Generic phylogeny, historical biogeography and character evolution of the cosmopolitan aquatic plant family Hydrocharitaceae

Megasporogenesis and megagametogenesis in Hydrocleys nymphoides, Alisma plantago-aquatica, and Sagittaria montevidensis (Alismataceae)

Ovule morphology, megasporogenesis, and megagametogenesis processes were examined in Hydrocleys nymphoides, Alisma plantago-aquatica, and Sagittaria montevidensis. Each of these species belongs to a different clade within the Alismataceae family. It is worth mentioning that the genus Hydrocleys previously belonged to the Limnocharitaceae family but is now classified within the Alismataceae. Flowers in different developmental stages were processed following classical histological methods for their observation with bright-field microscope. The three species present an anatropous and bitegmic mature ovule. This is tenuinucellate in A. plantago-aquatica and S. montevidensis and pseudo-crassinucellate in H. nymphoides. Although all three species have the same type of megasporogenesis, they differ in the megagametogenesis and in the total number of nuclei and cells that form the mature gametophyte. H. nymphoides has a female gametophyte composed of four cells and four nuclei, while A. plantago-aquatica and S. montevidensis have a female gametophyte of five cells and six nuclei. The results are discussed according to the phylogenetic position of each of the species. Moreover, new types of megagametophyte development are described: Hydrocleys and Sagittaria types. The reduction of the female gametophyte with respect to the Polygonum type is found in families belonging to the ANA grade and in other aquatic families within the order Alismatales. We infer that the reduction in the number of cells and nuclei in the female gametophyte is characteristic of species that inhabit aquatic environments. Future studies in aquatic species belonging to other families would be necessary to confirm this hypothesis.

Origin and Early Evolution of Hydrocharitaceae and the Ancestral Role of Stratiotes

The combined morphological features of Stratiotes (Hydrocharitaceae) pollen, observed with light and electron microscopy, make it unique among all angiosperm pollen types and easy to identify. Unfortunately, the plant is (and most likely was) insect-pollinated and produces relatively few pollen grains per flower, contributing to its apparent absence in the paleopalynological record. Here, we present fossil Stratiotes pollen from the Eocene of Germany (Europe) and Kenya (Africa), representing the first reliable pre-Pleistocene pollen records of this genus worldwide and the only fossils of this family discovered so far in Africa. The fossil Stratiotes pollen grains are described and compared to pollen from a single modern species, Stratiotes aloides L. The paleophytogeographic significance and paleoecological aspects of these findings are discussed in relation to the Hydrocharitaceae fossil records and molecular phylogeny, as well as the present-day distribution patterns of its modern genera.

Responses of three invasive alien aquatic plant species to climate warming and plant density

Climate warming may impact plant invasion success directly, as well as indirectly through changes among interactions within plant communities. However, the responses of invasive alien aquatic species to plant density and rising temperatures remain largely unknown. We tested the effects of plant density and neighbour plant identity at different temperatures to better understand the performance of a community of invasive species exposed to climate warming. A microcosm experiment was conducted with three invasive aquatic plants species-Elodea canadensis, Egeria densa and Lagarosiphon major-, at mono and polycultures with low and high plant density, at 16 °C, 19 °C and 23 °C. The results clearly demonstrated that rising temperature influenced, either as a single parameter or as a combined factor, at least one of the measured traits of the three invasive species. Leaf area of E. densa, root number of L. major and growth of E. densa and L. major were influenced by temperature, plant density and neighbour identity. Plant density influenced all traits with the exception of leaf area of E. canadensis and lateral branch production of E. densa. Neighbour identity had no effect on growth rate and leaf area of E. canadensis, on lateral branch and roots production of E. densa and on leaf area of L. major. These findings establish that rising temperature could enhance competition or facilitation among E. canadensis, L. major and E. densa and could cancel the beneficial effects of the presence of a neighbour species; however, the magnitude of this effect was strongly dependent on plant density. Rising temperature due to climate change will likely play a crucial role in interactions between invasive species within plant communities and in the further spread of these invasive aquatic plants.

Chloroplast genomic comparison provides insights into the evolution of seagrasses

BACKGROUND

Seagrasses are a polyphyletic group of monocotyledonous angiosperms that have evolved to live entirely submerged in marine waters. Thus, these species are ideal for studying plant adaptation to marine environments. Herein, we sequenced the chloroplast (cp) genomes of two seagrass species (Zostera muelleri and Halophila ovalis) and performed a comparative analysis of them with 10 previously published seagrasses, resulting in various novel findings.

RESULTS

The cp genomes of the seagrasses ranged in size from 143,877 bp (Zostera marina) to 178,261 bp (Thalassia hemprichii), and also varied in size among different families in the following order: Hydrocharitaceae > Cymodoceaceae > Ruppiaceae > Zosteraceae. The length differences between families were mainly related to the expansion and contraction of the IR region. In addition, we screened out 2,751 simple sequence repeats and 1,757 long repeat sequence types in the cp genome sequences of the 12 seagrass species, ultimately finding seven hot spots in coding regions. Interestingly, we found nine genes with positive selection sites, including two ATP subunit genes (atpA and atpF), three ribosome subunit genes (rps4, rps7, and rpl20), one photosystem subunit gene (psbH), and the ycf2, accD, and rbcL genes. These gene regions may have played critical roles in the adaptation of seagrasses to diverse environments. In addition, phylogenetic analysis strongly supported the division of the 12 seagrass species into four previously recognized major clades. Finally, the divergence time of the seagrasses inferred from the cp genome sequences was generally consistent with previous studies.

CONCLUSIONS

In this study, we compared chloroplast genomes from 12 seagrass species, covering the main phylogenetic clades. Our findings will provide valuable genetic data for research into the taxonomy, phylogeny, and species evolution of seagrasses.

Current advances in seagrass research: A review from Viet Nam

Seagrass meadows provide valuable ecosystem services but are fragile and threatened ecosystems all over the world. This review highlights the current advances in seagrass research from Viet Nam. One goal is to support decision makers in developing science-based conservation strategies. In recent years, several techniques were applied to estimate the size of seagrass meadows. Independent from the method used, there is an alarming decline in the seagrass area in almost all parts of Viet Nam. Since 1990, a decline of 46.5% or 13,549 ha was found. Only in a few protected and difficult-to-reach areas was an increase observed. Conditions at those sites could be investigated in more detail to make suggestions for conservation and recovery of seagrass meadows. Due to their lifestyle and morphology, seagrasses take up compounds from their environment easily. Phytoremediation processes of Thalassia hemprichii and Enhalus acoroides are described exemplarily. High accumulation of heavy metals dependent on their concentration in the environment in different organs can be observed. On the one hand, seagrasses play a role in phytoremediation processes in polluted areas; on the other hand, they might suffer at high concentrations, and pollution will contribute to their overall decline. Compared with the neighboring countries, the total C _org stock from seagrass beds in Viet Nam was much lower than in the Philippines and Indonesia but higher than that of Malaysia and Myanmar. Due to an exceptionally long latitudinal coastline of 3,260 km covering cool to warm water environments, the seagrass species composition in Viet Nam shows a high diversity and a high plasticity within species boundaries. This leads to challenges in taxonomic issues, especially with the Halophila genus, which can be better deduced from genetic diversity/population structures of members of Hydrocharitaceae. Finally, the current seagrass conservation and management efforts in Viet Nam are presented and discussed. Only decisions based on the interdisciplinary cooperation of scientists from all disciplines mentioned will finally lead to conserve this valuable ecosystem for mankind and biodiversity.

Correlative adaptation between Rubisco and CO2-concentrating mechanisms in seagrasses

Submerged angiosperms sustain some of the most productive and diverse ecosystems worldwide. However, their carbon acquisition and assimilation mechanisms remain poorly explored, missing an important step in the evolution of photosynthesis during the colonization of aquatic environments by angiosperms. Here we reveal a convergent kinetic adaptation of Rubisco in phylogenetically distant seagrass species that share catalytic efficiencies and CO₂ and O₂ affinities up to three times lower than those observed in phylogenetically closer angiosperms from terrestrial, freshwater and brackish-water habitats. This Rubisco kinetic convergence was found to correlate with the effectiveness of seagrass CO₂-concentrating mechanisms (CCMs), which probably evolved in response to the constant CO₂ limitation in marine environments. The observed Rubisco kinetic adaptation in seagrasses more closely resembles that seen in eukaryotic algae operating CCMs rather than that reported in terrestrial C₄ plants. Our results thus demonstrate a general pattern of co-evolution between Rubisco function and biophysical CCM effectiveness that traverses distantly related aquatic lineages.

Phylogenomic Analyses of Alismatales Shed Light into Adaptations to Aquatic Environments

Land plants first evolved from freshwater algae, and flowering plants returned to water as early as the Cretaceous and multiple times subsequently. Alismatales is the largest clade of aquatic angiosperms including all marine angiosperms, as well as terrestrial plants. We used Alismatales to explore plant adaptations to aquatic environments by analyzing a data set that included 95 samples (89 Alismatales species) covering four genomes and 91 transcriptomes (59 generated in this study). To provide a basis for investigating adaptations, we assessed phylogenetic conflict and whole-genome duplication (WGD) events in Alismatales. We recovered a relationship for the three main clades in Alismatales as (Tofieldiaceae, Araceae) + core Alismatids. We also found phylogenetic conflict among the three main clades that was best explained by incomplete lineage sorting and introgression. Overall, we identified 18 putative WGD events across Alismatales. One of them occurred at the most recent common ancestor of core Alismatids, and three occurred at seagrass lineages. We also found that lineage and life-form were both important for different evolutionary patterns for the genes related to freshwater and marine adaptation. For example, several light- or ethylene-related genes were lost in the seagrass Zosteraceae, but are present in other seagrasses and freshwater species. Stomata-related genes were lost in both submersed freshwater species and seagrasses. Nicotianamine synthase genes, which are important in iron intake, expanded in both submersed freshwater species and seagrasses. Our results advance the understanding of the adaptation to aquatic environments and WGDs using phylogenomics.

Plastome phylogenomics and historical biogeography of aquatic plant genus Hydrocharis (Hydrocharitaceae)

BACKGROUND

Hydrocharis L. and Limnobium Rich. are small aquatic genera, including three and two species, respectively. The taxonomic status, phylogenetic relationships and biogeographical history of these genera have remained unclear, owing to the lack of Central African endemic H. chevalieri from all previous studies. We sequenced and assembled plastomes of all three Hydrocharis species and Limnobium laevigatum to explore the phylogenetic and biogeographical history of these aquatic plants.

RESULTS

All four newly generated plastomes were conserved in genome structure, gene content, and gene order. However, they differed in size, the number of repeat sequences, and inverted repeat borders. Our phylogenomic analyses recovered non-monophyletic Hydrocharis. The African species H. chevalieri was fully supported as sister to the rest of the species, and L. laevigatum was nested in Hydrocharis as a sister to H. dubia. Hydrocharis-Limnobium initially diverged from the remaining genera at ca. 53.3 Ma, then began to diversify at ca. 30.9 Ma. The biogeographic analysis suggested that Hydrocharis probably originated in Europe and Central Africa.

CONCLUSION

Based on the phylogenetic results, morphological similarity and small size of the genera, the most reasonable taxonomic solution to the non-monophyly of Hydrocharis is to treat Limnobium as its synonym. The African endemic H. chevalieri is fully supported as a sister to the remaining species. Hydrocharis mainly diversified in the Miocene, during which rapid climate change may have contributed to the speciation and extinctions. The American species of former Limnobium probably dispersed to America through the Bering Land Bridge during the Miocene.

Complete genus-level plastid phylogenomics of Alismataceae with revisited historical biogeography

Alismataceae, an ancient lineage of monocots, has attracted attention due to its complex evolutionary history, ornamental value, and ecological role. However, the phylogenetic relationships and evolutionary history of the family have not been conclusively resolved. Here, we constructed the first complete genus-level plastid phylogeny of Alismataceae by using 78 genes and updated the historical biogeography based on the phylogenomic tree. Our results divide the Alismataceae into three major clades with robust support values; one clade comprises the former Limnocharitaceae, and the second clade includes the mainly temperate genera Alisma, Baldellia, Damasonium and Luronium, and the monotypic African genus Burnatia as a sister of the temperate genera. The remaining genera are either tropical or have some temperate species in addition to tropical ones, and they constitute the third major clade. Molecular dating and biogeographic analyses suggest that Alismataceae arose in Neotropical, West Palearctic, and Afrotropical regions during the Cretaceous, followed by the split into three main clades due to a combination of vicariance and dispersal events. Unlike earlier studies, we inferred that the mainly temperate clade likely originated from Afrotropical and West Palearctic regions during the Eocene. The most recent common ancestor of the other two clades lived in the Neotropical area during the Late Cretaceous. Long-distance dispersal and vicariance together seem to contribute to the transoceanic distribution of this family.

Liao L, Papenbrock J. Analysis of rDNA reveals a high genetic diversity of Halophila major in the Wallacea region

The genus Halophila shows the highest species diversity within the seagrass genera. Southeast Asian countries where several boundary lines exist were considered as the origin of seagrasses. We hypothesize that the boundary lines, such as Wallace's and Lydekker's Lines, may act as marine geographic barriers to the population structure of Halophila major. Seagrass samples were collected at three islands in Vietnamese waters and analyzed by the molecular maker ITS. These sequences were compared with published ITS sequences from seagrasses collected in the whole region of interest. In this study, we reveal the haplotype and nucleotide diversity, linking population genetics, phylogeography, phylogenetics and estimation of relative divergence times of H. major and other members of the Halophila genus. The morphological characters show variation. The results of the ITS marker analysis reveal smaller groups of H. major from Myanmar, Shoalwater Bay (Australia) and Okinawa (Japan) with high supporting values. The remaining groups including Sri Lanka, Viet Nam, the Philippines, Thailand, Malaysia, Indonesia, Two Peoples Bay (Australia) and Tokushima (Japan) showed low supporting values. The Wallacea region shows the highest haplotype and also nucleotide diversity. Non-significant differences were found among regions, but significant differences were presented among populations. The relative divergence times between some members of section Halophila were estimated 2.15-6.64 Mya.

Phylogenomics of the aquatic plant genus Ottelia (Hydrocharitaceae): Implications for historical biogeography

Ottelia Pers. is the second largest genus of the family Hydrocharitaceae, including approximately 23 extant species. The genus exhibits a diversity of both bisexual and unisexual flowers, and complex reproductive system comprising cross-pollinated to cleistogamous flowers. Ottelia has been regarded as a pivotal group to study the evolution of Hydrocharitaceae, but the phylogenic relationships and evolutionary history of the genus remain unresolved. Here, we reconstructed a robust phylogenetic framework for Ottelia using 40 newly assembled complete plastomes. Our results resolved Ottelia as a monophyletic genus consisting of two major clades, which correspond to the main two centers of diversity in Asia and Africa. According to the divergence time estimation analysis, the crown group Ottelia began to diversify around 13.09 Ma during the middle Miocene. The biogeographical analysis indicated the existence of the most recent common ancestor somewhere in Africa/Australasia/Asia. Basing on further insights from the morphological evolution of Ottelia, we hypothesized that the ancestral center of origin was in Africa, from where the range expanded by transoceanic dispersal to South America and Australasia, and further from Australasia to Asia. We suggested that the climatic change and global cooling since the mid-Miocene, such as the development of East Asian monsoon climate and tectonic movement of the Yunnan-Guizhou Plateau (YGP), might have played a crucial role in the evolution of Ottelia in China.

Cryptic diversity within the African aquatic plant Ottelia ulvifolia (Hydrocharitaceae) revealed by population genetic and phylogenetic analyses

Revealing cryptic diversity is of great importance for effective conservation and understanding macroevolution and ecology of plants. Ottelia, a typical example of aquatic plants, possesses extremely variable morphology and the presence of cryptic diversity makes its classification problematic. Previous studies have revealed cryptic Ottelia species in Asia, but very little is known about the molecular systematics of this genus in Africa, a center of species diversity of Ottelia. In this study, we sampled Ottelia ulvifolia, an endemic species of tropical Africa, from Zambia and Cameroon. We used six chloroplast DNA regions, nrITS and six polymorphic microsatellite markers to estimate the molecular diversity and population genetic structure in O. ulvifolia. The phylogenetic inference, STACEY and STRUCTURE analyses supported at least three clusters within O. ulvifolia, each representing unique flower types (i.e., bisexual yellow flower, unisexual yellow flower and bisexual white flower types). Although abundant genetic variation (> 50%) was observed within the populations, excessive anthropogenic activities may result in genetic drift and bottlenecks. Here, three cryptic species of O. ulvifolia complex are defined, and insights are provided into the taxonomy of Ottelia using the phylogenetic species concept.

Ottelia fengshanensis, a new bisexual species of Ottelia (Hydrocharitaceae) from southwestern China

Ottelia fengshanensis, a new species (Hydrocharitaceae) from southwest China is here described and illustrated. Comparing its morphological features to putative close relatives O. guanyangensis, it has 3-4 flowers (vs. 2-5) each spathe, hexagonal-cylindric fruit, white styles (vs. yellow), green leaves (vs. dark green) and fruit tiny winged (vs. winged obviously). Molecular phylogenetic investigation of four DNA sequences (ITS, rbcL, trnK5' intron and trnS-trnG) and the Poisson Tree Processes model for species delimitation (PTP) analysis, further resolves O. fengshanensis as a new species that is close to O. guanyangensis with distinct support.

Molecular phylogenetic species delimitation in the aquatic genus Ottelia (Hydrocharitaceae) reveals cryptic diversity within a widespread species

Ottelia, a pantropical genus of aquatic plants belonging to the family Hydrocharitaceae, includes several narrowly distributed taxa in Asia. Although the Asian species have received comparatively more research attention than congeners in other areas, various key taxonomic questions remain unaddressed, especially with regards to apparent cryptic diversity within O. alismoides, a widespread species complex native to Asia, northern Australia and tropical Africa. Here we test taxonomic concepts and evaluate species boundaries using a phylogenetic framework. We sampled five of the seven species of Ottelia in Asia as well as each species endemic to Africa and Australia; multiple samples of O. alismoides were obtained from across Asia. Phylogenetic trees based on five plastid DNA markers and the nuclear ITS region shared almost identical topologies. A Bayesian coalescent method of species delimitation using the multi-locus data set discerned one species in Africa, one in Australia and four in Asia with the highest probability. The results lead us to infer that a population sampled in Thailand represents a hitherto unrecognised cryptic taxon within the widespread species complex, although the apparent lack of unambiguous diagnostic characters currently precludes formal description. Conversely, no molecular evidence for distinguishing O. cordata and O. emersa was obtained, and so the latter is synonymised under the former. Two accessions that exhibit inconsistent positions among our phylogenetic trees may represent cases of chloroplast capture, however incomplete lineage sorting or polyploidy are alternative hypotheses that ought to be tested using other molecular markers.

Molecular phylogenetic species delimitation in the aquatic genus Ottelia (Hydrocharitaceae) reveals cryptic diversity within a widespread species

Ottelia, a pantropical genus of aquatic plants belonging to the family Hydrocharitaceae, includes several narrowly distributed taxa in Asia. Although the Asian species have received comparatively more research attention than congeners in other areas, various key taxonomic questions remain unaddressed, especially with regards to apparent cryptic diversity within O. alismoides, a widespread species complex native to Asia, northern Australia and tropical Africa. Here we test taxonomic concepts and evaluate species boundaries using a phylogenetic framework. We sampled five of the seven species of Ottelia in Asia as well as each species endemic to Africa and Australia; multiple samples of O. alismoides were obtained from across Asia. Phylogenetic trees based on five plastid DNA markers and the nuclear ITS region shared almost identical topologies. A Bayesian coalescent method of species delimitation using the multi-locus data set discerned one species in Africa, one in Australia and four in Asia with the highest probability. The results lead us to infer that a population sampled in Thailand represents a hitherto unrecognised cryptic taxon within the widespread species complex, although the apparent lack of unambiguous diagnostic characters currently precludes formal description. Conversely, no molecular evidence for distinguishing O. cordata and O. emersa was obtained, and so the latter is synonymised under the former. Two accessions that exhibit inconsistent positions among our phylogenetic trees may represent cases of chloroplast capture, however incomplete lineage sorting or polyploidy are alternative hypotheses that ought to be tested using other molecular markers.

Genomic comparison of two independent seagrass lineages reveals habitat-driven convergent evolution

Seagrasses are marine angiosperms that live fully submerged in the sea. They evolved from land plant ancestors, with multiple species representing at least three independent return-to-the-sea events. This raises the question of whether these marine angiosperms followed the same adaptation pathway to allow them to live and reproduce under the hostile marine conditions. To compare the basis of marine adaptation between seagrass lineages, we generated genomic data for Halophila ovalis and compared this with recently published genomes for two members of Zosteraceae, as well as genomes of five non-marine plant species (Arabidopsis, Oryza sativa, Phoenix dactylifera, Musa acuminata, and Spirodela polyrhiza). Halophila and Zosteraceae represent two independent seagrass lineages separated by around 30 million years. Genes that were lost or conserved in both lineages were identified. All three species lost genes associated with ethylene and terpenoid biosynthesis, and retained genes related to salinity adaptation, such as those for osmoregulation. In contrast, the loss of the NADH dehydrogenase-like complex is unique to H. ovalis. Through comparison of two independent return-to-the-sea events, this study further describes marine adaptation characteristics common to seagrass families, identifies species-specific gene loss, and provides molecular evidence for convergent evolution in seagrass lineages.

Aquatic Plant Genomics: Advances, Applications, and Prospects

Genomics is a discipline in genetics that studies the genome composition of organisms and the precise structure of genes and their expression and regulation. Genomics research has resolved many problems where other biological methods have failed. Here, we summarize advances in aquatic plant genomics with a focus on molecular markers, the genes related to photosynthesis and stress tolerance, comparative study of genomes and genome/transcriptome sequencing technology.

Flora do Rio de Janeiro: Hydrocharitaceae

Resumo A família Hydrocharitaceae é cosmopolita; composta por 17 gêneros e 127 espécies. São ervas aquáticas fixas ou livres, flutuantes, submersas ou emersas, em água doce, salobra ou salgada. No Brasil ocorrem seis gêneros e cerca de 15 espécies. A fim de conhecer a real diversidade da família no estado do Rio de Janeiro foi realizado o levantamento em herbários e coletas. Registraram-se seis espécies autóctones e duas alóctone em cultivo. Apalanthe granatensis, encontrada em represas, rios e lagoas de água doce, possui flores bissexuais. Egeria densa, semelhante a A. granatensis, distingue-se pelas flores unissexuais. Halophila decipiens, ocorre em praias da Baia de Guanabara; caracteriza-se pelo caule rizomatoso e folhas elípticas. Limnobium laevigatum, ocorre flutuando livremente em lagoas e emersa em alagados, possui folhas flutuantes com espessamento aerenquimatoso abaxial e/ou folhas emersas sem espessamento. Najas arguta var. arguta, exibe caule inerme, folhas concentradas no ápice dos ramos e semente fusiforme, Najas marina, apresenta dentes no caule e nervura principal e semente elíptica; ambas encontradas em lagoas costeiras. Elodea canadensis e Vallisneria sp. foram encontradas em cultivo.

A portrait of the C4 photosynthetic family on the 50th anniversary of its discovery: species number, evolutionary lineages, and Hall of Fame

Fifty years ago, the C₄ photosynthetic pathway was first characterized. In the subsequent five decades, much has been learned about C₄ plants, such that it is now possible to place nearly all C₄ species into their respective evolutionary lineages. Sixty-one independent lineages of C₄ photosynthesis are identified, with additional, ancillary C₄ origins possible in 12 of these principal lineages. The lineages produced ~8100 C₄ species (5044 grasses, 1322 sedges, and 1777 eudicots). Using midpoints of stem and crown node dates in their respective phylogenies, the oldest and most speciose C₄ lineage is the grass lineage Chloridoideae, estimated to be near 30 million years old. Most C₄ lineages are estimated to be younger than 15 million years. Older C₄ lineages tend to be more speciose, while those younger than 7 million years have <43 species each. To further highlight C₄ photosynthesis for a 50th anniversary snapshot, a Hall of Fame comprised of the 40 most significant C₄ species is presented. Over the next 50 years, preservation of the Earth's C₄ diversity is a concern, largely because of habitat loss due to elevated CO₂ effects, invasive species, and expanded agricultural activities. Ironically, some members of the C₄ Hall of Fame are leading threats to the natural C₄ flora due to their association with human activities on landscapes where most C₄ plants occur.

Reproductive Allocation in Three Macrophyte Species from Different Lakes with Variable Eutrophic Conditions

Reproductive allocation is a key process in the plant life cycle and aquatic plants exhibit great diversity in their reproductive systems. In the present study, we conduct a field investigation of three aquatic macrophytes: Stuckenia pectinata, Myriophyllum spicatum, and Potamogeton perfoliatus. Our results showed that widespread species, including S. pectinata and M. spicatum had greater plasticity in their allocation patterns in the form of increased sexual and asexual reproduction, and greater potential to set seeds and increase fitness in more eutrophic environments. P. perfoliatus also exhibited a capacity to adopt varied sexual reproductive strategies such as setting more offspring for the future, although only in clear conditions with low nutrient levels. Our results establish strategies and mechanisms of some species for tolerating and surviving in varied eutrophic lake conditions.

A portrait of the C4 photosynthetic family on the 50th anniversary of its discovery: species number, evolutionary lineages, and Hall of Fame

Fifty years ago, the C4 photosynthetic pathway was first characterized. In the subsequent five decades, much has been learned about C4 plants, such that it is now possible to place nearly all C4 species into their respective evolutionary lineages. Sixty-one independent lineages of C4 photosynthesis are identified, with additional, ancillary C4 origins possible in 12 of these principal lineages. The lineages produced ~8100 C4 species (5044 grasses, 1322 sedges, and 1777 eudicots). Using midpoints of stem and crown node dates in their respective phylogenies, the oldest and most speciose C4 lineage is the grass lineage Chloridoideae, estimated to be near 30 million years old. Most C4 lineages are estimated to be younger than 15 million years. Older C4 lineages tend to be more speciose, while those younger than 7 million years have <43 species each. To further highlight C4 photosynthesis for a 50th anniversary snapshot, a Hall of Fame comprised of the 40 most significant C4 species is presented. Over the next 50 years, preservation of the Earth's C4 diversity is a concern, largely because of habitat loss due to elevated CO2 effects, invasive species, and expanded agricultural activities. Ironically, some members of the C4 Hall of Fame are leading threats to the natural C4 flora due to their association with human activities on landscapes where most C4 plants occur.

Green Web or megabiased clock? Plant fossils from Gondwanan Patagonia speak on evolutionary radiations

Evolutionary divergence-age estimates derived from molecular 'clocks' are frequently correlated with paleogeographic, paleoclimatic and extinction events. One prominent hypothesis based on molecular data states that the dominant pattern of Southern Hemisphere biogeography is post-Gondwanan clade origins and subsequent dispersal across the oceans in a metaphoric 'Green Web'. We tested this idea against well-dated Patagonian fossils of 19 plant lineages, representing organisms that actually lived on Gondwana. Most of these occurrences are substantially older than their respective, often post-Gondwanan molecular dates. The Green Web interpretation probably results from directional bias in molecular results. Gondwanan history remains fundamental to understanding Southern Hemisphere plant radiations, and we urge significantly greater caution when using molecular dating to interpret the biological impacts of geological events.

The phylogeographic structure of Hydrilla verticillata (Hydrocharitaceae) in China and its implications for the biogeographic history of this worldwide-distributed submerged macrophyte

BACKGROUND

Aquatic vascular plants are a distinctive group, differing from terrestrial plants in their growth forms and habitats. Among the various aquatic plant life forms, the evolutionary processes of freshwater submerged species are most likely distinct due to their exclusive occurrence in the discrete and patchy aquatic habitats. Using the chloroplast trnL-F region sequence data, we investigated the phylogeographic structure of a submerged macrophyte, Hydrilla verticillata, the single species in the genus Hydrilla, throughout China, in addition to combined sample data from other countries to reveal the colonisation and diversification processes of this species throughout the world.

RESULTS

We sequenced 681 individuals from 123 sampling locations throughout China and identified a significant phylogeographic structure (NST > GST, p < 0.01), in which four distinct lineages occurred in different areas. A high level of genetic differentiation among populations (global FST = 0.820) was detected. The divergence of Hydrilla was estimated to have occurred in the late Miocene, and the diversification of various clades was dated to the Pleistocene epoch. Biogeographic analyses suggested an East Asian origin of Hydrilla and its subsequent dispersal throughout the world.

CONCLUSIONS

The presence of all four clades in China indicates that China is most likely the centre of Hydrilla genetic diversity. The worldwide distribution of Hydrilla is due to recent vicariance and dispersal events that occurred in different clades during the Pleistocene. Our findings also provide useful information for the management of invasive Hydrilla in North America.

Biochemical and biophysical CO2 concentrating mechanisms in two species of freshwater macrophyte within the genus Ottelia (Hydrocharitaceae)

Two freshwater macrophytes, Ottelia alismoides and O. acuminata, were grown at low (mean 5 μmol L(-1)) and high (mean 400 μmol L(-1)) CO2 concentrations under natural conditions. The ratio of PEPC to RuBisCO activity was 1.8 in O. acuminata in both treatments. In O. alismoides, this ratio was 2.8 and 5.9 when grown at high and low CO2, respectively, as a result of a twofold increase in PEPC activity. The activity of PPDK was similar to, and changed with, PEPC (1.9-fold change). The activity of the decarboxylating NADP-malic enzyme (ME) was very low in both species, while NAD-ME activity was high and increased with PEPC activity in O. alismoides. These results suggest that O. alismoides might perform a type of C4 metabolism with NAD-ME decarboxylation, despite lacking Kranz anatomy. The C4-activity was still present at high CO2 suggesting that it could be constitutive. O. alismoides at low CO2 showed diel acidity variation of up to 34 μequiv g(-1) FW indicating that it may also operate a form of crassulacean acid metabolism (CAM). pH-drift experiments showed that both species were able to use bicarbonate. In O. acuminata, the kinetics of carbon uptake were altered by CO2 growth conditions, unlike in O. alismoides. Thus, the two species appear to regulate their carbon concentrating mechanisms differently in response to changing CO2. O. alismoides is potentially using three different concentrating mechanisms. The Hydrocharitaceae have many species with evidence for C4, CAM or some other metabolism involving organic acids, and are worthy of further study.

Genetic species identification and population structure of Halophila (Hydrocharitaceae) from the Western Pacific to the Eastern Indian Ocean

Background

The Indo-Pacific region has the largest number of seagrass species worldwide and this region is considered as the origin of the Hydrocharitaceae. Halophila ovalis and its closely-related species belonging to the Hydrocharitaceae are well-known as a complex taxonomic challenge mainly due to their high morphological plasticity. The relationship of genetic differentiation and geographic barriers of H. ovalis radiation was not much studied in this region. Are there misidentifications between H. ovalis and its closely related species? Does any taxonomic uncertainty among different populations of H. ovalis persist? Is there any genetic differentiation among populations in the Western Pacific and the Eastern Indian Ocean, which are separated by the Thai-Malay peninsula? Genetic markers can be used to characterize and identify individuals or species and will be used to answer these questions.

Results

Phylogenetic analyses of the nuclear ribosomal internal transcribed spacer region based on materials collected from 17 populations in the Western Pacific and the Eastern Indian Ocean showed that some specimens identified as H. ovalis belonged to the H. major clade, also supported by morphological data. Evolutionary divergence between the two clades is between 0.033 and 0.038, much higher than the evolutionary divergence among H. ovalis populations. Eight haplotypes were found; none of the haplotypes from the Western Pacific is found in India and vice versa. Analysis of genetic diversity based on microsatellite analysis revealed that the genetic diversity in the Western Pacific is higher than in the Eastern Indian Ocean. The unrooted neighbor-joining tree among 14 populations from the Western Pacific and the Eastern Indian Ocean showed six groups. The Mantel test results revealed a significant correlation between genetic and geographic distances among populations. Results from band-based and allele frequency-based approaches from Amplified Fragment Length Polymorphism showed that all samples collected from both sides of the Thai-Malay peninsula were clustered into two clades: Gulf of Thailand and Andaman Sea.

Conclusions

Our study documented the new records of H. major for Malaysia and Myanmar. The study also revealed that the Thai-Malay peninsula is a geographic barrier between H. ovalis populations in the Western Pacific and the Eastern Indian Ocean.

Systematics, biogeography, and character evolution of Sparganium (Typhaceae): diversification of a widespread, aquatic lineage

PREMISE OF THE STUDY

Sparganium (Typhaceae) is a genus of aquatic monocots containing ±14 species, with flowers aggregated in unisexual, spherical heads, and habit ranging from floating to emergent. Sparganium presents an opportunity to investigate diversification, character evolution, and biogeographical relationships in a widespread temperate genus of aquatic monocots. We present a fossil-calibrated, molecular phylogeny of Sparganium based on analysis of two chloroplast and two nuclear markers. Within this framework, we examine character evolution in both habit and stigma number and infer the ancestral area and biogeographic history of the genus. •

METHODS

Sequence data from two cpDNA and two nDNA markers were analyzed using maximum parsimony, maximum likelihood, and Bayesian inference. We used the program BEAST to simultaneously estimate phylogeny and divergence times, S-DIVA and Lagrange for biogeographical reconstruction, and BayesTraits to examine locule number and habit evolution. •

KEY RESULTS

Two major clades were recovered with strong support: one composed of S. erectum and S. eurycarpum; and the other containing all remaining Sparganium. We realigned the subgenera to conform to these clades. Divergence time analysis suggests a Miocene crown origin but Pliocene diversification. Importantly, the floating-leaved habit has arisen multiple times in the genus, from emergent ancestors-contrary to past hypotheses. •

CONCLUSIONS

Cooling trends during the Tertiary are correlated with the isolation of temperate Eurasian and North American taxa. Vicariance, long-distance dispersal, and habitat specialization are proposed as mechanisms for Sparganium diversification.

Eurasian origin of Alismatidae inferred from statistical dispersal-vicariance analysis

Alismatidae is a wetland or aquatic herb lineage of monocots with a cosmopolitan distribution. Although considerable progress in systematics and biogeography has been made in the past several decades, geographical origin of this group remains unresolved. In this study, we used statistical dispersal-vicariance analysis implemented in program RASP to investigate the biogeography of Alismatidae. Six areas of endemism were used to describe the distribution: North America, South America, Eurasia, Africa, Southeast Asia and Australia. 18,000 trees retained from Bayesian inference of rbcL served as a framework to reconstruct the ancestral areas. The results suggested that the most recent common ancestor of Alismatidae most probably occurred in Eurasia, followed by a split into two major clades. The clade comprising Hydrocharitaceae, Butomaceae and Alismataceae mainly diversified in Eurasia and Africa. The other clade comprising the remaining families dispersed to southern hemisphere. Australia played an important role in diversification of this clade. Several families were suggested to have occurred in Australia, such as Ruppiaceae, Cymodoceaceae, Posidoniaceae and Zosteraceae.