Meiotic pairing was studied at metaphase I in three different cv. Chinese Spring × rye hybrid combinations (5B deficient, 3D deficient, and normal ABDR) to establish the arm homoeology of wheat and rye chromosomes. The majority of individual wheat chromosomes and their arms, as well as the arms of chromosomes 1R and 5R, were identified by means of C-banding. The results on pairing relationships support the genome reallocation of chromosomes 4A and 4B. The short arms of wheat chromosomes belonging to homoeologous groups 1, 3, 5, and 6 and of chromosome pairs 4A–4D and 7A–7D showed full pairing homoeology as well as the long arms of wheat chromosomes of groups 1, 3, 6, and 7 and of chromosome pairs 4A–4D and 5B–5D. Chromosomes 2A, 2B, and 2D were homoeologous, but the homoeologies of their arms were not identified. Reduced homoeologies of the 4BL arm to 7AS and 7DS, of the 5AL arm to 4AL and 4DL, and of the 7BS arm to 5BL and 5DL were identified. Arms 4BL, 5AL, and 7BS are involved in a double translocation that arose during the evolution of common wheat. The homoeology relationships of chromosome arm 4BS were not identified since this arm seldom paired. The homoeologous pairing pattern between wheat chromosomes was characterized by a remarkable predominance of A–D associations, altered only by structural changes in groups 4 and 5. Chromosome arm 1RL showed full pairing homoeology to 1AL, 1BL, and 1DL, while 5RL was homoeologous to 5AL and partially homoeologous to 4AL and 4DL. It is concluded that 5RL carries a translocated segment from 4RL. Key words: homoeologous pairing, translocations, wheat evolution, C-banding.

Interspecific differentiation values (G_ST) between two closely related oak species (Quercus petraea and Q. robur) were compiled across different studies with the aim to explore the distribution of differentiation at the genome level. The study was based on a total set of 389 markers (isozymes, AFLPs, SCARs, microsatellites, and SNPs) for which allelic frequencies were estimated in pairs of populations sampled throughout the sympatric distribution of the two species. The overall distribution of G_ST values followed an L-shaped curve with most markers exhibiting low species differentiation (G_ST < 0.01) and only a few loci reaching >10% levels. Twelve percent of the loci exhibited significant G_ST deviations to neutral expectations, suggesting that selection contributed to species divergence. Coding regions expressed higher differentiation than noncoding regions. Among the 389 markers, 158 could be mapped on the 12 linkage groups of the existing Q. robur genetic map. Outlier loci with large G_ST values were distributed over 9 linkage groups. One cluster of three outlier loci was found within 0.51 cM; but significant autocorrelation of G_ST was observed at distances <2 cM. The size and distribution of genomic regions involved in species divergence are discussed in reference to hitchhiking effects and disruptive selection.

Background: Expressed Sequence Tags (ESTs) are a source of simple sequence repeats (SSRs) that can be used to develop molecular markers for genetic studies. The availability of ESTs for Quercus robur and Quercus petraea provided a unique opportunity to develop microsatellite markers to accelerate research aimed at studying adaptation of these long-lived species to their environment. As a first step toward the construction of a SSR-based linkage map of oak for quantitative trait locus (QTL) mapping, we describe the mining and survey of EST-SSRs as well as a fast and cost-effective approach (bin mapping) to assign these markers to an approximate map position. We also compared the level of polymorphism between genomic and EST-derived SSRs and address the transferability of EST-SSRs in Castanea sativa (chestnut). Results: A catalogue of 103,000 Sanger ESTs was assembled into 28,024 unigenes from which 18.6% presented one or more SSR motifs. More than 42% of these SSRs corresponded to trinucleotides. Primer pairs were designed for 748 putative unigenes. Overall 37.7% (283) were found to amplify a single polymorphic locus in a reference full-sib pedigree of Quercus robur. The usefulness of these loci for establishing a genetic map was assessed using a bin mapping approach. Bin maps were constructed for the male and female parental tree for which framework linkage maps based on AFLP markers were available. The bin set consisting of 14 highly informative offspring selected based on the number and position of crossover sites. The female and male maps comprised 44 and 37 bins, with an average bin length of 16.5 cM and 20.99 cM, respectively. A total of 256 EST-SSRs were assigned to bins and their map position was further validated by linkage mapping. EST-SSRs were found to be less polymorphic than genomic SSRs, but their transferability rate to chestnut, a phylogenetically related species to oak, was higher. Conclusion: We have generated a bin map for oak comprising 256 EST-SSRs. This resource constitutes a first step toward the establishment of a gene-based map for this genus that will facilitate the dissection of QTLs affecting complex traits of ecological importance.