A macro-ecological perspective on crassulacean acid metabolism (CAM) photosynthesis evolution in Afro-Madagascan drylands: Eulophiinae orchids as a case study

The Cissus quadrangularis genome reveals its adaptive features in an arid habitat

Cissus quadrangularis is a tetraploid species belonging to the Vitaceae family and is known for the Crassulacean acid metabolism (CAM) pathway in the succulent stem, while the leaves perform C3 photosynthesis. Here, we report a high-quality genome of C. quadrangularis comprising a total size of 679.2 Mb which was phased into two subgenomes. Genome annotation identified 51 857 protein-coding genes, while approximately 47.75% of the genome was composed of repetitive sequences. Gene expression ratios of two subgenomes demonstrated that the sub-A genome as the dominant subgenome played a vital role during the drought tolerance. Genome divergence analysis suggests that the tetraploidization event occurred around 8.9 million years ago. Transcriptome data revealed that pathways related to cutin, suberine, and wax metabolism were enriched in the stem during drought treatment, suggesting that these genes contributed to the drought adaption. Additionally, a subset of CAM-related genes displayed diurnal expression patterns in the succulent stems but not in leaves, indicating that stem-biased expression of existing genes contributed to the CAM evolution. Our findings provide insights into the mechanisms of drought adaptation and photosynthesis transition in plants.

The CAM lineages of planet Earth

BACKGROUND AND SCOPE

The growth of experimental studies of crassulacean acid metabolism (CAM) in diverse plant clades, coupled with recent advances in molecular systematics, presents an opportunity to re-assess the phylogenetic distribution and diversity of species capable of CAM. It has been more than two decades since the last comprehensive lists of CAM taxa were published, and an updated survey of the occurrence and distribution of CAM taxa is needed to facilitate and guide future CAM research. We aimed to survey the phylogenetic distribution of these taxa, their diverse morphology, physiology and ecology, and the likely number of evolutionary origins of CAM based on currently known lineages.

RESULTS AND CONCLUSIONS

We found direct evidence (in the form of experimental or field observations of gas exchange, day-night fluctuations in organic acids, carbon isotope ratios and enzymatic activity) for CAM in 370 genera of vascular plants, representing 38 families. Further assumptions about the frequency of CAM species in CAM clades and the distribution of CAM in the Cactaceae and Crassulaceae bring the currently estimated number of CAM-capable species to nearly 7 % of all vascular plants. The phylogenetic distribution of these taxa suggests a minimum of 66 independent origins of CAM in vascular plants, possibly with dozens more. To achieve further insight into CAM origins, there is a need for more extensive and systematic surveys of previously unstudied lineages, particularly in living material to identify low-level CAM activity, and for denser sampling to increase phylogenetic resolution in CAM-evolving clades. This should allow further progress in understanding the functional significance of this pathway by integration with studies on the evolution and genomics of CAM in its many forms.

CAM plants: their importance in epiphyte communities and prospects with global change

BACKGROUND AND SCOPE

The epiphytic life form characterizes almost 10 % of all vascular plants. Defined by structural dependence throughout their life and their non-parasitic relationship with the host, the term epiphyte describes a heterogeneous and taxonomically diverse group of plants. This article reviews the importance of crassulacean acid metabolism (CAM) among epiphytes in current climatic conditions and explores the prospects under global change.

RESULTS AND CONCLUSIONS

We question the view of a disproportionate importance of CAM among epiphytes and its role as a 'key innovation' for epiphytism but do identify ecological conditions in which epiphytic existence seems to be contingent on the presence of this photosynthetic pathway. Possibly divergent responses of CAM and C3 epiphytes to future changes in climate and land use are discussed with the help of experimental evidence, current distributional patterns and the results of several long-term descriptive community studies. The results and their interpretation aim to stimulate a fruitful discussion on the role of CAM in epiphytes in current climatic conditions and in altered climatic conditions in the future.

Low internal air space in plants with crassulacean acid metabolism may be an anatomical spandrel

Crassulacean acid metabolism (CAM) is a photosynthetic adaptation found in at least 38 plant families. Typically, the anatomy of CAM plants is characterized by large photosynthetic cells and a low percentage of leaf volume consisting of internal air space (% IAS). It has been suggested that reduced mesophyll conductance (gm) arising from low % IAS benefits CAM plants by preventing the movement of CO2 out of cells and ultimately minimizing leakage of CO2 from leaves into the atmosphere during day-time decarboxylation. Here, we propose that low % IAS does not provide any adaptive benefit to CAM plants, because stomatal closure during phase III of CAM will result in internal concentrations of CO2 becoming saturated, meaning low gm will not have any meaningful impact on the flux of gases within leaves. We suggest that low % IAS is more likely an indirect consequence of maximizing the cellular volume within a leaf, to provide space for the overnight storage of malic acid during the CAM cycle.

The diverse diaspora of CAM: a pole-to-pole sketch

BACKGROUND

Crassulacean acid metabolism (CAM) photosynthesis is a successful adaptation that has evolved often in angiosperms, gymnosperms, ferns and lycophytes. Present in ~5 % of vascular plants, the CAM diaspora includes all continents apart from Antarctica. Species with CAM inhabit most landscapes colonized by vascular plants, from the Arctic Circle to Tierra del Fuego, from below sea level to 4800 m a.s.l., from rainforests to deserts. They have colonized terrestrial, epiphytic, lithophytic, palustrine and aquatic systems, developing perennial, annual or geophyte strategies that can be structurally arborescent, shrub, forb, cladode, epiphyte, vine or leafless with photosynthetic roots. CAM can enhance survival by conserving water, trapping carbon, reducing carbon loss and/or via photoprotection.

SCOPE

This review assesses the phylogenetic diversity and historical biogeography of selected lineages with CAM, i.e. ferns, gymnosperms and eumagnoliids, Orchidaceae, Bromeliaceae, Crassulaceae, Euphorbiaceae, Aizoaceae, Portulacineae (Montiaceae, Basellaceae, Halophytaceae, Didiereaceae, Talinaceae, Portulacaceae, Anacampserotaceae and Cactaceae) and aquatics.

CONCLUSIONS

Most extant CAM lineages diversified after the Oligocene/Miocene, as the planet dried and CO2 concentrations dropped. Radiations exploited changing ecological landscapes, including Andean emergence, Panamanian Isthmus closure, Sundaland emergence and submergence, changing climates and desertification. Evidence remains sparse for or against theories that CAM biochemistry tends to evolve before pronounced changes in anatomy and that CAM tends to be a culminating xerophytic trait. In perennial taxa, any form of CAM can occur depending upon the lineage and the habitat, although facultative CAM appears uncommon in epiphytes. CAM annuals lack strong CAM. In CAM annuals, C3 + CAM predominates, and inducible or facultative CAM is common.

Elevated nocturnal respiratory rates in the mitochondria of CAM plants: current knowledge and unanswered questions

Crassulacean acid metabolism (CAM) is a metabolic adaptation that has evolved convergently in 38 plant families to aid survival in water-limited niches. Whilst primarily considered a photosynthetic adaptation, CAM also has substantial consequences for nocturnal respiratory metabolism. Here, we outline the history, current state and future of nocturnal respiration research in CAM plants, with a particular focus on the energetics of nocturnal respiratory oxygen consumption. Throughout the 20th century, research interest in nocturnal respiration occurred alongside initial discoveries of CAM, although the energetic and mechanistic implications of nocturnal oxygen consumption and links to the operation of the CAM cycle were not fully understood. Recent flux balance analysis (FBA) models have provided new insights into the role that mitochondria play in the CAM cycle. Several FBA models have predicted that CAM requires elevated nocturnal respiratory rates, compared to C3 species, to power vacuolar malic acid accumulation. We provide physiological data, from the genus Clusia, to corroborate these modelling predictions, thereby reinforcing the importance of elevated nocturnal respiratory rates for CAM. Finally, we outline five unanswered questions pertaining to nocturnal respiration which must be addressed if we are to fully understand and utilize CAM plants in a hotter, drier world.

The Agavoideae: an emergent model clade for CAM evolutionary biology

Crassulacean acid metabolism - or CAM photosynthesis - was described in the early to mid-20th century, and our understanding of this metabolic pathway was later expanded upon through detailed biochemical analyses of carbon balance. Soon after, scientists began to study the ecophysiological implications of CAM, and a large part of this early work was conducted in the genus Agave, in the subfamily Agavoideae of the family Asparagaceae. Today, the Agavoideae continues to be important for the study of CAM photosynthesis, from the ecophysiology of CAM species, to the evolution of the CAM phenotype and to the genomics underlying CAM traits. Here we review past and current work on CAM in the Agavoideae, in particular highlighting the work of Park Nobel in Agave, and focusing on the powerful comparative system the Agavoideae has become for studying the origins of CAM. We also highlight new genomics research and the potential for studying intraspecific variation within species of the Agavoideae, particularly species in the genus Yucca. The Agavoideae has served as an important model clade for CAM research for decades, and undoubtedly will continue to help push our understanding of CAM biology and evolution in the future.

Atmospheric CO2 decline and the timing of CAM plant evolution

BACKGROUND AND AIMS

CAM photosynthesis is hypothesized to have evolved in atmospheres of low CO2 concentration in recent geological time because of its ability to concentrate CO2 around Rubisco and boost water use efficiency relative to C3 photosynthesis. We assess this hypothesis by compiling estimates of when CAM clades arose using phylogenetic chronograms for 73 CAM clades. We further consider evidence of how atmospheric CO2 affects CAM relative to C3 photosynthesis.

RESULTS

Where CAM origins can be inferred, strong CAM is estimated to have appeared in the past 30 million years in 46 of 48 examined clades, after atmospheric CO2 had declined from high (near 800 ppm) to lower (<450 ppm) values. In turn, 21 of 25 clades containing CAM species (but where CAM origins are less certain) also arose in the past 30 million years. In these clades, CAM is probably younger than the clade origin. We found evidence for repeated weak CAM evolution during the higher CO2 conditions before 30 million years ago, and possible strong CAM origins in the Crassulaceae during the Cretaceous period prior to atmospheric CO2 decline. Most CAM-specific clades arose in the past 15 million years, in a similar pattern observed for origins of C4 clades.

CONCLUSIONS

The evidence indicates strong CAM repeatedly evolved in reduced CO2 conditions of the past 30 million years. Weaker CAM can pre-date low CO2 and, in the Crassulaceae, strong CAM may also have arisen in water-limited microsites under relatively high CO2. Experimental evidence from extant CAM species demonstrates that elevated CO2 reduces the importance of nocturnal CO2 fixation by increasing the contribution of C3 photosynthesis to daily carbon gain. Thus, the advantage of strong CAM would be reduced in high CO2, such that its evolution appears less likely and restricted to more extreme environments than possible in low CO2.

Diversification of flowering plants in space and time

The rapid diversification and high species richness of flowering plants is regarded as 'Darwin's second abominable mystery'. Today the global spatiotemporal pattern of plant diversification remains elusive. Using a newly generated genus-level phylogeny and global distribution data for 14,244 flowering plant genera, we describe the diversification dynamics of angiosperms through space and time. Our analyses show that diversification rates increased throughout the early Cretaceous and then slightly decreased or remained mostly stable until the end of the Cretaceous-Paleogene mass extinction event 66 million years ago. After that, diversification rates increased again towards the present. Younger genera with high diversification rates dominate temperate and dryland regions, whereas old genera with low diversification dominate the tropics. This leads to a negative correlation between spatial patterns of diversification and genus diversity. Our findings suggest that global changes since the Cenozoic shaped the patterns of flowering plant diversity and support an emerging consensus that diversification rates are higher outside the tropics.

High-quality Cymbidium mannii genome and multifaceted regulation of crassulacean acid metabolism in epiphytes

Epiphytes with crassulacean acid metabolism (CAM) photosynthesis are widespread among vascular plants, and repeated evolution of CAM photosynthesis is a key innovation for micro-ecosystem adaptation. However, we lack a complete understanding of the molecular regulation of CAM photosynthesis in epiphytes. Here, we report a high-quality chromosome-level genome assembly of a CAM epiphyte, Cymbidium mannii (Orchidaceae). The 2.88-Gb orchid genome with a contig N50 of 22.7 Mb and 27 192 annotated genes was organized into 20 pseudochromosomes, 82.8% of which consisted of repetitive elements. Recent expansions of long terminal repeat retrotransposon families have made a major contribution to the evolution of genome size in Cymbidium orchids. We reveal a holistic scenario of molecular regulation of metabolic physiology using high-resolution transcriptomics, proteomics, and metabolomics data collected across a CAM diel cycle. Patterns of rhythmically oscillating metabolites, especially CAM-related products, reveal circadian rhythmicity in metabolite accumulation in epiphytes. Genome-wide analysis of transcript and protein level regulation revealed phase shifts during the multifaceted regulation of circadian metabolism. Notably, we observed diurnal expression of several core CAM genes (especially βCA and PPC) that may be involved in temporal fixation of carbon sources. Our study provides a valuable resource for investigating post-transcription and translation scenarios in C. mannii, an Orchidaceae model for understanding the evolution of innovative traits in epiphytes.

The climate changes promoted the chloroplast genomic evolution of Dendrobium orchids among multiple photosynthetic pathways

Dendrobium orchids have multiple photosynthetic pathways, which can be used as a model system for studying the evolution of crassulacean acid metabolism (CAM). In this study, based on the results of the net photosynthetic rates (P_n), we classified Dendrobium species into three photosynthetic pathways, then employed and compared their chloroplast genomes. The Dendrobium chloroplast genomes have typical quartile structures, ranging from 150,841-153,038 bp. The apparent differences in GC content, sequence variability, and IR junctions of SSC/IR_B junctions (J_SBs) were measured within chloroplast genomes among different photosynthetic pathways. The phylogenetic analysis has revealed multiple independent CAM origins among the selected Dendrobium species. After counting insertions and deletions (InDels), we found that the occurrence rates and distribution densities among different photosynthetic pathways were inconsistent. Moreover, the evolution patterns of chloroplast genes in Dendrobium among three photosynthetic pathways were also diversified. Considering the diversified genome structure variations and the evolution patterns of protein-coding genes among Dendrobium species, we proposed that the evolution of the chloroplast genomes was disproportional among different photosynthetic pathways. Furthermore, climatic correlation revealed that temperature and precipitation have influenced the distribution among different photosynthetic pathways and promoted the foundation of CAM pathway in Dendrobium orchids. Based on our study, we provided not only new insights into the CAM evolution of Dendrobium but also provided beneficial genetic data resources for the further systematical study of Dendrobium.

Advances and prospects of orchid research and industrialization

Orchidaceae is one of the largest, most diverse families in angiosperms with significant ecological and economical values. Orchids have long fascinated scientists by their complex life histories, exquisite floral morphology and pollination syndromes that exhibit exclusive specializations, more than any other plants on Earth. These intrinsic factors together with human influences also make it a keystone group in biodiversity conservation. The advent of sequencing technologies and transgenic techniques represents a quantum leap in orchid research, enabling molecular approaches to be employed to resolve the historically interesting puzzles in orchid basic and applied biology. To date, 16 different orchid genomes covering four subfamilies (Apostasioideae, Vanilloideae, Epidendroideae, and Orchidoideae) have been released. These genome projects have given rise to massive data that greatly empowers the studies pertaining to key innovations and evolutionary mechanisms for the breadth of orchid species. The extensive exploration of transcriptomics, comparative genomics, and recent advances in gene engineering have linked important traits of orchids with a multiplicity of gene families and their regulating networks, providing great potential for genetic enhancement and improvement. In this review, we summarize the progress and achievement in fundamental research and industrialized application of orchids with a particular focus on molecular tools, and make future prospects of orchid molecular breeding and post-genomic research, providing a comprehensive assemblage of state of the art knowledge in orchid research and industrialization.

Uncovering haplotype diversity in cultivated Mexican vanilla species

PREMISE

Although vanilla is one of the best-known spices, there is only a limited understanding of its biology and genetics within Mexico, where its cultivation originated and where phenotypic variability is high. This study aims to augment our understanding of vanilla's genetic resources by assessing species delimitation and genetic, geographic, and climatic variability within Mexican cultivated vanilla.

METHODS

Using nuclear and plastid DNA sequence data from 58 Mexican samples collected from three regions and 133 ex situ accessions, we assessed species monophyly using phylogenetic analyses and genetic distances. Intraspecific genetic variation was summarized through the identification of haplotypes. Within the primarily cultivated species, Vanilla planifolia, haplotype relationships were further verified using plastome and rRNA gene sequences. Climatic niche and haplotype composition were assessed across the landscape.

RESULTS

Three species (Vanilla planifolia, V. pompona, and V. insignis) and 13 haplotypes were identified among Mexican vanilla. Within V. planifolia haplotypes, hard phylogenetic incongruences between plastid and nuclear sequences suggest past hybridization events. Eight haplotypes consisted exclusively of Mexican samples. The dominant V. planifolia haplotype occurred throughout all three regions as well as outside of its country of origin. Haplotype richness was found to be highest in regions around Papantla and Chinantla.

CONCLUSIONS

Long histories of regional cultivation support the consideration of endemic haplotypes as landraces shaped by adaptation to local conditions and/or hybridization. Results may aid further genomic investigations of vanilla's genetic resources and ultimately support the preservation of genetic diversity within the economically important crop.

Diversification Slowdown in the Cirrhopetalum Alliance (Bulbophyllum, Orchidaceae): Insights From the Evolutionary Dynamics of Crassulacean Acid Metabolism

Evolutionary slowdowns in diversification have been inferred in various plant and animal lineages. Investigation based on diversification models integrated with environmental factors and key characters could provide critical insights into this diversification trend. We evaluate diversification rates in the Cirrhopetalum alliance (Bulbophyllum, Orchidaceae subfam. Epidendroideae) using a time-calibrated phylogeny and assess the role of Crassulacean acid metabolism (CAM) as a hypothesised key innovation promoting the spectacular diversity of orchids, especially those with an epiphytic habit. An explosive early speciation in the Cirrhopetalum alliance is evident, with the origin of CAM providing a short-term advantage under the low atmospheric CO2 concentrations (pCO2) associated with cooling and aridification in the late Miocene. A subsequent slowdown of diversification in the Cirrhopetalum alliance is possibly explained by a failure to keep pace with pCO2 dynamics. We further demonstrate that extinction rates in strong CAM lineages are ten times higher than those of C3 lineages, with CAM not as evolutionarily labile as previously assumed. These results challenge the role of CAM as a "key innovation" in the diversification of epiphytic orchids.

CAM photosynthesis: the acid test

There is currently considerable interest in the prospects for bioengineering crassulacean acid metabolism (CAM) photosynthesis - or key elements associated with it, such as increased water-use efficiency - into C₃ plants. Resolving how CAM photosynthesis evolved from the ancestral C₃ pathway could provide valuable insights into the targets for such bioengineering efforts. It has been proposed that the ability to accumulate organic acids at night may be common among C₃ plants, and that the transition to CAM might simply require enhancement of pre-existing fluxes, without the need for changes in circadian or diurnal regulation. We show, in a survey encompassing 40 families of vascular plants, that nocturnal acidification is a feature entirely restricted to CAM species. Although many C₃ species can synthesize malate during the light period, we argue that the switch to night-time malic acid accumulation requires a fundamental metabolic reprogramming that couples glycolytic breakdown of storage carbohydrate to the process of net dark CO₂ fixation. This central element of the CAM pathway, even when expressed at a low level, represents a biochemical capability not seen in C₃ plants, and so is better regarded as a discrete evolutionary innovation than as part of a metabolic continuum between C₃ and CAM.

Evolution of crassulacean acid metabolism (CAM) as an escape from ecological niche conservatism in Malagasy Bulbophyllum (Orchidaceae)

Despite growing evidence that niche shifts are more common in flowering plants than previously thought, little is known of whether such shifts are promoted by changes in photosynthetic pathways. Here we combine the most complete phylogeny for epiphytic Malagasy Bulbophyllum orchids (c. 210 spp.) with climatic niche and carbon isotope ratios to infer the group's spatial-temporal history, and the role of strongly expressed crassulacean acid metabolism (CAM) in facilitating niche shifts and diversification. We find that most extant species still retain niche (Central Highland) and photosynthesis (C₃ ) states as present in the single mid-Miocene (c. 12.70 million yr ago (Ma)) ancestor colonizing Madagascar. However, we also infer a major transition to CAM, linked to a late Miocene (c. 7.36 Ma) invasion of species from the sub-humid highland first into the island's humid eastern coastal, and then into the seasonally dry 'Northwest Sambirano' rainforests, yet without significant effect on diversification rates. These findings indicate that CAM in tropical epiphytes may be selectively advantageous even in high rainfall habitats, rather than presenting a mere adaptation to dry environments or epiphytism per se. Overall, our study qualifies CAM as an evolutionary 'gateway' trait that considerably widened the spatial-ecological amplitude of Madagascar's most species-rich orchid genus.

Crassulacean acid metabolism (CAM) supersedes the turgor loss point (TLP) as an important adaptation across a precipitation gradient, in the genus Clusia

As future climates continue to change, precipitation deficits are expected to become more severe across tropical ecosystems. As a result, it is important that we identify plant physiological traits that act as adaptations to drought, and determine whether these traits act synergistically or independently of each other. In this study, we assessed the role of three leaf-level putative adaptations to drought: crassulacean acid metabolism (CAM), the turgor loss point (TLPΨ) and water storage hydrenchyma tissue. Using the genus Clusia as a model, we were able to explore the extent to which these leaf physiological traits co-vary, and also how they contribute to species' distributions across a precipitation gradient in Central and South America. We found that CAM is independent of the TLPΨ and hydrenchyma depth in Clusia. In addition, we provide evidence that constitutive CAM is an adaptation to year-long water deficits, whereas facultative CAM appears to be more important for surviving acute dry seasons. Finally, we find that the other leaf traits tested did not correlate with environmental precipitation, suggesting that the reduced transpirational rates associated with CAM obviate the need to adapt the TLPΨ and hydrenchyma depth in this genus.

Exploring C4-CAM plasticity within the Portulaca oleracea complex

Portulaca oleracea is a C₄ herb capable of performing CAM under drought stress. It is distributed worldwide and is either considered a polymorphic species or a complex of subspecies, due to its numerous morphological variations. We evaluated CAM plasticity within P. oleracea genotypes since the complexity surrounding this species may be reflected in intraspecific variations in photosynthetic behavior. Eleven subspecies of P. oleracea from distant geographical locations and one cultivar were morphologically and physiologically characterized. C₄ and CAM photosynthesis were monitored in plants exposed to well-watered, droughted and rewatered treatments, and data obtained were compared among individual genotypes. All subspecies expressed CAM in a fully-reversible manner. Transcript abundance of C₄–CAM signature genes was shown to be a useful indicator of the C₄–CAM–C₄ switches in all genotypes. C₄-related genes were down-regulated and subsequently fully expressed upon drought and rewatering, respectively. CAM-marker genes followed the opposite pattern. A gradient of morphological traits and drought-induced nighttime malate accumulation was observed across genotypes. Therefore, different combinations of CAM expression levels, plant sizes and shapes are available within the P. oleracea complex, which can be a valuable tool in the context of C₄/CAM photosynthesis research.

A perspective on crassulacean acid metabolism photosynthesis evolution of orchids on different continents: Dendrobium as a case study

Members of the Orchidaceae, one of the largest families of flowering plants, evolved the crassulacean acid metabolism (CAM) photosynthesis strategy. It is thought that CAM triggers adaptive radiation into new niche spaces, yet very little is known about its origin and diversification on different continents. Here, we assess the prevalence of CAM in Dendrobium, which is one of the largest genera of flowering plants and found in a wide range of environments, from the high altitudes of the Himalayas to relatively arid habitats in Australia. Based on phylogenetic time trees, we estimated that CAM, as determined by δ 13C values less negative than -20.0‰, evolved independently at least eight times in Dendrobium. The oldest lineage appeared in the Asian clade during the middle Miocene, indicating the origin of CAM was associated with a pronounced climatic cooling that followed a period of aridity. Divergence of the four CAM lineages in the Asian clade appeared to be earlier than divergence of those in the Australasian clade. However, CAM species in the Asian clade are much less diverse (25.6%) than those in the Australasian clade (57.9%). These findings shed new light on CAM evolutionary history and the aridity levels of the paleoclimate on different continents.

Understanding trait diversity associated with crassulacean acid metabolism (CAM)

Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis that exploits a temporal CO₂ pump with nocturnal CO₂ uptake and concentration to reduce photorespiration, improve water-use efficiency (WUE), and optimize the adaptability of plants to climates with seasonal or intermittent water limitations. CAM plants display a plastic continuum in the extent to which species engage in net nocturnal CO₂ uptake that ranges from 0 to 100%. CAM plants also display diverse enzyme and organic acid and carbohydrate storage systems, which likely reflect the multiple, independent evolutionary origins of CAM. CAM is often accompanied by a diverse set of anatomical traits, such as tissue succulence and water-storage and water-capture strategies to attenuate drought. Other co-adaptive traits, such as thick cuticles, epicuticular wax, low stomatal density, high stomatal responsiveness, and shallow rectifier-like roots limit water loss under conditions of water deficit. Recommendations for future research efforts to better explore and understand the diversity of traits associated with CAM and CAM Biodesign efforts are presented.

Multiple Geographical Origins of Environmental Sex Determination enhanced the diversification of Darwin's Favourite Orchids

Environmental sex determination (ESD) - a change in sexual function during an individual life span driven by environmental cues - is an exceedingly rare sexual system among angiosperms. Because ESD can directly affect reproduction success, it could influence diversification rate as compared with lineages that have alternative reproductive systems. Here we test this hypothesis using a solid phylogenetic framework of Neotropical Catasetinae, the angiosperm lineage richest in taxa with ESD. We assess whether gains of ESD are associated with higher diversification rates compared to lineages with alternative systems while considering additional traits known to positively affect diversification rates in orchids. We found that ESD has evolved asynchronously three times during the last ~5 Myr. Lineages with ESD have consistently higher diversification rates than related lineages with other sexual systems. Habitat fragmentation due to mega-wetlands extinction, and climate instability are suggested as the driving forces for ESD evolution.

Australia lacks stem succulents but is it depauperate in plants with crassulacean acid metabolism (CAM)?

In the flora of Australia, the driest vegetated continent, crassulacean acid metabolism (CAM), the most water-use efficient form of photosynthesis, is documented in only 0.6% of native species. Most are epiphytes and only seven terrestrial. However, much of Australia is unsurveyed, and carbon isotope signature, commonly used to assess photosynthetic pathway diversity, does not distinguish between plants with low-levels of CAM and C3 plants. We provide the first census of CAM for the Australian flora and suggest that the real frequency of CAM in the flora is double that currently known, with the number of terrestrial CAM species probably 10-fold greater. Still unresolved is the question why the large stem-succulent life - form is absent from the native Australian flora even though exotic large cacti have successfully invaded and established in Australia.

Frequent but asymmetric niche shifts in Bulbophyllum orchids support environmental and climatic instability in Madagascar over Quaternary time scales

BACKGROUND

Species or clades may retain or shift their environmental niche space over evolutionary time. Understanding these processes offers insights into the environmental processes fuelling lineage diversification and might also provide information on past range dynamics of ecosystems. However, little is known about the relative contributions of niche conservatism versus niche divergence to species diversification in the tropics. Here, we examined broad-scale patterns of niche evolution within a Pliocene-Pleistocene clade of epiphytic Bulbophyllum orchids (30 spp.) whose collective distribution covers the northwest and eastern forest ecosystems of Madagascar.

RESULTS

Using species occurrence data, ecological niche models, and multivariate analyses of contributing variables, we identified a three-state niche distribution character for the entire clade, coinciding with three major forest biomes viz. phytogeographical provinces in Madagascar: A, Northwest 'Sambirano'; B, 'Eastern Lowlands'; and C, 'Central Highlands'. A time-calibrated phylogeny and Bayesian models of niche evolution were then used to detect general trends in the direction of niche change over the clade's history (≤5.3 Ma). We found highest transitions rates between lowlands (A and B) and (mostly from B) into the highland (C), with extremely low rates out of the latter. Lowland-to-highland transitions occurred frequently during the Quaternary, suggesting that climate-induced vegetational shifts promoted niche transitions and ecological speciation at this time.

CONCLUSIONS

Our results reveal that niche transitions occurred frequently and asymmetrically within this Madagascan orchid clade, and in particular over Quaternary time scales. Intrinsic features germane to Bulbophyllum (e.g., high dispersal ability, drought tolerance, multiple photosynthetic pathways) as well as extrinsic factors (ecological, historical) likely interacted to generate the niche transition patterns observed. In sum, our results support the emerging idea of dramatic environmental and climatic fluctuations in Madagascar during the recent geological past, which overturns the long-held paradigm of long-term stability in tropical forest settings. The generality of the patterns and timings reported here awaits the availability of additional comparative studies in other Madagascan endemics.