Publications

Premise

Pandanus Parkinson (Pandanaceae) is a large genus of paleotropical tree‐like monocots. Previous studies using small DNA regions questioned the monophyly of the seven Pandanus subgenera, but low phylogenetic branch support hindered further investigations. We aimed to (1) test Pandanus subgeneric monophyly, (2) identify clade morphological synapomorphies, (3) investigate correlations between leaf anatomy of water storage tissue and climatic differentiation across clades, and (4) construct hypotheses on the genus' spatiotemporal history.

Leaf traits are crucial to seedling growth and survival, and their plasticity can influence seedling fitness in changing environments. Seedlings of Artemisia tridentata, a keystone shrub of the western North American sagebrush steppe, show heteromorphic leaf development. Early leaves are larger and less pubescent than those produced later, suggesting a shift from characteristics favouring rapid growth to those increasing drought tolerance. To investigate this hypothesis, we determined the specific leaf area (SLA) and the osmotic potential at full turgor (π0) of early and late leaves, and measured their stomatal conductance and photosynthetic rates as leaf water potential (Ψl) declined under imposed drought. We also examined whether water stress could trigger late leaf development. At high Ψl and per area, early and late leaves had similar photosynthetic rates. However, the SLA of early leaves was three times higher than that of late leaves, yielding higher photosynthetic rates per unit mass in the former. Late leaves had lower π0 and were less sensitive to drought, exhibiting a lower Ψl at 50% of maximum photosynthesis than early leaves. Drought triggered the shedding of early leaves and the initiation of late-like leaves. Formation of these leaves continued upon return to well-watered conditions, possibly indicating stress memory. The overall results suggest that early leaves enhance growth during wet springs following germination, while late leaves prolong photosynthesis as water potentials decline during summer drought. The adaptive value of early leaves may be diminishing due to changing environmental conditions that are accelerating the onset of drought.

Speciation processes in plants can be difficult to evaluate, but are essential to understanding evolutionary processes that lead to diversification. Determining the juncture at which a genetically and/or morphologically divergent population can be reliably considered a separate species is often challenging. This is particularly so with respect to recent divergences amongst closely related taxa wherein factors such as incomplete lineage sorting may yield confounding results. Taxa in the Cymopterus terebinthinus (Apiaceae) species complex have long puzzled botanists. Named entities in this group display similar, yet apparently distinct morphologies that have been classified as varieties under various generic names highlighting long‐standing nomenclatural instability. Previous phylogenetic studies have challenged the monophyly of this complex. This study aims to clarify taxonomic boundaries and infer evolutionary relationships among the four C. terebinthinus varieties and C. petraeus by applying phylogenetic inference and incorporating ecological, morphological, and geographical evidence. We sampled from populations of all varieties of C. terebinthinus and C. petraeus for target capture with the Angiosperms353 bait kit. We performed phylogenetic analyses with maximum likelihood (RAxML and IQ‐TREE) and coalescent‐based phylogenetic analysis (ASTRAL). We also conducted principal component analysis of soil samples and climatic variables. We find that C. terebinthinus and its varietal infrataxa comprise a monophyletic clade that includes C. petraeus. Clade groupings correspond to previous taxonomic assignments and morphology. Clades are often closely associated with geographical variables and at times correlated with ecological variables. Exceptions to this are here attributed to various evolutionary factors that often confound other phylogenetic analyses such as incomplete lineage sorting, introgression, and paralogous loci. Our findings suggests that geographical factors might play a major role in genetic and morphological differentiation in this complex. Despite finding well‐supported clades that correspond to defined morphological characters; further sampling among C. petraeus populations is required to make taxonomic decisions.

Pandanus ramromensis Callm., Y.W.Low & Buerki (Pandanaceae) from the summit of Khao Ram Rome (Nakhon Si Thammarat Province) in Peninsular Thailand is described here. The new species resembles Pandanus kedahensis H.St.John in its ecology and habit but differs by the dimensions of its leaves, leaf shape, syncarps and styles. The new species is provided with line drawings and field photographs, and is assigned a preliminary conservation status of Vulnerable (VU) using the IUCN Red List criteria.

Angiosperms are the cornerstone of most terrestrial ecosystems and human livelihoods^1,2. A robust understanding of angiosperm evolution is required to explain their rise to ecological dominance. So far, the angiosperm tree of life has been determined primarily by means of analyses of the plastid genome^3,4. Many studies have drawn on this foundational work, such as classification and first insights into angiosperm diversification since their Mesozoic origins^5–7. However, the limited and biased sampling of both taxa and genomes undermines confidence in the tree and its implications. Here, we build the tree of life for almost 8,000 (about 60%) angiosperm genera using a standardized set of 353 nuclear genes⁸. This 15-fold increase in genus-level sampling relative to comparable nuclear studies⁹ provides a critical test of earlier results and brings notable change to key groups, especially in rosids, while substantiating many previously predicted relationships. Scaling this tree to time using 200 fossils, we discovered that early angiosperm evolution was characterized by high gene tree conflict and explosive diversification, giving rise to more than 80% of extant angiosperm orders. Steady diversification ensued through the remaining Mesozoic Era until rates resurged in the Cenozoic Era, concurrent with decreasing global temperatures and tightly linked with gene tree conflict. Taken together, our extensive sampling combined with advanced phylogenomic methods shows the deep history and full complexity in the evolution of a megadiverse clade.

In memory of Dr. Christopher Davidson