Floral biology and ovule and seed ontogeny of Nymphaea thermarum, a water lily at the brink of extinction with potential as a model system for basal angiosperms
Nymphaea thermarum is a member of the Nymphaeales, of one of the most ancient lineages of flowering plants. This species was only recently described and then declared extinct in the wild, so little is known about its reproductive biology. In general, the complete ontogeny of ovules and seeds is not well documented among species of Nymphaea and has never been studied in the subgenus Brachyceras, the clade to which N. thermarum belongs. Early male and female function indicate that N. thermarum is predisposed towards self-pollination, a phenomenon that is likely to have evolved multiple times within Nymphaea. While formation of distinct micropylar and chalazal developmental domains in the endosperm, along with a copious perisperm, characterize the seeds of most members of the Nymphaeales, seed ontogenies vary between and among the constituent families. Floral biology, life history traits and small genome size make N. thermarum uniquely promising as an early-diverging angiosperm model system for genetic and molecular studies.
Pollen tube cell walls of wild and domesticated tomatoes contain arabinosylated and fucosylated xyloglucan
In flowering plants, fertilization relies on the delivery of the sperm cells carried by the pollen tube to the ovule. During the tip growth of the pollen tube, proper assembly of the cell wall polymers is required to maintain the mechanical properties of the cell wall. Xyloglucan (XyG) is a cell wall polymer known for maintaining the wall integrity and thus allowing cell expansion. In most angiosperms, the XyG of somatic cells is fucosylated, except in the Asterid clade (including the Solanaceae), where the fucosyl residues are replaced by arabinose, presumably due to an adaptive and/or selective diversification. However, it has been shown recently that XyG of Nicotiana alata pollen tubes is mostly fucosylated. The objective of this paper was to determine whether such structural differences between somatic and gametophytic cells are a common feature of Nicotiana and Solanum (more precisely tomato) genera. The results show that the male gametophyte (pollen tube) and the sporophyte have structurally different XyG. This suggests that fucosylated XyG may have an important role in the tip growth of pollen tubes, and that they must have a specific set of functional XyG fucosyltransferases, which are yet to be characterized.
Pteris L. (Pteridaceae) is a cosmopolitan fern genus growing either terrestrially or lithophytically (on rocks) in forests, coastal areas and xeric niches. Most species of this genus occur in tropical and subtropical areas, but a few live in temperate regions. Species of Pteris are usually distributed at lower altitudes, below 2500 m, but Pteris coriacea Desv. can be found up to 3500 m. Some species have ornamental value, especially those with pale marks on the leaves. A few species are cultivated worldwide and some have become naturalized. With 250–300 species, Pteris is one of the most diverse fern genera. The uncertainty about the total number of species highlights the need for further taxonomic and phylogenetic studies.
A recent article in Annals of Botany finds that the biogeographic history of Pteris highlights long-distance dispersal as a major process shaping the worldwide distribution of the genus. Colonizing into different niches was followed by subsequent morphological diversification, and dispersal events followed by allopatric and parapatric speciation have contributed to the species diversity. This phylogeny should contribute to a new, more reliable infrageneric classification, based not just on a few morphological characters but also on ecological traits and geographic distribution.
Chao, Y.S., Rouhan, G., Amoroso, V.B., and Chiou, W. L. (2014) Molecular phylogeny and biogeography of the fern genus Pteris (Pteridaceae). Annals of Botany, 114(1): 109-124.
The cyanobacterial genus Nostoc includes many species that are highly diverse with respect to morphology, functional properties, biotic relations and habitat distribution. Nostoc species have filaments with normal photosynthetic cells and N2-fixing heterocysts and they periodically form resistant akinetes for survival and short motile filaments (hormogonia) for reproduction. Some species are free-living and many species engage in loose or obligate cooperation with land plants and fungi (e.g. lichens). A third, fascinating Nostoc type forms large gelatinous colonies of variable shape and structure in rice fields, freshwater lakes, ponds and streams and on alternating wet and dry soils or rock surfaces. The large gelatinous species require special adaptations to obtain sufficient light, nutrients and dissolved inorganic carbon in water and to survive the extreme variations in temperature, water supply and irradiance on naked soils and rock surfaces.
The gelatinous Nostoc species have filaments with normal photosynthetic cells and N2-fixing heterocysts embedded in an extensive gelatinous matrix of polysaccharides and many other organic substances providing biological and environmental protection. Large colony size imposes constraints on the use of external resources and the gelatinous matrix represents extra costs and reduced growth rates.
This review in Annals of Botany evaluates the mechanisms behind the low rates of growth and mortality, protection against environmental hazards and the persistence and longevity of gelatinous Nostoc colonies, and their ability to economize with highly limiting resources.
As free-living organisms and as symbionts in lichens, Nostoc species are both pioneers and permanent members of the vegetation of deserts, semideserts, dry grasslands and rock surfaces ranging in geographical distribution from polar to tropical regions. Their N input to these biomes can be of utmost importance. In a carefully mapped Low-Arctic tundra landscape, N2 fixation by cyanobacteria was twice the annual wet deposition of nitrogen. It remains unexplored how the high water-absorbing capacity and N2 fixation of Nostoc can facilitate the colonization of bare or newly exposed mineral surfaces by mosses and higher plants, thereby forming more stable vegetation and more organic soils. With the exposure of new mineral surfaces behind retreating glaciers on a warming Earth, this ecosystem service of Nostoc deserves future attention. Despite profound ecological differences between species, active growth of temperate specimens is mostly restricted to the same temperature range. Future studies should aim to unravel the processes behind the extreme persistence and low metabolism of Nostoc species under ambient resource supply on sediment and soil surfaces.
Sand-Jensen, K. (2014) Ecophysiology of gelatinous Nostoc colonies: unprecedented slow growth and survival in resource-poor and harsh environments. Annals of Botany, 114(1): 17-33.
High School Students’ Learning and Perceptions of Phylogenetics of Flowering Plants. CBE Life Sci Educ vol. 13 no. 4 653-665 doi: 10.1187/cbe.14-04-0074
Basic phylogenetics and associated “tree thinking” are often minimized or excluded in formal school curricula. Informal settings provide an opportunity to extend the K–12 school curriculum, introducing learners to new ideas, piquing interest in science, and fostering scientific literacy. Similarly, university researchers participating in science, technology, engineering, and mathematics (STEM) outreach activities increase awareness of college and career options and highlight interdisciplinary fields of science research and augment the science curriculum. To aid in this effort, we designed a 6-h module in which students utilized 12 flowering plant species to generate morphological and molecular phylogenies using biological techniques and bioinformatics tools. The phylogenetics module was implemented with 83 high school students during a weeklong university STEM immersion program and aimed to increase student understanding of phylogenetics and coevolution of plants and pollinators. Student response reflected positive engagement and learning gains as evidenced through content assessments, program evaluation surveys, and program artifacts. We present the results of the first year of implementation and discuss modifications for future use in our immersion programs as well as in multiple course settings at the high school and undergraduate levels.
Student Interpretations of Phylogenetic Trees in an Introductory Biology Course. CBE Life Sci Educ vol. 13 no. 4 666-676 doi: 10.1187/cbe.14-01-0003
Phylogenetic trees are widely used visual representations in the biological sciences and the most important visual representations in evolutionary biology. Therefore, phylogenetic trees have also become an important component of biology education. We sought to characterize reasoning used by introductory biology students in interpreting taxa relatedness on phylogenetic trees, to measure the prevalence of correct taxa-relatedness interpretations, and to determine how student reasoning and correctness change in response to instruction and over time. Counting synapomorphies and nodes between taxa were the most common forms of incorrect reasoning, which presents a pedagogical dilemma concerning labeled synapomorphies on phylogenetic trees. Students also independently generated an alternative form of correct reasoning using monophyletic groups, the use of which decreased in popularity over time. Approximately half of all students were able to correctly interpret taxa relatedness on phylogenetic trees, and many memorized correct reasoning without understanding its application. Broad initial instruction that allowed students to generate inferences on their own contributed very little to phylogenetic tree understanding, while targeted instruction on evolutionary relationships improved understanding to some extent. Phylogenetic trees, which can directly affect student understanding of evolution, appear to offer introductory biology instructors a formidable pedagogical challenge.
Plant functional types in Earth System Models
Earth system models describe the physical, chemical and biological processes that govern our global climate. While it is difficult to single out one component as being more important than another in these sophisticated models, terrestrial vegetation is a critical player in the biogeochemical and biophysical dynamics of the Earth system. There is much debate, however, as to how plant diversity and function should be represented in these models.
A recent review in Annals of Botany traces the origins of the plant functional types (PFT) concept from its origin in the early 1800s to its current use in regional and global dynamic vegetation models (DVMs). Special attention is given to the representation and parameterization of PFTs and to validation and benchmarking of predicted patterns of vegetation distribution in high-latitude ecosystems. These ecosystems are sensitive to changing climate and thus provide a useful test case for model-based simulations of past, current and future distribution of vegetation.
Wullschleger, Stan D., Howard E. Epstein, Elgene O. Box, Eugénie S. Euskirchen, Santonu Goswami, Colleen M. Iversen, Jens Kattge, Richard J. Norby, Peter M. van Bodegom, and Xiaofeng Xu. (2014) Plant functional types in Earth system models: past experiences and future directions for application of dynamic vegetation models in high-latitude ecosystems. Annals of Botany 114 (1): 1-16. doi: 10.1093/aob/mcu077
Asymmetric cell divisions in plant development
Asymmetric cell divisions define plant development. High-throughput genomic and modelling approaches can elucidate their regulation, which in turn could enable the engineering of plant traits such as stomatal density, lateral root development and wood formation. Asymmetric divisions are formative divisions that generate daughter cells of distinct identity. These divisions are coordinated by either extrinsic (‘niche-controlled’) or intrinsic regulatory mechanisms and are fundamentally important in plant development.
A recent review in Annals of Botany describes how asymmetric cell divisions are regulated during development and in different cell types in both the root and the shoot of plants. It further highlights ways in which omics and modelling approaches have been used to elucidate these regulatory mechanisms. For example, the regulation of embryonic asymmetric divisions is described, including the first divisions of the zygote, formative vascular divisions and divisions that give rise to the root stem cell niche. Asymmetric divisions of the root cortex endodermis initial, pericycle cells that give rise to the lateral root primordium, procambium, cambium and stomatal cells are also discussed. The authors provide a perspective on the role of other hormones or regulatory molecules in asymmetric divisions, the presence of segregated determinants and the usefulness of modelling approaches in understanding network dynamics within these very special cells.
Kajala, Kaisa, Priya Ramakrishna, Adam Fisher, Dominique C. Bergmann, Ive De Smet, Rosangela Sozzani, Dolf Weijers, and Siobhan M. Brady. Omics and modelling approaches for understanding regulation of asymmetric cell divisions in arabidopsis and other angiosperm plants. (2014) Annals of Botany 113(7): 1083-1105.
Bats are responsible for pollinating several species of plants. A new paper in Annals of Botany reports for the first time bat-pollination of a species in the genus Tillandsia.
Bromeliaceae is a species-rich neotropical plant family, of which Tillandsia is the most diverse genus and includes more than a third of all bromeliad species. The flowers of some species show characteristics typical for pollination by nocturnal animals, particularly bats and moths. The authors find that nectar production is restricted to the night hours, and the most frequent visitor and the only pollinator is the nectarivorous bat Anoura geoffroyi. This is the first report of chiropterophily within the genus Tillandsia, and the results suggest an ongoing evolutionary switch from pollination by birds or moths to bats.
Aguilar-Rodríguez, P.A., Krömer, T., García-Franco, J.G., Knauer, A., & Kessler, M. (2014) First record of bat-pollination in the species-rich genus Tillandsia (Bromeliaceae). Annals of Botany, 113 (6): 1047-1055. doi: 10.1093/aob/mcu031
(Sorry, we’ll try to make next year’s Halloween special much scarier than this.)
Hydrometeorology and ecophysiology of cloud forests
Tropical montane cloud forests (TMCFs) are characterized by a unique set of biological and hydroclimatic features, including frequent and/or persistent fog, cool temperatures, and high biodiversity and endemism. These forests are one of the most vulnerable ecosystems to climate change given their small geographic range, high endemism and dependence on a rare microclimatic envelope. The frequency of atmospheric water deficits for some cloud forests is likely to increase in the future, but the consequences for the integrity and distribution of these ecosystems are uncertain. In order to investigate plant and ecosystem responses to climate change, we need to know how cloud forest species function in response to current climate, which factors shape function and ecology most and how these will change into the future.
A recent review in Annals of Botany focuses on recent ecophysiological research of cloud forest plants to establish a link between hydrometeorological conditions and vegetation distribution, functioning and survival. The hydraulic characteristics of cloud forest trees are discussed, together with the prevalence and ecological consequences of foliar uptake of fog water, a key process that allows efficient acquisition of water during cloud immersion periods, minimizing water deficits and favouring survival of species prone to drought-induced hydraulic failure.
Fog occurrence is the single most important microclimatic feature affecting the distribution and function of cloud forest plants. Plants in cloud forests are very vulnerable to drought (possessing a small hydraulic safety margin), and the presence of fog and water uptake minimizes the occurrence of tree water deficits and thus favours the survival of cloud forest trees where such deficits may occur. Characterizing the interplay between microclimatic dynamics and plant water relations is key to foster more realistic projections about climate change effects on cloud forest function and distribution.
Oliveira, R.S., Eller, C. B., Bittencourt, P.R., and Mulligan, M. (2014). The hydroclimatic and ecophysiological basis of cloud forest distributions under current and projected climates. Annals of Botany, 113(6): 909-920. doi: 10.1093/aob/mcu060
Frequency-dependent pollinator discrimination acts against female plants in the gynodioecious Geranium maculatum
Gynodioecy, the co-occurrence of female and hermaphroditic individuals, is thought to be an intermediate step between hermaphroditism and separate sexes, a major transition in flowering plants. This paper suggests that suggest that pollinator discrimination negatively affects females’ relative fitness when they are rare. Thus the initial spread of females in a population, the first step in the evolution of gynodioecy, may be made more difficult due to pollinator discrimination.
Factors affecting stress tolerance in recalcitrant embryonic axes from seeds of four Quercus (Fagaceae) species native to the USA or China
Oaks (Quercus species) are often considered ‘foundation’ components of temperate and/or subtropical forest ecosystems. However, the populations of some species are declining and there is considerable urgency to develop ex situ conservation strategies. In this study, the storage physiology of seeds within Quercus was explored in order to determine factors that affect survival during cryopreservation and to provide a quantitative assessment of seed recalcitrance to support future studies of this complex trait.
Multiple origins of circumboreal taxa in Pyrola (Ericaceae), a group with a Tertiary relict distribution
Two major categories of Northern Hemisphere intercontinental disjunctions are Tertiary relict disjunctions and circumboreal distributions. Tertiary relict disjunctions tend to be older and involve groups from warm temperate to sub-tropical regions, reflecting the warm climates of the Tertiary epoch. Conversely, circumboreal distributions typically involve cold temperate to Arctic-Alpine species, and tend to be younger, reflecting the recent development of these biomes due to global cooling over the past 5 million years. This paper reconstructs the biogeographic history of Pyrola based on a clear phylogenetic analysis and to explore how the genus attained its circumboreal distribution.
Maintenance of species cohesion despite ongoing hybridization
The maintenance of species cohesion despite ongoing gene exchange via natural hybridization in plants is a phenomenon attracting increasing research attention. Natural hybridization can create bridges for gene flow, offering a platform for adaptive evolution by introducing variation and novel traits into populations, potentially resulting in introgression and admixture of genotypes. With advances in genetic technologies, researchers have been able to uncover greater complexity within hybrid populations and we can now delve deeper into how interspecific gene exchange can be ongoing despite the presence of strong reproductive barriers.
When species cohesion is maintained despite ongoing natural hybridization, many questions are raised about the evolutionary processes operating in the species complex. A recent study in Annals of Botany examines the extensive natural hybridization between the Australian native shrubs Lomatia myricoides and L. silaifolia (Proteaceae). These species exhibit striking differences in morphology and ecological preferences, exceeding those found in most studies of hybridization to date. The results show that morphological and ecological distinctions between plant species can be maintained despite ongoing gene flow via natural hybridization. Localized gene flow and introgression are expected to be ongoing between L. myricoides and L. silaifolia and their hybrids wherever they occur in sympatry, due to the permeability of this species barrier.
McIntosh, E. J., Rossetto, M., Weston, P. H., & Wardle, G. M. (2014) Maintenance of strong morphological differentiation despite ongoing natural hybridization between sympatric species of Lomatia (Proteaceae). Annals of Botany, 113 (5): 861-872. doi: 10.1093/aob/mct314
Nuclear microsatellite markers (nSSRs), genotyping methods and morphometric analyses were used to uncover patterns of hybridization and the role of gene flow in morphological differentiation between sympatric species.
The complexity of hybridization patterns differed markedly between sites, however, signals of introgression were present at all sites. One site provided evidence of a large hybrid swarm and the likely presence of multiple hybrid generations and backcrosses, another site a handful of early generational hybrids and a third site only traces of admixture from a past hybridization event. The presence of cryptic hybrids and a pattern of morphological bimodality amongst hybrids often disguised the extent of underlying genetic admixture.
Distinct parental habitats and phenotypes are expected to form barriers that contribute to the rapid reversion of hybrid populations to their parental character state, due to limited opportunities for hybrid/intermediate advantage. Furthermore, strong genomic filters may facilitate continued gene flow between species without the danger of assimilation. Stochastic fire events facilitate temporal phenological isolation between species and may partly explain the bi-directional and site-specific patterns of hybridization observed. Furthermore, the findings suggest that F1 hybrids are rare, and backcrosses may occur rapidly following these initial hybridization events.