Boudewijn Michiels

With a little help from my friends: hybrid fertility of exotic Populus x canadensis enhanced by related native Populus nigra

Hybridization and genetic swamping by planted exotic Populus taxa are putative threats for native Populus resources. We investigated the potential for hybridization between the exotic hybrid Populus x canadensis Moench and its wild relative, the European black poplar (Populus nigra L.), by a series of pollination experiments in the greenhouse. We also tested the effect of mixtures of pollen from the taxon’s own pollen and pollen of the foreign taxon on mating success in reciprocal crosses. We observed reduced pollen and seed viability of the hybrid clones of P. x canadensis compared to the clones of the parental species P. nigra. Surprisingly, when in combination with pollen of its wild relative P. nigra, pollen of exotic P. x canadensis sired significantly better on its own flowers. In poplar breeding, the pollen mentor technique which uses pollen mixes of artificially inactivated conspecific pollen and untreated incompatible foreign pollen, has been successfully used to overcome incompatibility barriers. The results of this study suggest that in the wild, where mixed pollen loads are common, similar effects as the pollen mentor effects may enhance the invasiveness of exotic P. x canadensis. This study helps to explain former field observations and contributes to the understanding of potential environmental impacts of commercial exotic and transgene poplar plantations.

Dissection of leaf morphological traits from isolated and declined relict populations of Ulmus laevis reveals putative random ecotype evolution

Ulmus laevis Pallas is a rare and endangered tree species in the northern part of Belgium, restricted to a minimal amount of natural populations that mainly consist of a strongly reduced number of individuals. Probably isolated for several generations, random phenotypic variation in the strongly declined populations can be hypothesised. We analysed U. laevis trees grown in a field trial with a single-tree-plot design (completely randomised). The plants were vegetatively propagated through cuttings from nearly all known natural relict populations in Flanders. We analysed three short-shoot leaves from different ramets from different genotypes from eleven natural populations. Leaf size and shape variables were computed using landmark and procruste methods. We visualised the variability in leaf morphology in a principal component analysis (PCA) including shape, size, and categorical morphological variables. The variance structure of the morphological variables was studied applying mixed modelling methods. The overall PCA distinguished two deviating natural populations. The Zandhoven population showed a deviating simple leaf margin toothing, correlated with larger numbers of split secondary veins. The Halle population is characterised by absence of pubescence at the lower side of the leaves. Both relict populations with deviating morphological leaf characteristics may point to an ecotype evolution putatively due to isolation combined with greatly diminished population sizes. The variance structure analysis of individual leaf variables showed that the deviating categorical leaf characters are under stronger genetic control (specialist characters) compared to the more plastic variables.

Population differentiation and phenotypic plasticity in temperature response of bud burst in Frangula alnus provenances of different latitude

Flushing in spring marks an important adaptive process in a tree’s growth cycle. We studied bud burst in three provenances of a common small tree Frangula alnus Mill., originating from Italy, Belgium and Sweden. We observed timing of bud burst and leaf senescence in a common garden, and timing and duration of bud burst in greenhouse conditions (cuttings) with different temperature regimes, all located in Belgium. The early bud burst response of the southern European provenance together with the later leaf senescence compared to the local provenance indicated population differentiation that may, at least partly, be driven by local adaptation to a longer growing season. The duration of the process was longer in the cold greenhouse compared to the local provenance, whereas it responded similar as the local provenance in the warm greenhouse, suggesting adaptation to warmer conditions. Unexpectedly, the northern European provenance expressed a plastic reaction to warmer temperatures in the field trial and in the greenhouse conditions flushing in both cases earlier than the local provenance. The duration of the process in the warm greenhouse compared to the local provenance was shorter, whereas in the cold greenhouse it was similar. This result again suggested that not only the onset, but also the duration of the bud burst process was a plastic reaction to the warmer conditions compared to its site of origin. Together, our results suggest two mechanisms driving bud burst in F. alnus provenances, a co-gradient and a counter-gradient variation depending on the latitude of origin.