Natural populations of Dichroplus elongatus often exhibit diverse forms of supernumerary heterochromatin. In Tafi Viejo, Tucumkn Province (Argentina), parallel polymorphisms for extra segments on pairs M,, S, (proximal) and S,, (distal) and a mitotically unstable B chromosome were detected. The segments produce intrachromosome effects excluding chiasmata from their neigbourhood and displacing them to regions distal to the extra heterochromatin. The individuals bearing simultaneously the B and the segment on M6 show a decrease in the frequency of interstitial chismata (Xi) while those carrying only segments on M6 or S10 have an increase of this variable when compared with standard males. The between cell variance of Xi is also higher in M6 segment carriers. B individuals present an increase of abnormal spermatid frequency which may be explained by the simultaneous occurrence of two mechanisms: 1) lagging of supernumeraries and 2) physiological effects of B's affecting the meiosis even in cells lacking them. The intra and interchromosome effects may be important for the plasticity of populations allowing recombination to explore new chromosome regions. The B polymorphism seems to be stable which points to an advantage of B carriers and/or the occurrence of accumulation mechanisms counterbalancing their negative effects on fertility.

Open-pollinated (OPs) and composite populations (CPs) represent a valuable resource for sunflower breeding programmes. However, little is known about the levels and distribution of genetic variation within each of these populations. In this study, quantitative and qualitative traits along with molecular markers were used to evaluate 14 populations from the Instituto Nacional de Tecnología Agropecuaria (INTA) sunflower germplasm collection. These populations were chosen to represent historically important accessions that still play a central role within the INTA sunflower breeding programme due to their extensive variation in diverse agronomically important traits. Nine quantitative and eight qualitative agro-morphological descriptors were recorded and compared with those of a larger set of accessions representative of the phenotypic diversity of the sunflower collection. Molecular characterization was conducted on a total of 311 individuals using 16 microsatellite markers. Overall, the average gene diversity was 0.56 and the average number of alleles per locus was 6.25. No statistically significant differences in genetic diversity were detected between the OPs and CPs. Global estimates ofFSTrevealed very high levels of differentiation among accessions (FST= 0.413,P< 0.05). Population structure analyses were consistent with the observed levels of differentiation and identified two major groups. The results of this work show that high global diversity is preserved within the accessions analysed here. Additionally, this study provides a set of reliable and discriminant markers for the cost-effective molecular characterization of sunflower accessions, along with the guidelines for the delineation of sampling strategies for OPs and CPs, thus aiding the efficient management and exploitation of sunflower germplasm collections.