Chemotaxonomy of some Paleozoic vascular plants. Part I: Chemical compositions and preliminary cluster analyses

The chemical compositions of plant fossils having trimerophyte (Pertica,Psilophyton princeps, P. cf.jorbesii), rhyniophyte (Taeniocrada), and zosterophyll (Sawdonia) morphological characteristics are chemically analyzed and chemotaxonomically related to vascular ( ?Eohostimella, Renalia, Chahuria) and putative non-vascular plant fossils (Botryococcus, Parka, Pachytheca, Prototaxites,Nematothallus, Spongiophyton, Protosalvinia, Orestovia) whose taxonomic affinities are unknown or speculative. Separation of the material examined into clusters representing higher taxa (i.e., algal, nonalgal, trimerophyte, rhyniophyte, zosterophyll plant groupings) is effected by the weighting of chemical data during cluster analyses. The weighting of phenolic and monohydroxycarboxylic acid constituents is shown to cluster vascular plant material, while the criteria of carbon chainlength ranges and maxima separate vascular from non-vascular plant fossils. Multivariate analysis of the data, using chemical and geological factors, results in the clustering of four groups: (1)Botryococcus, Parka, Pachytheca, (2)Spongiophyton, Prototaxites, Nematothallus, Orestovia, (3)Eohostimella,Taeniocrada, Renalia, and (4)Psilophyton spp.,Pertica, Chaleuria. Sawdonia andProtosalvinia appear as data points showing no observable affinity with any of the above fossils.Protosalvinia, Renalia, andChaleuria are interpreted as being chemotaxonomically intermediate. These data are interpreted as indicating taxonomic affinities on a very broad scale ; possible evolutionary trends in specific chemical compounds, as they relate to vascular and non-vascular plant geochemistry, are discussed.     

Chemical examinations of some non-vascular Paleozoic plants

The organic chemical constituents of compression fossils ofNematothallus,Spongiophyton, Orestovia andEohostimella are identified and compared with those isolated from living and fossil forms ofBotryococcus (a green alga) andTaeniocrada (a vascular plant fossil). The range and maxima in the carbon numbers observed in the normal, saturated acids isolated fromNematothallus, Orestovia, andSpongiophyton are similar to those of fossilBotryococcus, while those acids contained within compression fossils ofEohostimella are similar to the hydrocarbon composition ofTaeniocrada. Isoprenoid, branched hydrocarbons and steroids identified fromNematothallus, Orestovia, andSpongiophyton suggest these genera have algal affinities, while the presence of thick cuticles and in some cases cutin-like compounds appear to show adaptation to a terrestrial environment. Phenolic compounds retained within rock matrices associated withEohostimella are similar to those isolated fromTaeniocrada suggesting chemical, as well as morphological parallels with the land plant habit. These data are interpreted as indicating an early polyphyletic exploitation of the terrestrial habitat during the Paleozoic.     

Chemotaxonomy of some problematic palaeozoic plants

Chemical data are given on the remains of nine fossil genera ranging from Precambrian to Carboniferous in age (Chuaria, Sporangites, Taeniocrada, Orestovia, Parka, Protosalvinia, Spongiophyton, Solenites, Botryococcus). It is suggested that such chemical data are useful additional criteria in making taxonomic assignments in palaeontology. Analyses indicate a high level of organic chemical diversity, although the original composition has evidently been altered by temperature and pressure over a long period of time. The labile chemical constituents are retained within a more chemically inert carbon matrix, which shows a progressive alteration of the ratios of C, N, O and H with respect relative age, similar to that seen in the coalification process.The chemical composition of Sporangites specimens is more suggestive of an animal rather than a plant affinity; it is here suggested that this genus represents in part the remains of some animal egg test. Hydrolytic products of Solenites and Taeniocrada indicate the presence of aliphatic hydroxy acids suggesting the presence of cutin and suberin. The chemical compositions of Orestovia, Parka, Protosalvinia, Spongiophyton and Botryococcus are interpreted as being consistent with an algal rather than a vascular plant affinity. Evidence for extreme diagenesis of the acritarch-like Chuaria greatly limits chemotaxonomic consideration. The possible role of thermal and biotic-thermal degradation patterns seen in palaeochemistry is discussed.     

Carbon sources for the Palaeozoic giant fungus Prototaxites inferred from modern analogues

A wide range of carbon isotope values in the Devonian fossil Prototaxites has been interpreted to support heterotrophy and the classification of Prototaxites as a giant fungus. This inference remains controversial because of the huge size of Prototaxites relative to co-occurring terrestrial vegetation and the lack of existing fungal analogues that display equally broad isotopic ranges. Here, we show wide isotopic variability in the modern saprotrophic fungus Arrhenia obscurata collected adjacent to shallow meltwater pools of a sparsely vegetated glacial succession in the Washington Cascades, USA. Soils collected specifically around the edges of these pools were up to 5‰ higher in δ¹³C than adjacent soils consistent with C₃ origin. Microbial sources of primary production appear to cause these high δ¹³C values, and the environment may be analogous to that of the Early Devonian landscapes, where Prototaxites individuals with extreme isotopic variance were found. Carbon isotopes are also compared in Prototaxites, Devonian terrestrial vascular plants, and Devonian algal-derived lake sediments. Prototaxites isotopic values show little correspondence with those of contemporaneous tracheophytes, providing further evidence that non-vascular land plants or aquatic microbes were important contributors to its carbon sources. Thus, a saprotrophic fungal identity is supported for Prototaxites, which may have relied on deposits of algal-derived organic matter in floodplain environments that were less dominated by vascular plants than a straight reading of the macrofossil record might suggest.     

Early devonian flora of the trout valley formation of northern maine

The data on the plant fossils of the Trout Valley Formation of north-central Maine, U.S.A., are summarized. Evidence from lithological correlation, from plant megafossils and paleopalynology suggest a late Early Devonian (Emsian) age for the Formation. Plant megafossils present are several species of Psilophyton, Pertica quadrifaria, Kaulangiophyton akantha, Prototaxites sp., Drepanophycus sp., Taeniocrada sp. and Thursophyton sp. The microfossil assemblage, through poorly preserved, contains Deltoidospora sp., Apiculiretusispora sp., Emphanisporites rotatus, E. annulatus, Clivosisporites cf. C. verrucata, Grandispora sp. and a chitinozoan, Sphaerochitina sp. The ancient landscape is suggested as having been an area a few square miles in extent with modest relief and comparable to a modern brackish or fresh-water marsh. Three other floras, one in Maine, a second in New Brunswick, Canada, and a third along the coast of Gaspé Bay, Quebec, are thought to have been at least partially contemporaneous with the Trout Valley flora.     

Organic chemistry of Protosalvinia (Foerstia) from the Chattanooga and New Albany Shales

Vertical samplings of Protosalvinia, a thalloid Upper Devonian alga from the Chattanooga and New Albany Shales, are chemically analyzed and correlated with the organic chemical constituents isolated from associated shale matrices. Normal, saturated acids (n-C₈ to n-C₃₆n-paraffins (C₁₀ to C₃₆), showing an odd carbon-number preference, branched-chain alkanes, and vanadyl porphyrins isolated from Protosalvinia vary in their concentrations with depth of burial and with the dominant associated morphology of Protosalvinia, i.e., P. arnoldii, P. ravenna, P. furcata. Organic constituents of shales, in general, reflect those detected in thalli; relative concentrations, molecular diversity, carbon chain-lengths and maxima of compounds extracted from both shale and fossil material are similar. Pristane, phytane and porphyrins are probably derived from a chlorophyllous organism, while δ¹³C data corroborate a photosynthetic system operating in the primary biosynthesis of shale geochemistry. Crude-oil extracts of Protosalvinia-rich strata contain higher alkane and lower aromatic hydrocarbon concentrations than those of an average crude oil. Chemical variations among forms of Protosalvinia suggest biochemical differences in original plant composition rather than diagenetic transitions; field observations of morphological trends seen in vertical samplings may be used in crude extrapolations of the organic chemical compositions of shale strata.     

Evidence of land plant affinity for the Devonian fossil Protosalvinia (Foerstia)

The Devonian plant fossil Protosalvinia (Foerstia) has been examined by solid-state ¹³C nuclear magnetic resonance spectroscopy (NMR) and pyrolysis-gas chromatography-mass spectrometry (PY-GC-MS). Results of these studies reveal that the chemical structure of Protosalvinia is remarkably similar to that of coalified wood. A well-defined phenolic carbon peak in the NMR spectra and the appearance of phenol and alkylated phenols in pyrolysis products are clearly indicative of lignin-like compounds. These data represent significant new information on the chemical nature of Protosalvinia and provide the first substantial organic geochemical evidence for land plant affinity. ▭ Protosalvinia, Foerstia, Upper Devonian, biostratigraphy, carbon-13 NMR, PY-GC-MS, lignin.     

螺旋粉虱产卵行为及产卵分泌物的形态和化学成分

螺旋粉虱Aleurodicus dispersus Russell具独特的产卵过程--成虫将卵产在叶片上排列成螺旋状并覆盖丝状蜡泌物。本研究利用数码摄像机拍摄记录螺旋粉虱的产卵过程,并采用体视显微镜、扫描电镜及气相色谱/质谱联用研究产卵分泌物的结构和化学成分。结果表明：单头雌成虫产卵时可形成典型螺旋轨迹,当其他雌成虫加入产卵时,其产卵行为包括跨越螺旋轨迹后继续在外围扩大螺旋圈、仅在螺旋轨迹表面穿过及在螺旋轨迹外围直接产卵3种情况。 在体视镜及扫描电镜下产卵分泌物呈交错的白色丝状结构。 另用GC/MS联用检测仪检测发现产卵分泌物主要是一些饱和长链烃（66.21%）和芳香酯类（26%）,同时有少量酸和酚。 其中烃类化合物含有3种碘代物（10.64%）,其他为直链烃类,碳原子数介于C₈～C₃₀;芳香酯类化合物有3种,碳原子数介于C₁₆～C₂₄,其中含量最大的是邻苯二甲酸二丁酯（21.19%）。 最后讨论了产卵分泌物在吸引初孵爬虫、抵御天敌及阻碍其他植食性者进入螺旋圈的可能性。     

Changes in Ester-Linked Phospholipid Fatty Acid Profiles of Subsurface Bacteria during Starvation and Desiccation in a Porous Medium

Ester-linked phospholipid fatty acid (PLFA) profiles of a Pseudomonas aureofaciens strain and an Arthrobacter protophormiae strain, each isolated from a subsurface sediment, were quantified in a starvation experiment in a silica sand porous medium under moist and dry conditions. Washed cells were added to sand microcosms and maintained under saturated conditions or subjected to desiccation by slow drying over a period of 16 days to final water potentials of approximately - 7.5 MPa for the P. aureofaciens and - 15 MPa for the A. protophormiae. In a third treatment, cells were added to saturated microcosms along with organic nutrients and maintained under saturated conditions. The numbers of culturable cells of both bacterial strains declined to below detection level within 16 days in both the moist and dried nutrient-deprived conditions, while direct counts and total PLFAs remained relatively constant. Both strains of bacteria maintained culturability in the nutrient-amended microcosms. The dried P. aureofaciens cells showed changes in PLFA profiles that are typically associated with stressed gram-negative cells, i.e., increased ratios of saturated to unsaturated fatty acids, increased ratios of trans- to cis-monoenoic fatty acids, and increased ratios of cyclopropyl fatty acids to their monoenoic precursors. P. aureofaciens starved under moist conditions showed few changes in PLFA profiles during the 16-day incubation, whereas cells incubated in the presence of nutrients showed decreases in the ratios of both saturated fatty acids to unsaturated fatty acids and cyclopropyl fatty acids to their monoenoic precursors. The PLFA profiles of A. protophormiae changed very little in response to either nutrient deprivation or desiccation. Diglyceride fatty acids, which have been proposed to be indicators of dead or lysed cells, remained relatively constant throughout the experiment. Only the A. protophormiae desiccated for 16 days showed an increase in the ratio of diglyceride fatty acids to PLFAs. The results of this laboratory experiment can be useful for interpreting PLFA profiles of subsurface communities of microorganisms for the purpose of determining their physiological status.     

Genome Features of the Endophytic Actinobacterium Micromonospora lupini Strain Lupac 08: On the Process of Adaptation to an Endophytic Life Style?

Endophytic microorganisms live inside plants for at least part of their life cycle. According to their life strategies, bacterial endophytes can be classified as “obligate” or “facultative”. Reports that members of the genus Micromonospora, Gram-positive Actinobacteria, are normal occupants of nitrogen-fixing nodules has opened up a question as to what is the ecological role of these bacteria in interactions with nitrogen-fixing plants and whether it is in a process of adaptation from a terrestrial to a facultative endophytic life. The aim of this work was to analyse the genome sequence of Micromonospora lupini Lupac 08 isolated from a nitrogen fixing nodule of the legume Lupinus angustifolius and to identify genomic traits that provide information on this new plant-microbe interaction. The genome of M. lupini contains a diverse array of genes that may help its survival in soil or in plant tissues, while the high number of putative plant degrading enzyme genes identified is quite surprising since this bacterium is not considered a plant-pathogen. Functionality of several of these genes was demonstrated in vitro, showing that Lupac 08 degraded carboxymethylcellulose, starch and xylan. In addition, the production of chitinases detected in vitro, indicates that strain Lupac 08 may also confer protection to the plant. Micromonospora species appears as new candidates in plant-microbe interactions with an important potential in agriculture and biotechnology. The current data strongly suggests that a beneficial effect is produced on the host-plant.     

Nitrogen acquisition by roots: physiological and developmental mechanisms ensuring plant adaptation to a fluctuating resource

Background

Nitrogen (N) is one of the key mineral nutrients for plants and its availability has a major impact on their growth and development. Most often N resources are limiting and plants have evolved various strategies to modulate their root uptake capacity to compensate for both spatial and temporal changes in N availability in soil. The main N sources for terrestrial plants in soils of temperate regions are in decreasing order of abundance, nitrate, ammonium and amino acids. N uptake systems combine, for these different N forms, high- and low-affinity transporters belonging to multige families. Expression and activity of most uptake systems are regulated locally by the concentration of their substrate, and by a systemic feedback control exerted by whole-plant signals of N status, giving rise to a complex combinatory network. Besides modulation of the capacity of transport systems, plants are also able to modulate their growth and development to maintain N homeostasis. In particular, root system architecture is highly plastic and its changes can greatly impact N acquisition from soil.

Scope

In this review, we aim at detailing recent advances in the identification of molecular mechanisms responsible for physiological and developmental responses of root N acquisition to changes in N availability. These mechanisms are now unravelled at an increasing rate, especially in the model plant Arabidopsis thaliana L.. Within the past decade, most root membrane transport proteins that determine N acquisition have been identified. More recently, molecular regulators in nitrate or ammonium sensing and signalling have been isolated, revealing common regulatory genes for transport system and root development, as well as a strong connection between N and hormone signalling pathways.

Conclusion

Deciphering the complexity of the regulatory networks that control N uptake, metabolism and plant development will help understanding adaptation of plants to sub-optimal N availability and fluctuating environments. It will also provide solutions for addressing the major issues of pollution and economical costs related to N fertilizer use that threaten agricultural and ecological sustainability.     

Unsaturated fatty acid composition and biosynthesis in Oscillatoria limnetica and other cyanobacteria

A number of cyanobacteria showing a high degree of adaptation to life under reduced oxygen tensions as witnessed by their potency of facultative anoxygenic CO₂ photoassimilation with sulfide as electron donor were found to lack polyunsaturated fatty acids in their lipids. Lack of polyunsaturated fatty acids was found in representatives of different taxonomic groups. One of the strains lacking polyenoic acids was Oscillatoria limnetica, which can alternatively grow acrobically or anaerobically with sulfide as electron donor. This organism was found to synthesize monounsaturated fatty acids by desaturation of their saturated counterparts, in the presence as well as in the absence of molecular oxygen.Abbreviations ACP Acyl carrier protein - DCMU 3(3,4-dichlorophenyl)-1,1-dimethylurea     

Biogenesis of mitochondria 19 the effects of unsaturated fatty acid depletion on the lipid composition and energy metabolism of a fatty acid desaturase mutant of

1. The lipid composition of a mutant ofSaccharomyces cerevisiae which cannot synthesize unsaturated fatty acid (UFA) can be extensively manipulated by growing the organism in the presence of added fatty acids.
2. Growth of the mutant is supported by a wide range of unsaturated fatty acids including oleic, palmitoleic, petroselenic, 11-eicosaenoic, ricinoleic, arachidonic, clupanodonic, linoleic and linolenic acids; 9- and 10-hydroxystearic acids support growth less effectively, but erucic, nervonic, elaidic and saturated fatty acids (C_8∶0?C_20∶0)^* are ineffective. All the fatty acids which support growth are incorporated into cell lipids, apparently without further metabolism.
3. The effects of altered lipid composition on the energy metabolism of yeast cells were investigated. Cells containing less than approximately 20% of their fatty acids as UFA cannot grow on non-fermentable substrates, and their growth on glucose is restricted to that which can be supported by fermentation alone.
4. UFA-depleted cells contain mitochondria which are apparently normal in morphology, furthermore they have normal levels of cytochromesa+a ₃,b,c ₁ andc and respire at normal rates. This suggests that the lesion in energy metabolism produced by UFA-depletion may be the loss of the ability of the mitochondria to couple respiration to phosphorylation.
5. UFA-depleted cells incorporate added UFA into their cell lipids and subsequently regain the ability to grow on non-fermentable substrates, showing that the lesion in energy metabolism is fully reversible.

     

Identifying Signatures of Natural Selection in Tibetan and Andean Populations Using Dense Genome Scan Data

High-altitude hypoxia (reduced inspired oxygen tension due to decreased barometric pressure) exerts severe physiological stress on the human body. Two high-altitude regions where humans have lived for millennia are the Andean Altiplano and the Tibetan Plateau. Populations living in these regions exhibit unique circulatory, respiratory, and hematological adaptations to life at high altitude. Although these responses have been well characterized physiologically, their underlying genetic basis remains unknown. We performed a genome scan to identify genes showing evidence of adaptation to hypoxia. We looked across each chromosome to identify genomic regions with previously unknown function with respect to altitude phenotypes. In addition, groups of genes functioning in oxygen metabolism and sensing were examined to test the hypothesis that particular pathways have been involved in genetic adaptation to altitude. Applying four population genetic statistics commonly used for detecting signatures of natural selection, we identified selection-nominated candidate genes and gene regions in these two populations (Andeans and Tibetans) separately. The Tibetan and Andean patterns of genetic adaptation are largely distinct from one another, with both populations showing evidence of positive natural selection in different genes or gene regions. Interestingly, one gene previously known to be important in cellular oxygen sensing, EGLN1 (also known as PHD2), shows evidence of positive selection in both Tibetans and Andeans. However, the pattern of variation for this gene differs between the two populations. Our results indicate that several key HIF-regulatory and targeted genes are responsible for adaptation to high altitude in Andeans and Tibetans, and several different chromosomal regions are implicated in the putative response to selection. These data suggest a genetic role in high-altitude adaption and provide a basis for future genotype/phenotype association studies necessary to confirm the role of selection-nominated candidate genes and gene regions in adaptation to altitude.     

The biosynthesis of cutin and suberin as an alternative source of enzymes for the production of bio-based chemicals and materials

Oxygenated fatty acids such as ricinoleic acid and vernolic acid can serve in the industry as synthons for the synthesis of a wide range of chemicals and polymers traditionally produced by chemical conversion of petroleum derivatives. Oxygenated fatty acids can also be useful to synthesize specialty chemicals such as cosmetics and aromas. There is thus a strong interest in producing these fatty acids in seed oils (triacylglycerols) of crop species. In the last 15 years or so, much effort has been devoted to isolate key genes encoding proteins involved in the synthesis of oxygenated fatty acids and to express them in the seeds of the model plant Arabidopsis thaliana or crop species. An often overlooked but rich source of enzymes catalyzing the synthesis of oxygenated fatty acids and their esterification to glycerol is the biosynthetic pathways of the plant lipid polyesters cutin and suberin. These protective polymers found in specific tissues of all higher plants are composed of a wide variety of oxygenated fatty acids, many of which have not been reported in seed oils (e.g. saturated ω-hydroxy fatty acids and α,ω-diacids). The purpose of this mini-review is to give an overview of the recent advances in the biosynthesis of cutin and suberin and discuss their potential utility in producing specific oxygenated fatty acids for specialty chemicals. Special emphasis is given to the role played by specific acyltransferases and P450 fatty acid oxidases. The use of plant surfaces as possible sinks for the accumulation of high value-added lipids is also highlighted.     

Acylserotonins – a new class of plant lipids with antioxidant activity and potential pharmacological applications

During analysis of components of baobab (Adansonia digitata) seed oil, several new fluorescent compounds were detected in HPLC chromatograms that were not found previously in any seed oils investigated so far. After preparative isolation of these compounds, structural analysis by NMR spectroscopy, UHPLC-HR-MS, GC-FID and spectroscopic methods were applied and allowed identification of these substances as series of N-acylserotonins containing saturated C22 to C26 fatty acids with minor contribution of C27 to C30 homologues. The main component was N-lignocerylserotonin and the content of odd carbon-atom-number fatty acids was unusually high among the homologues. The suggested structure of the investigated compounds was additionally confirmed by their chemical synthesis. Synthetic N-acylserotonins showed pronounced inhibition of membrane lipid peroxidation of liposomes prepared from chloroplast lipids, especially when the peroxidation was initiated by a water-soluble azo-initiator, AIPH. Comparative studies of the reaction rate constants of the N-acylserotonins and tocopherols with a stable radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) in solvents of different polarity revealed that N-acylserotonins showed similar activity to δ-tocopherol in this respect. The described compounds have been not reported before either in plants or in animals. This indicates that we have identified a new class of plant lipids with antioxidant properties that could have promising pharmacological activities.     

Photometabolism of Heterocyclic Aromatic Compounds by Rhodopseudomonas palustris OU 11

Rhodopseudomonas palustris OU 11 (ATCC 51186; DSM 7375) isolated from a pond of chemical industry effluent could anaerobically photometabolize heterocyclic aromatic compounds belonging to the pyridine and pyrazine groups only after a period of adaptation on pyrazinoic acid of 5 to 6 weeks. Growth on heterocyclic compounds was light dependent. The effects of various concentrations of heterocyclic compounds on growth suggest that higher concentrations of these compounds inhibit growth and are toxic.     

Chemical constituents of the medicinal plant Indigofera spicata Forsk (Fabaceae) and their chemophenetic significance

The chemical investigation of the whole plant of Indigofera spicata Forsk (Fabaceae), a medicinal plant from Cameroon, resulted in the isolation of a new benzofuran, named spibenzofuran (1a), together with ten known secondary metabolites including one benzofuran (2), one flavonoid (3), one saponin (6), two triterpenes (4 and 5), two steroids (8 and 11), one phthalate (7) and two fatty acids (9 and 10). All these compounds have been isolated for the first time from this plant. The structures of the isolated compounds were established by spectroscopic analysis including HR-ESI-MS, 1D and 2D NMR and by comparison of our data with the reported data. The isolated compounds might be considered as the chemophenetic markers of this species, and antibacterial and urease inhibitory activities of some isolated compounds were assessed.     

Minor aristolochic acids from Aristolochia argentina and mass spectral analysis of aristolochic acids

The minor aristolochic acids isolated from Aristolochia argentina were identified as 6,7-dimethoxy, 6-hydroxy-7-methoxy, 2-hydroxy-8-methoxy and 7-hydroxy-8-methoxy disubstituted derivatives of the 3,4-methylenedioxy-10-nitro-1-phenanthroic acid, respectively. A. argentina also contains the previously reported aristoloside. The mass spectra of the aristolochic acids, their esters and decarboxylation products have been examined. A number of successive fragmentation processes leading to the formation of aromatic hydrocarbons were observed. Cleavage of the nitro group is a prominent process in the mass spectra of the aristolochic acids and their esters. Evidence is presented that the formation of the [MNO₂]⁺ ion occurs by an intramolecular aromatic substitution reaction with participation of the CO₂R group. The different behaviour of the decarboxylated aristolochic acids is also discussed. A mechanism is proposed for the favourable loss of CH₂O in the 8-methoxy isomer.     

Candidate genes for adaptation to an aquatic habitat recovered from Ranunculus bungei and Ranunculus sceleratus

The molecular mechanisms responsible for plant adaptations to aquatic habitats remain poorly known. In this study, we aimed to detect selection on orthologous genes among the submerged aquatic plant Ranunculus bungei, semi-aquatic R. sceleratus, and terrestrial R. brotherusii and R. cantoniensis, in order to identify candidate genes involved in the adaptive transition from terrestrial habitats to aquatic habitats and if they differ between R. bungei and R. sceleratus. We used 884 previously reported 1:1:1:1 orthologous genes among the four species (one ortholog is represented in each species). We identified 177 genes of R. bungei and R. sceleratus that are potentially under positive selection. Among them, a set of candidate genes for adaptation to an aquatic habitat were recognized. These include several genes that are associated with photosynthesis, vacuoles, ethylene and oxygen receptors, and aerenchyma formation. Gene Ontology analyses suggest that the submerged R. bungei has a higher percentage of positively selected genes associated with the chloroplast and other plastids, while the semi-aquatic R. sceleratus has a higher percentage associated with metabolic processes. This study represents a step forward towards understanding the molecular mechanism of plant adaptations to aquatic habitats.