Leaf shape and size variation in bur oaks: an empirical study and simulation of sampling strategies.

PREMISE: Leaf shape and size figure strongly in plants' adaptation to their environments. Among trees, oaks are notoriously variable in leaf morphology. Our study examines the degree to which within-tree, among-tree, and among-site variation contribute to latitudinal variation in leaf shape and size of bur oak (Quercus macrocarpa: Fagaceae), one of North America's most geographically widespread oak species. METHODS: Samples were collected from four sites each at northern, central, and southern latitudes of the bur oak range. Ten leaf size traits were measured, and variance in these traits and eight ratios based on these traits was partitioned into tree and population components. Population means were regressed on latitude. We then parameterized a series of leaf collection simulations using empirical covariance among leaves on trees and trees at sites. We used the simulations to assess the efficiency of different collecting strategies for estimating among-population differences in leaf shape and size. RESULTS: Leaf size was highly responsive to latitude. Site contributed more than tree to total variation in leaf shape and size. Simulations suggest that power to detect among-site variance in leaf shape and size increases with either more leaves per tree (10-11 leaves from each of 5 trees) or more trees per site (5 leaves from each of 10+ trees). CONCLUSIONS: Our study demonstrates the utility of simulating sampling and controlling for variance in sampling for leaf morphology, whether the questions being addressed are ecological, evolutionary, or taxonomic. Simulation code is provided as an R package (traitsPopSim) to help researchers plan morphological sampling strategies.

Patterns and predictors of fleshy fruit phenology at five international botanical gardens.

PREMISE OF THE STUDY: To improve our understanding of the patterns and drivers of fleshy fruit phenology, we examined the sequence, patterns across years and locations, and drivers of fruiting times at five botanical gardens on three continents. METHODS: We monitored four stages of fruit phenology for 406 temperate, fleshy-fruited, woody plant species in 2014 and 2015. KEY RESULTS: Across all gardens, ripe fruits were present from May to March of the following year, with peak fruiting durations ranging from under 1 week to over 150 days. Species-level first fruiting and onset of peak fruiting dates were strongly associated with one another within sites and were more consistent between years and sites than the end of peak fruiting and last fruiting date. The order of fruiting among species between years and gardens was moderately consistent, and both peak fruiting times and fruiting durations were found to be phylogenetically conserved. CONCLUSIONS: The consistent order of fruiting among species between years and locations indicates species-specific phenological responses to environmental conditions. Wide variation in fruiting times across species and in the duration of peak fruiting reinforces the importance of understanding how plant phenology impacts dispersers and monitoring the health and consistency of these interactions.

Ulmus americana (Ulmaceae) is a polyploid complex.

PREMISE OF THE STUDY: Exotic diseases are threatening many North American tree species, and management of diseases requires understanding the biology of the host as well as the pathogen. Ulmus americana is a widespread dominant tree of eastern North America that has been widely planted as an ornamental and shade tree. Populations of the species have suffered heavy mortality from Dutch elm disease, caused by an introduced fungus. Ulmus americana is generally reported to be tetraploid, but the discovery of triploid trees in cultivation suggested that lower ploidy levels may exist in the wild, so the species was surveyed for nuclear DNA content. METHODS: Ploidy level was estimated by flow cytometry for 81 individuals from wild populations of U. americana from throughout the range of the species and for four cultivated trees of interest. KEY RESULTS: Most specimens were tetraploid, as previously reported for the species, but 21% of the wild trees sampled were diploid, a ploidy level not previously confirmed for the species. Tetraploids are found throughout the range of the species. Diploids are most common on the Atlantic coastal plain, Cumberland Plateau, and in southern Ohio, but isolated diploids were also found in central Texas, Oklahoma, and eastern Missouri. Diploids and tetraploids grew in proximity in several areas, but no wild triploids were found in the course of this survey. CONCLUSIONS: The species is genetically heterogeneous, but further research is needed to understand the origin and relations of the different ploidy levels. Understanding the ploidy situation in U. americana will be important in the search for further genotypes that are resistant to Dutch elm disease.

Convergent molecular evolution among ash species resistant to the emerald ash borer.

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 United States' 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 hybridization 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 invaded by this beetle.