Canadian Forest Service Publications

Testing the ‘hybrid susceptibility’ and ‘phenological sink’ hypotheses using the P. balsamifera – P. deltoides hybrid zone and septoria leaf spot [Septoria musiva]. 2013. LeBoldus, J.M.; Isabel, N.; Floate, K.D.; Blenis, P.; Thomas, B.R. PLOS ONE 8:e84437.

Year: 2013

Issued by: Laurentian Forestry Centre

Catalog ID: 35402

Language: English

Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1371/journal.pone.0084437

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Hybrid genotypes that arise between plant species frequently have increased susceptibility to arthropod pests and fungal pathogens. This pattern has been attributed to the breakdown of plant defenses (‘Hybrid susceptibility’ hypothesis) and (or) to extended periods of susceptibility attributed to plant phenologies in zones of species overlap and (or) hybridization (‘phenological sink’ hypothesis). We examined these hypotheses by assessing the susceptibility of parental and hybrid Populus host genotypes to a leaf spot disease caused by the fungal pathogen Septoria musiva. For this purpose, 214 genotypes were obtained from morphologically pure zones of P. balsamifera and P. deltoides, and from an intervening zone of overlap and hybridization on the drainage of the Red Deer River, Alberta, Canada. Genotypes were identified as P. balsamifera, P. deltoides, or hybrid using a suite of 27 species-specific SNP markers. Initially the genetic structure of the hybrid zone was characterized with 27.7% of trees classified as admixed individuals. To test the hybrid susceptibility hypothesis, a subset of 52 genotypes was inoculated with four isolates of S. musiva. Levels of susceptibility were P. balsamifera > F1 hybrid > P. deltoides. A further 53 genotypes were grown in a common garden to assess the effect of genotype on variation in leaf phenology. Leaf phenology was more variable within the category of hybrid genotypes than within categories of either parental species. Leaf phenology was also more variable for the category of trees originating in the hybrid (P. balsamifera – P. deltoides [hybrid and parental genotypes combined]) zone than in adjacent pure zones of the parental species. The results from the inoculation experiment support the hybrid intermediacy hypothesis. The results from the common garden experiment support the ‘phenological sink’ hypothesis. These findings have greatly increased our understanding of the epidemiology and ecology of fungal pathogens in plant hybrid zones.

Plain Language Summary

There are natural hybrid zones, i.e., contact zones between two or more tree species belonging to the same genus, in many locations. The trees in these zones are often the result of cross breeding between two species growing in proximity to one another. These species are called parental species. It has also been reported in some studies that in these zones populated by hybrid trees, insects and diseases have a much greater impact on these hybrids than they do on the parental species.

In this study, the researchers wanted to verify this hypothesis by assessing the susceptibility of various individual trees from one of these zones to Septoria musiva, a disease causing leaf spot. They established a plot by obtaining cuttings (genetic copies) of parental species poplars and hybrid trees from this zone. They infected the cuttings with various strains of Septoria. Balsam poplar is naturally more affected by this disease than eastern cottonwood. In balsam poplar, the disease can attack the trunk and cause the formation of cankers that end up splitting the tree, which could reduce the tree’s productivity. However, this is not the case with eastern cottonwood.

The researchers observed that although hybrid poplars were affected by the disease, they were less affected than balsam poplars, but more so than eastern cottonwoods. Early or late leaf development in the spring is one of the factors that could explain the poplars’ susceptibility to the disease. However, in hybrid zones, there is considerable variability in leaf development between trees owing to the presence of both hybrid trees and parental species. This may partially explain the more frequent presence of the disease in these zones.