BackgroundSalix (willow) and Populus (poplar) are members of the Salicaceae family and they share many ecological as well as genetic and genomic characteristics. The interest of using willow for biomass production is growing, which has resulted in increased pressure on breeding of high yielding and resistant clones adapted to different environments. The main purpose of this work was to develop dense genetic linkage maps for mapping of traits related to yield and resistance in willow. We used the Populus trichocarpa genome to extract evenly spaced markers and mapped the orthologous loci in the willow genome. The marker positions in the two genomes were used to study genome evolution since the divergence of the two lineages some 45 mya.ResultsWe constructed two linkage maps covering the 19 linkage groups in willow. The most detailed consensus map, S1, contains 495 markers with a total genetic distance of 2477 cM and an average distance of 5.0 cM between the markers. The S3 consensus map contains 221 markers and has a total genetic distance of 1793 cM and an average distance of 8.1 cM between the markers. We found high degree of synteny and gene order conservation between willow and poplar. There is however evidence for two major interchromosomal rearrangements involving poplar LG I and XVI and willow LG Ib, suggesting a fission or a fusion in one of the lineages, as well as five intrachromosomal inversions. The number of silent substitutions were three times lower (median: 0.12) between orthologs than between paralogs (median: 0.37 - 0.41).ConclusionsThe relatively slow rates of genomic change between willow and poplar mean that the genomic resources in poplar will be most useful in genomic research in willow, such as identifying genes underlying QTLs of important traits. Our data suggest that the whole-genome duplication occurred long before the divergence of the two genera, events which have until now been regarded as contemporary. Estimated silent substitution rates were 1.28 × 10-9 and 1.68 × 10-9 per site and year, which are close to rates found in other perennials but much lower than rates in annuals.

Quantitative trait loci (QTL) for growth traits and water-use efficiency have been identified in two water regimes (normal and drought-treated) and for a treatment index. A tetraploid hybrid F2 population originating from a cross between a Salix dasyclados clone (SW901290) and a Salix viminalis clone ('Jorunn') was used in the study. The growth response of each individual including both above and below ground dry-matter production (i.e. shoot length, shoot diameter, aboveground and root dry weight, internode length, root dry weight/total dry weight, relative growth rate and leaf nitrogen content) was analysed in a replicated block experiment with two water treatments. A composite interval mapping approach was used to estimate number of QTL, the magnitude of the QTL and their position on genetic linkage maps. QTL specific for each treatment and for the treatment index were found, but QTL common across the treatments and the treatment index were also detected. Each QTL explained from 8% to 29% of the phenotypic variation, depending on trait and treatment. Clusters of QTL for different traits were mapped close to each other at several linkage groups, indicating either a common genetic base or tightly linked QTL. Common QTL identified between treatments and treatment index in the complex trait dry weight can be useful tools in the breeding and selection for drought stress tolerance in Salix.

Background: Sex chromosomes have evolved independently multiple times in eukaryotes and are therefore considered a prime example of convergent genome evolution. Sex chromosomes are known to emerge after recombination is halted between a homologous pair of chromosomes, and this leads to a range of non-adaptive modifications causing gradual degeneration and gene loss on the sex-limited chromosome. However, the proximal causes of recombination suppression and the pace at which degeneration subsequently occurs remain unclear. Results: Here, we use long-and short-read single-molecule sequencing approaches to assemble and annotate a draft genome of the basket willow, Salix viminalis, a species with a female heterogametic system at the earliest stages of sex chromosome emergence. Our single-molecule approach allowed us to phase the emerging Z and W haplotypes in a female, and we detected very low levels of Z/W single-nucleotide divergence in the non-recombining region. Linked-read sequencing of the same female and an additional male (ZZ) revealed the presence of two evolutionary strata supported by both divergence between the Z and W haplotypes and by haplotype phylogenetic trees. Gene order is still largely conserved between the Z and W homologs, although the W-linked region contains genes involved in cytokinin signaling regulation that are not syntenic with the Z homolog. Furthermore, we find no support across multiple lines of evidence for inversions, which have long been assumed to halt recombination between the sex chromosomes. Conclusions: Our data suggest that selection against recombination is a more gradual process at the earliest stages of sex chromosome formation than would be expected from an inversion and may result instead from the accumulation of transposable elements. Our results present a cohesive understanding of the earliest genomic consequences of recombination suppression as well as valuable insights into the initial stages of sex chromosome formation and regulation of sex differentiation.

In a greenhouse experiment we examined the effect of willow genotype and irrigation regime (moderate drought and well-watered) on plant growth parameters, foliar nitrogen, and phenolic concentrations, as well as on the preference and performance of the blue leaf beetle, Phratora vulgatissima (L.) (Coleoptera: Chrysomelidae) . The 10 vegetatively propagated willow genotypes in the experiments were F2 full-sibling hybrids, originated from a cross between Salix viminalis (L.) (Salicaceae) (high in condensed tannins) and Salix dasyclados (L.) (Salicaceae) (rich in phenolic glycosides). Insect bioassays were conducted on detached leaves in Petri dishes as well as with free-living insects on intact potted plants. The 10-week long irrigation treatments caused statistically significant phenotypic differences in the potted willow saplings. Total biomass was somewhat higher in the well-watered treatment. The root to total biomass ratio was higher in the drought-treatment plants. There was significant genotypic variation in foliar nitrogen concentrations, and they were higher in the droughttreatment plants. There was also a strong genotypic variation in each of the phenolic substances analyzed. Condensed tannins, which accounted for the greatest proportion of total phenolic mass, were higher in the well-watered treatment. There was, however, no difference in levels of the other phenolics (salicylates, cinnamic acid, flavonoids, and chlorogenic acid) between irrigation treatments. The sum of these phenolics was higher in the well-watered treatment. There was a strong variation in P. vulgatissima larval development on different willow genotypes, and larval performance was negatively correlated with levels of salicylates and cinnamic acid. There was, however, no effect of irrigation treatment on larval performance. Phratora vulgatissima preferred to feed on well-watered plants, and we found a preference for oviposition there, but neither feeding nor oviposition site preference was affected by willow genotype. Adult feeding and oviposition preferences were not correlated with larval performance.

Background and AimsThe major objective was to identify plant traits functionally important for optimization of shoot growth and nitrogen (N) economy under drought. Although increased leaf N content (area basis) has been observed in dry environments and theory predicts increased leaf N to be an acclimation to drought, experimental evidence for the prediction is rare.MethodsA pedigree of 200 full-sibling hybrid willows was pot-grown in a glasshouse in three replicate blocks and exposed to two water regimes for 3 weeks. Drought conditions were simulated as repeated periods of water shortage. The total leaf mass and area, leaf area efficiency (shoot growth per unit leaf area, EA), area-based leaf N content (NA), total leaf N pool (NL) and leaf N efficiency (shoot growth per unit leaf N, EN) were assessed.Key ResultsIn the water-stress treatment, shoot biomass growth was N limited in the genotypes with low NL, but increasingly limited by other factors in the genotypes with greatest NL. The NA was increased by drought, and drought-induced shift in NA varied between genotypes (significant G × E). Judged from the EA–NA relationship, optimal NA was 16 % higher in the water-stress compared with the well-watered treatment. Biomass allocation to leaves and shoots varied between treatments, but the treatment response of the leaf : shoot ratio was similar across all genotypes.ConclusionsIt is concluded that N-uptake efficiency and leaf N efficiency are important traits to improve growth under drought. Increased leaf N content (area basis) is an acclimation to optimize N economy under drought. The leaf N content is an interesting trait for breeding of willow bioenergy crops in a climate change future. In contrast, leaf biomass allocation is a less interesting breeding target to improve yield under drought.