Arnaud Dowkiw

Partial resistance to Melampsora larici-populina leaf rust in hybrid poplars: Genetic variability in inoculated excised leaf disk bioassay and relationship with complete resistance

ABSTRACT Inoculated excised leaf disk technique allows decomposition of poplar partial resistance to Melampsora larici-populina leaf rust into key epidemiological components such as latent period (LP), uredinia number (UN), uredinia size (US), and spore production (SP) for a given M. larici-populina strain under controlled environmental conditions. Three hundred thirty-six genotypes from an interspecific Populus deltoides x P. trichocarpa F(1) progeny segregating for complete resistance to M. larici-populina strain 93ID6 were inoculated with M. larici-populina strain 93CV1. This strain was able to infect the whole family, except few probable recombinants. LP, final UN, and final US after one infectious cycle proved to be relevant complementary descriptors of partial resistance. Area under the disease progress curve and other parameters of uredinia appearance dynamics did not yield additional information. Indirect assessment of SP by US scoring was reliable and easy to access compared with direct spore counting. UN was the only trait for which a doubling of the inoculum pressure level had a significant effect, leading to greater differentiation between genotypes. Consistent with previous studies is the clear relationship between presence of complete resistance against M. larici-populina strain 93ID6 and higher partial resistance to M. larici-populina strain 93CV1 (32% longer LP, 76% smaller UN, and 34% smaller US). In the subpopulation compatible with 93ID6, bimodal distribution of genotypic means for US suggested implication of a major gene inherited from the P. trichocarpa parent. Residual variation was noted for the three epidemiological components, suggesting that additional genes might condition these quantitative traits.

Presence of defeated qualitative resistance genes frequently has major impact on quantitative resistance to Melampsora larici-populina leaf rust in P. × interamericana hybrid poplars

Qualitative resistance to Melampsora larici-populina leaf rust inherited from North American species Populus deltoides did not allow for durable control of this pathogen in interspecific hybrid cultivars. Despite significant levels of strain-specificity, quantitative resistance would exert lower selection pressures on the pathogen populations, and hence could be more durable. Previous studies restricted to a large P. × interamericana (i.e., P. deltoides × Populus trichocarpa) F1 family revealed that the presence of R1, a segregating defeated qualitative resistance gene inherited from P. deltoides, had major beneficial effects on quantitative resistance. The present study was based on 14 F1 families from a 4 × 5 P. deltoides × P. trichocarpa factorial mating design where at least four defeated qualitative resistances segregate 1:1. Even though quantitative resistance assessments were conducted in the laboratory with a M. larici-populina strain able to overcome these qualitative resistances, their presence had a significant effect on the mean level and on the genetic variability for quantitative resistance. One unprecedented result is the identification of a defeated qualitative resistance which presence is associated with lower levels of quantitative resistance. Possible inferences on the nature of the genetic relationship between both resistance types are discussed.

Qualitative and quantitative resistances to leaf rust finely mapped within two nucleotide-binding site leucine-rich repeat (NBS-LRR)-rich genomic regions of chromosome 19 in poplar.

• R(US) is a major dominant gene controlling quantitative resistance, inherited from Populus trichocarpa, whereas R(1) is a gene governing qualitative resistance, inherited from P. deltoides. • Here, we report a reiterative process of concomitant fine-scale genetic and physical mapping guided by the P. trichocarpa genome sequence. The high-resolution linkage maps were developed using a P. deltoides × P. trichocarpa progeny of 1415 individuals. R(US) and R(1) were mapped in a peritelomeric region of chromosome 19. Markers closely linked to R(US) were used to screen a bacterial artificial chromosome (BAC) library constructed from the P. trichocarpa parent, heterozygous at the locus R(US) . • Two local physical maps were developed, one encompassing the R(US) allele and the other spanning r(US) . The alignment of the two haplophysical maps showed structural differences between haplotypes. The genetic and physical maps were anchored to the genome sequence, revealing genome sequence misassembly. Finally, the R(US) locus was localized within a 0.8-cM interval, whereas R(1) was localized upstream of R(US) within a 1.1-cM interval. • The alignment of the genetic and physical maps with the local reorder of the chromosome 19 sequence indicated that R(US) and R(1) belonged to a genomic region rich in nucleotide-binding site leucine-rich repeat (NBS-LRR) and serine threonine kinase (STK) genes.

Rising Out of the Ashes: Additive Genetic Variation for Crown and Collar Resistance to Hymenoscyphus fraxineus in Fraxinus excelsior

Since the early 1990s, ash dieback due to the invasive ascomycete Hymenoscyphus fraxineus is threatening Fraxinus excelsior in most of its natural range. Previous studies reported significant levels of genetic variability in susceptibility in F. excelsior either in field or inoculation experiments. The present study was based on a field experiment planted in 1995, 15 years before onset of the disease. Crown and collar status were monitored on 777 trees from 23 open-pollinated progenies originating from three French provenances. Health status was modeled using a Bayesian approach where spatiotemporal effects were explicitly taken into account. Moderate narrow-sense heritability was found for crown dieback (h2 = 0.42). This study is first to show that resistance at the collar level is also heritable (h2 = 0.49 for collar lesions prevalence and h2 = 0.42 for their severity) and that there is significant genetic correlation (r = 0.40) between the severities of crown and collar symptoms. There was no evidence for differences between provenances. Family effects were detected, but computing individual breeding values showed that most of the genetic variation lies within families. In agreement with previous reports, early flushing correlates with healthier crown. Implications of these results in disease management and breeding are discussed.

Polygamy or subdioecy? The impact of diallelic self-incompatibility on the sexual system in Fraxinus excelsior (Oleaceae)

How flowering plants have recurrently evolved from hermaphroditism to separate sexes (dioecy) is a central question in evolutionary biology. Here, we investigate whether diallelic self-incompatibility (DSI) is associated with sexual specialization in the polygamous common ash (Fraxinus excelsior), which would ultimately facilitate the evolution towards dioecy. Using interspecific crosses, we provide evidence of strong relationships between the DSI system and sexual phenotype. The reproductive system in F. excelsior that was previously viewed as polygamy (co-occurrence of unisexuals and hermaphrodites with varying degrees of allocation to the male and female functions) and thus appears to actually behave as a subdioecious system. Hermaphrodites and females belong to one SI group and functionally reproduce as females, whereas males and male-biased hermaphrodites belong to the other SI group and are functionally males. Our results offer an alternative mechanism for the evolution of sexual specialization in flowering plants.

Post-pollination paternal reproductive success in Populus nigra : a male affair

Populus nigra is a riparian forest species that is considered vulnerable in several parts of its natural range due to habitat loss and fragmentation. Resulting small population sizes are a source of demographic stochasticity, loss of genetic diversity, and inbreeding which can be strongly enhanced if adult trees exhibit contrasted and stable differential reproductive success (DRS). This study was designed to test for the existence of post-pollination paternal DRS in P. nigra and its stability across female genotypes. An equal-weight pollen mix was used to hand-pollinate four female genotypes, and the paternity of the resulting seeds was elucidated using SSR markers. Pollen germination rates and number of grains per weight unit were taken into account as potential predictors of the DRS observed. Using a four-pollen mix allowed testing male-by-female interactions statistically. Although most parental genotypes originated from a single population along the Loire River in France, one pollen contributor was the Lombardy poplar (P. nigra var. “Italica”), an ornamental male cultivar supposed to originate from the Middle East and present worldwide. Moreover, two of the females involved were identified as spontaneous F1s from this clone. The material studied was thus genetically diverse enough to detect male-by-female interactions should they be common in the species. Significant post-pollination paternal DRS was found which could not be explained by the pollen characteristics measured. The most significant result was the absence of male-by-female interactions on the paternal DRS observed. These results are discussed in terms of in situ conservation strategies. They may also have implications in terms of evolution, a stable paternal DRS across females being a prerequisite for sexual selection to occur.

La sélection participative en France : des plantes annuelles aux ligneux

Breeding good varieties of uncommon forest species may be achieved through a lengthy, expensive conventional breeding programme. For some annual plants and tropical forest species, participatory breeding or domestication achieves good results and better dissemination of the varieties. This approach was initiated in France taking the wild cherry tree as a model, for which there are many existing schemes and cultivars that can be used as controls to evaluate the varieties and select new individuals while fine-tuning the methodology. The model can be applied to other species: we show that the wild cherry tree controls may be used for other species when soil and climate requirements are similar. Highly intensive breeding in nurseries would rapidly enable seed orchards to be set up