The Hardness (Ha) locus controls grain hardness in hexaploid wheat (Triticum aestivum) and its relatives (Triticum and Aegilops species) and represents a classical example of a trait whose variation arose from gene loss after polyploidization. In this study, we investigated the molecular basis of the evolutionary events observed at this locus by comparing corresponding sequences of diploid, tertraploid, and hexaploid wheat species (Triticum and Aegilops). Genomic rearrangements, such as transposable element insertions, genomic deletions, duplications, and inversions, were shown to constitute the major differences when the same genomes (i.e., the A, B, or D genomes) were compared between species of different ploidy levels. The comparative analysis allowed us to determine the extent and sequences of the rearranged regions as well as rearrangement breakpoints and sequence motifs at their boundaries, which suggest rearrangement by illegitimate recombination. Among these genomic rearrangements, the previously reported Pina and Pinb genes loss from the Ha locus of polyploid wheat species was caused by a large genomic deletion that probably occurred independently in the A and B genomes. Moreover, the Ha locus in the D genome of hexaploid wheat (T. aestivum) is 29 kb smaller than in the D genome of its diploid progenitor Ae. tauschii, principally because of transposable element insertions and two large deletions caused by illegitimate recombination. Our data suggest that illegitimate DNA recombination, leading to various genomic rearrangements, constitutes one of the major evolutionary mechanisms in wheat species.

A set of 41 wheat microsatellite markers (WMS), giving 42 polymorphic loci (two loci on each chromosome), was used to describe genetic diversity in a sample of 559 French bread wheat accessions (landraces and registered varieties) cultivated between 1800 and 2000. A total of 609 alleles were detected. Allele number per locus ranged from 3 to 28, with a mean allele number of 14.5. On the average, about 72% of the total number of alleles were observed with a frequency of less than 5% and were considered to be rare alleles. WMS markers used showed different levels of gene diversity: the highest PIC value occurred in the B genome (0.686) compared to 0.641 and 0.659 for the A and D genomes, respectively. When comparing landraces with registered varieties gathered in seven temporal groups, a cluster analysis based on an F(st) matrix provided a clear separation of landraces from the seven variety groups, while a shift was observed between varieties registered before and after 1970. There was a decrease of about 25% in allelic richness between landraces and varieties. In contrast, when considering only registered varieties, changes in diversity related to temporal trends appeared more qualitative than quantitative, except at the end of the 1960s, when a bottleneck might have occurred. New varieties appear to be increasingly similar to each other in relation to allelic composition, while differences between landraces are more and more pronounced over time. Finally, considering a sub-sample of 193 varieties representative of breeding material selected during the twentieth century by the six most important plant breeding companies, few differences in diversity were observed between the different breeding programmes. The observed structure of diversity in French bread wheat collections is discussed in terms of consequences, both for plant breeders and for managers of crop genetic resources.

Bread wheat (Triticum aestivum), one of the world's major crops, is genetically very diverse. In order to select a representative sample of the worldwide wheat diversity, 3,942 accessions originating from 73 countries were analysed with a set of 38 genomic simple sequence repeat (SSR) markers. The number of alleles at each locus ranged from 7 to 45 with an average of 23.9 alleles per locus. The 908 alleles detected were used together with passport data to select increasingly large sub-samples that maximised both the number of observed alleles at SSR loci and the number of geographical origins. A final core of 372 accessions (372CC) was selected with this M strategy. All the different geographical areas and more than 98% of the allelic diversity at the 38 polymorphic loci were represented in this core. The method used to build the core was validated, by using a second set of independent markers [44 expressed sequence tag (EST)-SSR markers] on a larger sample of 744 accessions: 96.74% of the alleles observed at these loci had already been captured in the 372CC. So maximizing the diversity with a first set of markers also maximised the diversity at a second independent set of locus. To relate the genetic structure of wheat germplasm to its geographical origins, the two sets of markers were used to compute a dissimilarity matrix between geographical groups. Current worldwide wheat diversity is clearly divided according to wheat's European and Asian origins, whereas the diversity within each geographical group might be the result of the combined effects of adaptation of an initial germplasm to different environmental conditions and specific breeding practices. Seeds from each accession of the 372CC were multiplied and are now available to the scientific community. The genomic DNA of the 372CC, which can be entirely contained in a 384-deep-well storage plate, will be a useful tool for future studies of wheat genetic diversity.

Transgenic maize (Zea mays) plants were generated with a construct harboring a maize caffeic acid O-methyltransferase (COMT) cDNA in the antisense (AS) orientation under the control of the maize Adh1 (alcohol dehydrogenase) promoter. Adh1-driven ␤-glucuronidase expression was localized in vascular tissues and lignifying sclerenchyma, indicating its suitability in transgenic experiments aimed at modifying lignin content and composition. One line of AS plants, COMT-AS, displayed a significant reduction in COMT activity (15%-30% residual activity) and barely detectable amounts of COMT protein as determined by western-blot analysis. In this line, transgenes were shown to be stably integrated in the genome and transmitted to the progeny. Biochemical analysis of COMT-AS showed: (a) a strong decrease in Klason lignin content at the flowering stage, (b) a decrease in syringyl units, (c) a lower p-coumaric acid content, and (d) the occurrence of unusual 5-OH guaiacyl units. These results are reminiscent of some characteristics already observed for the maize bm3 (brownmidrib3) mutant, as well as for COMT down-regulated dicots. However, as compared with bm3, COMT down-regulation in the COMT-AS line is less severe in that it is restricted to sclerenchyma cells. To our knowledge, this is the first time that an AS strategy has been applied to modify lignin biosynthesis in a grass species.Lignins are complex phenolic polymers present in all vascular plants. They provide rigidity to conducting xylem elements and fiber cells. Lignins are composed of C 6 C 3 units, principally p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, and are present in various proportions according to botanical, physiological, and cytological criteria (Lewis and Yamamoto, 1990). Throughout the plant kingdom, grass lignins appear to be particularly specialized because they contain not only H, G, and S units, but also additional p-hydroxycinnamic units such as p-coumaric and ferulic acids (Higuchi et al., 1967). Ferulic acid may be ester linked to wall polysaccharides and/or ether linked to G units, thereby forming bridges between lignins and polysaccharides (Jacquet et al., 1995), whereas p-coumaric acid is primarily ester linked to S lignin units in lignified walls (Ralph et al., 1994; Grabber et al., 1996).Lignification in dicotyledons has been extensively studied and most of the known lignin biosynthetic genes have been employed in genetic engineering experiments (for review, see Grima-Pettenati and Goffner, 1999). There are almost no molecular data on lignification in grasses (Collazo et al., 1992;McAlister et al., 1998;Pichon et al., 1998;Selman-Housein et al., 1999;Spangenberg et al., 2001). Although lignification in grass species is likely to share a high degree of similarity to other angiosperms, the aforementioned structural specificity of grass cell walls may also involve a certain degree of grass-specific regulatory mechanisms. An in-depth knowledge of lignification in Graminaeae is of utmost importance because lignins are one of the ...

In flowering plants, double fertilization occurs when the egg cell and the central cell are each fertilized by one sperm cell. In maize, some lines produce pollen capable of inducing in situ gynogenesis thereby leading to maternal haploids that originate exclusively from the female plant. In this paper, we present a genetic analysis of in situ gynogenesis in maize. Using a cross between non-inducing and inducing lines, we identified a major locus on maize chromosome 1 controlling in situ gynogenesis (ggi1, for gynogenesis inducer 1). Fine mapping of this locus was performed, and BAC physical contigs spanning the locus were identified using the rice genome as anchor. Genetic component analysis showed that (a) a segregation distortion against the inducer parent was present at this locus, (b) segregation resulted only from male deficiency and (c) there was a correlation between the rate of segregation distortion and the level of gynogenetic induction. In addition, our results showed that the genotype of the pollen determined its capacity to induce the formation of a haploid female embryo, indicating gametophytic expression of the character with incomplete penetrance. We propose the occurrence of a gametophytic-specific process which leads to segregation distortion at the ggi1 locus associated with gynogenetic induction with incomplete penetrance.