diversification and multiplication of these genes. In addition, the PPR genes of this duplicated region are not syntenically located, whereas the non-PPR-encoding genes maintain their syntenic locations. The same is true for orthologous comparisons between Arabidopsis and other plant species. PPR genes are therefore malleable and capable of alteration in response to changing environmental pressures, such as the evolution of sterility inducing genes.

No description available.