Different scales of Ty1/copia-like retrotransposon proliferation in the genomes of three diploid hybrid sunflower species.

Activation of transposable elements in species' genomes represents an important mechanism of new mutation and of potential rapid change in genome size. Thus, it is increasingly recognized that transposable elements likely have played a significant role in shaping species' evolution. In an earlier report, we showed that the genomes of three sunflower species of ancient hybrid origin have experienced large-scale proliferation events of sequences within the Ty3/gypsy-like superfamily of long terminal repeat (LTR) retrotransposons. In this report, we investigate whether another superfamily of LTR retrotransposon (Ty1/copia-like elements) have experienced similar derepression and proliferation events in the genomes of these sunflower hybrid taxa. We show that Ty1/copia-like elements also have undergone copy number increases following or associated with the origins of these species, although the scale of proliferation is less than that for Ty3/gypsy-like elements. Surveys of sequence heterogeneity of Ty1/copia-like elements in the genomes of the three hybrid and two parental species' genomes reveal that a single sub-lineage of these elements exhibits characteristics of recent amplification, and likely served as the proliferative source lineage. These findings indicate that the genomic and/or environmental conditions associated with the origins of these sunflower hybrid taxa were conducive to derepression of at least two major groups of transposable elements.

Ecological genomics: understanding gene and genome function in the natural environment.

The field of ecological genomics seeks to understand the genetic mechanisms underlying responses of organisms to their natural environments. This is being achieved through the application of functional genomic approaches to identify and characterize genes with ecological and evolutionary relevance. By its very nature, ecological genomics is an interdisciplinary field. In this review, we consider the significance of this new area of study from both an ecological and genomic perspective using examples from the recent literature. We submit that by considering more fully an ecological context, researchers may gain additional insights into the underlying genetic basis of ecologically relevant phenotypic variation. Likewise, genomic approaches are beginning to offer new insights into higher-level biological phenomena that previously occupied the realm of ecological investigation only. We discuss various approaches that are likely to be useful in ecological genomic studies and offer thoughts on where this field is headed in the future.

Map-based cloning of quantitative trait loci: progress and prospects.

Map-based cloning has been considered problematic for isolating quantitative trait loci (QTLs) due to the confounding phenotypic effects of environment and other QTLs. However, five recent studies, all in plants, have succeeded in cloning QTLs using map-based methods. We review the important features of these studies and evaluate the prospects for broader application of the techniques. Successful map-based cloning requires that QTLs represent single genes that can be isolated in near-isogenic lines, and that genotypes can be unambiguously inferred by progeny testing. In plants or animals for which map-based cloning of genes with discrete phenotypes is feasible, the modified procedures required for QTLs should not be limiting in most cases. The choice between map-based cloning and alternative methods will depend on details of the species and traits being studied.

The effects of mating design on introgression between chromosomally divergent sunflower species.

Population genetic theory suggests that mating designs employing one or more generations of sib-crossing or selfing prior to backcrossing are more effective than backcrossing alone for moving alleles across linkage groups where effective recombination rates are low (e.g., chromosomally divergent linkages). To test this hypothesis, we analyzed the effects of chromosomal structural differences and mating designs on the frequency and genomic distribution of introgressed markers using the domesticated sunflower, Helianthus annuus, and one of its wild relatives, H. petiolaris, as the experimental system. We surveyed 170 progeny, representing the end products of three different mating designs (design I, P-F1-BC1-BC2-F2-F3; design II, P-F1-F2-BC1-BC2-F3; and design III, P-F1-F2-F3-BC1-BC2), for 197 parental RAPD markers of known genomic location. Comparison of observed patterns of introgression with expectations based on simulations of unrestricted introgression revealed that much of the genome was protected from introgression regardless of mating design or chromosomal structural differences. Although the simulations indicated that all markers should introgress into multiple individuals in each of the three mating designs, 20 of 58 (34%) markers from collinear linkage groups, and 112 of 139 (81%) markers from rearranged linkage groups did not introgress. In addition, the average size of introgressed fragments (12.2 cM) was less than half that predicted by theoretical models (26-33 cM). Both of these observations are consistent with strong selection against introgressed linkage blocks, particularly in chromosomally divergent linkages. Nonetheless, mating designs II and III, which employed one and two generations of sib-mating, respectively, prior to backcrossing, were significantly more effective at moving alleles across both collinear and rearranged linkages than mating design I, in which the backcross generations preceded sib-mating. Thus, breeding strategies that include sib-crossing, in combination with backcrossing, should significantly increase the effectiveness of gene transfer across complex genic or chromosomal sterility barriers.

Rapid hybrid speciation in wild sunflowers.

Hybrid or "recombinational" speciation refers to the origin of a new homoploid species via hybridization between chromosomally or genetically divergent parental species. Theory predicts that this mode of speciation is punctuated, but there has been little empirical evidence to support this claim. Here, we test the hypothesis of rapid hybrid speciation by estimating the sizes of parental species chromosomal blocks in Helianthus anomalus, a wild sunflower species derived via hybridization between H. annuus and H. petiolaris. Analysis of the frequency spectrum of parental species chromosomal blocks with respect to predictions based on R. A. Fisher's [Fisher, R. A. (1953) Heredity 8, 187-197] junctions approach, suggests that H. anomalus arose rapidly, probably in fewer than 60 generations. This result is corroborated by independent lines of evidence demonstrating (i) a significant concordance between the genomes of H. anomalus and early generation H. annuus x H. petiolaris synthetic hybrids, and (ii) a rapid recovery of pollen fertility in these synthetic hybrid lineages. These results are not only consistent with theory but also provide a new and general method for estimating the tempo of hybrid speciation and dating the origin of hybrid zones.

Long-term analysis of biologic and technical aspects of fixed partial dentures with cantilevers.

PURPOSE: The aim of the present clinical study was to analyze the long-term treatment outcome of fixed partial dentures (FPD) with cantilevers. MATERIALS AND METHODS: Ninety-two patients with 115 cantilever FPDs on 239 abutment teeth were examined. Immediately following cementation, radiographs of the abutment teeth were obtained. At the follow-up examination 5 to 16 years after cementation, the abutment teeth were clinically and radiographically evaluated. Besides the assessment of periodontal parameters and vitality testing, the presence of carious lesions of the abutment teeth, loss of retention, fracture of bridgework, and fracture of abutment teeth were also recorded. The Student's t test was used to detect differences between abutment teeth and control teeth with natural crowns as well as over time within the groups. RESULTS: Plaque scores and bleeding on probing were low at both abutment and nonabutment teeth. Mean probing pocket depth and level of the clinical attachment never exceeded 3.0 mm in both groups. The radiographic bone levels at abutments were stable within 3% over the observation period. Of the 120 initially vital abutment teeth, 12 (10%) lost vitality during the observation period; one (1%) of the 119 initially nonvital abutments showed periapical pathology. Development of secondary caries was observed at 8% of the 239 abutment teeth. In total, 8% of the abutment teeth were affected by loss of retention, which made up more than half of all technical problems. Material fractures only occurred occasionally, affecting from 1% to less than 3% of the abutment teeth. Fracture of abutment teeth amounted to 3% and was twice as frequent at abutments adjacent to cantilevers compared to abutments not adjacent to cantilevers. CONCLUSION: The most frequent biologic (caries) and technical complications (loss of retention) made up more than half of all problems recorded. These problems may be at least partially avoided by performing optimal plaque control and by strictly observing the rules for preparation of retentive FPD abutments.