Leonardo O. Alvarado-Cárdenas

Universal hydraulics of the flowering plants: vessel diameter scales with stem length across angiosperm lineages, habits and climates

Angiosperm hydraulic performance is crucially affected by the diameters of vessels, the water conducting conduits in the wood. Hydraulic optimality models suggest that vessels should widen predictably from stem tip to base, buffering hydrodynamic resistance accruing as stems, and therefore conductive path, increase in length. Data from 257 species (609 samples) show that vessels widen as predicted with distance from the stem apex across angiosperm orders, habits and habitats. Standardising for stem length, vessels are only slightly wider in warm/moist climates and in lianas, showing that, rather than climate or habit, plant size is by far the main driver of global variation in mean vessel diameter. Terminal twig vessels become wider as plant height increases, while vessel density decreases slightly less than expected tip to base. These patterns lead to testable predictions regarding evolutionary strategies allowing plants to minimise carbon costs per unit leaf area even as height increases.

Convergent Vessel Diameter–Stem Diameter Scaling across Five Clades of New and Old World Eudicots from Desert to Rain Forest

Premise of research. Variation in average xylem vessel diameter across species has important functional consequences, but the causes of this variation remain unclear. Average vessel diameter is known to scale with stem size within and across species. Vessel diameter also seems to differ between clades and across environments, with dryland plants having narrower, more cavitation-resistant vessels. As a result, it is not clear to what extent phylogenetic affinity and environment are associated with differences in the vessel diameter–stem size relationship. Methodology. With linear models and correlations, we explored the influence of environment and phylogeny on the vessel diameter–stem diameter relationship in a molecular phylogenetic context across 83 species in four families spanning desert to rain forest in the Americas, Africa, Asia, and Madagascar. Pivotal results. Mean species vessel diameter was strongly predicted by trunk diameter (slope ∼0.33), and this slope was not affected by either phylogenetic affinity or environment. Clades differed only slightly in mean vessel diameter when controlling for stem size, and there was no tendency for plants of moist environments to have wider vessels. Of four climate indexes, only the temperature index contributed to explaining vessel diameter, although very weakly. Conclusions. Our results are congruent with models suggesting that natural selection should maximize vessel conductivity while minimizing cavitation risk via vessel taper in the context of conductive path length. Because neither environment nor phylogeny contributed to explaining vessel diameter–stem diameter scaling across species, our results appear congruent with the notion that selection favoring cavitation resistance via narrow vessels should lead to shorter statures independently of ancestry or habitat. The repeated finding of narrow vessels in dryland plants might therefore reflect the smaller average stem size of plants in drylands rather than the plants having vessels that are narrow for their stem diameters.

Systematics, distribution and conservation of Cascabela (Apocynaceae: Rauvolfioideae: Plumerieae) in Mexico

Cascabela is a Neotropical genus of Apocynaceae with six species: C. balsaensis, C. gaumeri, C. ovata, C. pinifolia, C. thevetia and C. thevetioides. Its taxonomic history has long been a subject of controversy, especially due to its circumscription as a genus closely related to Thevetia. In this work, we revised the taxonomy of Cascabela for Mexico and provide detailed descriptions and illustrations. We also applied species distribution models and geographical information tools to evaluate chorological aspects of the six species occurring in Mexico and provide a conservation category. According to our results, Cascabela has a typical tropical distribution within Mexico, and the Balsas basin was identified as an area of diversity and endemism for the genus. The species C. ovata and C. thevetia have the broadest distribution ranges, and we assigned them the category of least concern. We suggest that the restricted, endemic species C. balsaensis, C. pinifolia and C. thevetioides be considered for protection. The genus was corroborated as an element of deciduous forest, and its cultural importance is highlighted.

RESURRECTION OF ASCLEPIAS SCHAFFNERI (APOCYNACEAE, ASCLEPIADOIDEAE), A RARE, MEXICAN MILKWEED

ABSTRACT Relocation and further study of the local endemic Asclepias zacatecana McVaugh showed that it is conspecific with A. schaffneri A. Gray, a distinct species that previously had been synonymized with A. quinquedentata A. Gray. However, we consider a recently described species, A. rzedowskii W.D. Stevens, to be conspecific with A. quinquedentata. Asclepias schaffneri is lectotypified.

The diversity and evolution of pollination systems in large plant clades: Apocynaceae as a case study

Abstract Background and Aims Large clades of angiosperms are often characterized by diverse interactions with pollinators, but how these pollination systems are structured phylogenetically and biogeographically is still uncertain for most families. Apocynaceae is a clade of >5300 species with a worldwide distribution. A database representing >10 % of species in the family was used to explore the diversity of pollinators and evolutionary shifts in pollination systems across major clades and regions. Methods The database was compiled from published and unpublished reports. Plants were categorized into broad pollination systems and then subdivided to include bimodal systems. These were mapped against the five major divisions of the family, and against the smaller clades. Finally, pollination systems were mapped onto a phylogenetic reconstruction that included those species for which sequence data are available, and transition rates between pollination systems were calculated. Key Results Most Apocynaceae are insect pollinated with few records of bird pollination. Almost three-quarters of species are pollinated by a single higher taxon (e.g. flies or moths); 7 % have bimodal pollination systems, whilst the remaining approx. 20 % are insect generalists. The less phenotypically specialized flowers of the Rauvolfioids are pollinated by a more restricted set of pollinators than are more complex flowers within the Apocynoids + Periplocoideae + Secamonoideae + Asclepiadoideae (APSA) clade. Certain combinations of bimodal pollination systems are more common than others. Some pollination systems are missing from particular regions, whilst others are over-represented. Conclusions Within Apocynaceae, interactions with pollinators are highly structured both phylogenetically and biogeographically. Variation in transition rates between pollination systems suggest constraints on their evolution, whereas regional differences point to environmental effects such as filtering of certain pollinators from habitats. This is the most extensive analysis of its type so far attempted and gives important insights into the diversity and evolution of pollination systems in large clades.

Population genetic analysis and bioclimatic modeling in Agave striata in the Chihuahuan Desert indicate higher genetic variation and lower differentiation in drier and more variable environments

PREMISE OF THE STUDY Is there an association between bioclimatic variables and genetic variation within species? This question can be approached by a detailed analysis of population genetics parameters along environmental gradients in recently originated species (so genetic drift does not further obscure the patterns). The genus Agave, with more than 200 recent species encompassing a diversity of morphologies and distributional patterns, is an adequate system for such analyses. We studied Agave striata, a widely distributed species from the Chihuahuan Desert, with a distinctive iteroparous reproductive ecology and two recognized subspecies with clear morphological differences. We used population genetic analyses along with bioclimatic studies to understand the effect of environment on the genetic variation and differentiation of this species. METHODS We analyzed six populations of the subspecies A. striata subsp. striata, with a southern distribution, and six populations of A. striata subsp. falcata, with a northern distribution, using 48 ISSR loci and a total of 541 individuals (averaging 45 individuals per population). We assessed correlations between population genetics parameters (the levels of genetic variation and differentiation) and the bioclimatic variables of each population. We modeled each subspecies distribution and used linear correlations and multifactorial analysis of variance. KEY RESULTS Genetic variation (measured as expected heterozygosity) increased at higher latitudes. Higher levels of genetic variation in populations were associated with a higher variation in environmental temperature and lower precipitation. Stronger population differentiation was associated with wetter and more variable precipitation in the southern distribution of the species. The two subspecies have genetic differences, which coincide with their climatic differences and potential distributions. CONCLUSIONS Differences in genetic variation among populations and the genetic differentiation between A. striata subsp. striata and A. striata subsp. falcata is correlated with differences in environmental climatic variables along their distribution. We found two distinct gene pools that suggest active differentiation and perhaps incipient speciation. The detected association between genetic variation and environment variables indicates that climatic variables are playing an important role in the differentiation of A. striata.

Molecular phylogenetics and morphology of Beaucarnea (Ruscaceae) as distinct from Nolina, and the submersion of Calibanus into Beaucarnea

Despite the economic importance and longstanding worldwide commercialization of representatives of the Beaucarnea lineage, it remains poorly known systematically. One of the main systematic problems is lack of certainty regarding the validity of the genus itself. Some authors consider Beaucarnea a synonym of its close relative Nolina, whereas others consider Beaucarnea a distinct genus. In addition to the Beaucarnea vs. Nolina controversy, the boundary between Beaucarnea and Calibanus is an issue that has not yet been addressed. Here we show that Beaucarnea is a well-supported entity, distinct from Nolina on molecular and morphological grounds. Additionally, we demonstrate the absence of reciprocal monophyly between Beaucarnea and Calibanus and formally include Calibanus within Beaucarnea. These decisions were based on maximum parsimony, Bayesian, and maximum likelihood analyses of datasets including ten species of Beaucarnea, two species of Calibanus, six species of Nolina, and five species of Dasylirion, with sequences from the nuclear ITS and plastid trnL-F and ycf1 regions. Our taxonomic decisions were also based on morphological observations of herbarium specimens and on the literature. We illustrate the diagnostic features of the genera with phylogenetic character mapping. Finally, we offer a redescription of Beaucarnea to accommodate the former Calibanus species and give a key to the taxonomically valid Beaucarnea species.