Variation of chloroplast DNA and nuclear ribosomal DNA (DNA encoding ribosomal RNA) was studied for five species of white oak native to the eastern United States. Although these species differ in many morphological characters and have different (though overlapping) geographical ranges and ecological tolerances, they are interfertile and often grow in mixed stands, and hybrids are occasionally found in nature. AU individuals studied were morphologically typical members of their respective species-i.e., showed no evidence of recent hybrid ancestry. Restriction site markers in the chloroplast DNA reveal several clear cases of localized gene exchange between species, showing that there is appreciable gene flow between sympatric species in this group. One length variant of the nuclear ribosomal DNA, however, is species specific. The sharp morphological and ecological differences between the species, together with the one ribosomal DNA variant, suggest that nuclear genes may be exchanged less freely between species than are chloroplast genotypes.

SummaryLeaf out phenology affects a wide variety of ecosystem processes and ecological interactions and will take on added significance as leaf out times increasingly shift in response to warming temperatures associated with climate change. There is, however, relatively little information available on the factors affecting species differences in leaf out phenology.An international team of researchers from eight Northern Hemisphere temperate botanical gardens recorded leaf out dates of c. 1600 woody species in 2011 and 2012.Leaf out dates in woody species differed by as much as 3 months at a single site and exhibited strong phylogenetic and anatomical relationships. On average, angiosperms leafed out earlier than gymnosperms, deciduous species earlier than evergreen species, shrubs earlier than trees, diffuse and semi-ring porous species earlier than ring porous species, and species with smaller diameter xylem vessels earlier than species with larger diameter vessels. The order of species leaf out was generally consistent between years and among sites.As species distribution and abundance shift due to climate change, interspecific differences in leaf out phenology may affect ecosystem processes such as carbon, water, and nutrient cycling. Our open access leaf out data provide a critical framework for monitoring and modelling such changes going forward.

Recent studies show that molecular convergence plays an unexpectedly common role in the evolution of convergent phenotypes. We exploited this phenomenon to find candidate loci underlying resistance to the emerald ash borer (EAB; Agrilus planipennis ), the USA’s most costly invasive forest insect to date, within the pan-genome of ash trees (the genus Fraxinus ). We show that EAB-resistant taxa occur within three independent phylogenetic lineages. In genomes from these resistant lineages, we detect 53 genes with evidence of convergent amino acid evolution. Gene tree reconstruction indicates that for 48 of these candidates, the convergent amino acids are more likely to have arisen via independent evolution than by another process, such as hybridisation or incomplete lineage sorting. Seven of the candidate genes have putative roles connected to the phenylpropanoid biosynthesis pathway and 17 relate to herbivore recognition, defence signalling or programmed cell death. Evidence for loss-of-function mutations among these candidates is more frequent in susceptible species, than in resistant ones. Our results on evolutionary relationships, variability in resistance, and candidate genes for defence response within the ash genus could inform breeding for EAB resistance, facilitating ecological restoration in areas this beetle has invaded.