Vanina Guérin

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.

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.