Category: Plants & Algae

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"Algae" is a general term that refers to an extremely diverse group of photosynthetic organisms, most of which are unicellular. All algae are eukaryotic (DNA packaged in a double-membrane bound nucleus) and all possess some form of plastid. Plastids are organelles (e.g. chloroplasts) bound by two or more membranes and containing small circular DNA genomes. They are of cyanobacterial origin and appear to have been acquired by one or more endocytosis events early in eukaryote history.

Endocytosis of a photosynthetic cyanobacterium by its eukaryote 'host' resulted in endosymbiosis, so that over time most genetic information was transferred from the cyanobacterial plastid to the host genome. The bulk of evidence currently supports the hypothesis that only one ancestral endosymbiosis event occurred, from which three 'primary plastid' groups evolved: the green algae (or chlorophytes, including one very famous lineage - the plants), red algae (or rhodophytes) and glaucocystophytes. It should be noted, however, that some recent findings suggest that convergent acquisition of plastids from three different cyanobacterial lineages cannot yet be ruled out as an explanation for the primary algal groups. Subsequent endosymbioses, in which primary plastid algae were engulfed by other eukaryotes, are inferred to have occurred several times, resulting in algae with complex plastid membranes and characteristic molecular signatures. Major groups of these 'secondary' and 'tertiary' plastid-bearing algae include euglenoids, dinoflagellates, heterokonts (e.g. diatoms), haptophytes (e.g. coccoliths) and cryptomonads.

Among the algae and land plants (which evolved from green algae) we find many examples of convergent evolution. A few of these are highlighted here but please see the topic list below for many more details and cases.

Regarding convergences among unicellular algae, the warnowiid dinoflagellates stand out. Warnowiids use their chloroplast not for photosynthesis but instead to form a camera eye-like "ocelloid", evidently allowing them to sense visual stimuli while hunting for food. Another notable case is the sequestering of photosynthetic dinoflagellates or 'zooxanthellae' within marine organisms, from protists to corals, clams and sea slugs. In the terrestrial sphere, various green algae and cyanobacteria associate with a diversity of fungi to form lichens, in another rampantly convergent and ecologically successful symbiosis.

To optimise efficiency, a number land plants and a few algae have independently evolved CO2 concentration mechanisms (CCMs). 'C4 photosynthesis' and 'Crassulacean Acid Metabolism' or 'CAM photosynthesis' are CCMs that both share a key enzyme for CO2 fixation, PEP carboxylase. CCMs allow efficient photosynthesis in spite of low ambient CO2 levels or hot, dry conditions in which gas exchange ceases during the day (when stomata are closed to prevent water loss). C4 organisms include a few algae and many crop plants (e.g. maize and sugarcane) whereas CAM plants include succulent desert angiosperms (e.g. cacti, Euphorbia, Hoodia, Stapelia, Aloe and Agave) plus the aquatic lycophyte Isoetes. Interestingly, many cyanobacteria concentrate CO2 in a similar way, via organelle-like carboxysomes.

Several plants are specialised for obtaining additional nutrients (especially nitrogen), including three highly convergent adaptations: parasitism, carnivory and symbiosis with nitrogen-fixing bacteria. Many angiosperms live as parasites within other plants and recent genetic data support at least 11 independent origins of so-called endophytic parasitism. Endophytes include species such as the colossal Rafflesia from South East Asia, Pilostyles, Cytius, Lennoa and Hydnora from Africa. Most endophytes cannot photosynthesise but survive instead as filamentous 'haustoria' that extract nutrients from host tissues and only emerge to flower. Carnivory as a general strategy has evolved many times in plants that live in low-nitrogen environments (e.g. poor soil, high rainfall, rocky surfaces). Five different kinds of carnivorous plant are recognised: pitcher or pitfall traps, flypaper traps, snap traps, bladder traps of the bladderwort Utricularia and lobster-pot traps (e.g. in Genlisea and several pitcher plants to a degree). Pitcher plants display a rolled-leaf structure containing a soup of enzymes for digesting trapped prey (usually insects), and they evolved independently at least 4 times. New World Heliamphora and its relatives, tropical Nepenthes 'monkey cups', the Australian Cephalotus follicularis and rainforest bromeliads Brocchinia and Catopsis all have pitchers. Flypaper traps, based on sticky 'mucilage' secreted from glands on the surface of leaves or at the ends of tentacles, have evolved convergently at least 5 times. Flypaper plants include the well-known tropical 'sundew' Drosera, butterwort Pinguicula, Australian 'rainbow plant' Byblis and others. Nitrogen-fixing bacteria live as symbionts in the root nodules of many plants. These vital symbioses have evolved many times and rely on either diazotropic bacteria (anaerobic and actinobacteria) or cyanobacteria such as Nostoc and Anabaena. For example, legumes (e.g. Fabaceae) and "actinorhizal" shrubs and trees depend on diazotropic bacteria while hornworts and the aquatic pteridophyte Azolla form symbioses with Anabaena.

Fascinatingly, certain plants are able to generate heat by "thermogenesis", a biochemical process that has strong molecular parallels with heat generation in endothermic mammals. Most thermogenic plants belong to the Araceae or arum family where heat spreads volatile scents to attract (and temporarily trap) pollinating insects. The most famous must be the Titan Arum (Amorphopahllus titanum), with its immense and pungent column or 'spadix' of florets. Among other thermogenic plant families the scented Nymphaceae (water lilies) stand out as having an arum-like system of trapping insects overnight to ensure successful cross-pollination. Thermogenesis is often associated with emission of a scent is foul to us but irresistible to pollinating insects. Indeed, malodorous scent is highly convergent among plants and even some fungi. Angiosperms from various distinct groups are malodorous, for example Araceae and Trillim erectum ('Stinking Benjamin') in the monocots, Hydnora africana in the magnoliids, Rafflesia and the milkweed 'Carrion Flower' Stapelia from among the eudicots. Malodorous fungi attract insects for spore dispersal and include various species of 'Stinkhorn'.

Among desert plants classic cases of convergence are found between stem succulent cacti in the Americas and cactus-like Euphorbia species in Africa and South Asia, and also the striking similarity between leaf succulent agavaceans (e.g. Agave, Yucca) of the Americas and Aloe and its close relatives in Africa. The succulent stems of cacti and cactus-like Euphorbia are leafless, folded or 'plicated' and fully adapted for CAM photosynthesis, water storage, retention and defence (e.g. through prickly spines). Additionally, 'stone plants' of the family Aizoaceae (e.g. Lithops) avoid herbivores by resembling small pebbles, a strategy convergently adopted by the 'Living Rock Cactus' Ariocarpus.

Even the most fundamental aspects of plant anatomy have been found to show convergence. For example, angiosperms are defined by the possession of specialised xylem vessels for transport of water and solutes, and yet it is clear that they also evolved independently in species of the lycophyte Selaginella, some primitive euphyllophytes (e.g. monilophyte Equisetum and seed-ferns Marsilea and Pteridium) and in the gymnosperms (e.g. Gnetales and Permian fossils of Gnetum-like 'gigantopterids'). The astonishing efficiency of angiosperm xylem vessel elements was even matched by an alternative mechanism in the conifers, where torus-margo pits provide analogous and in some cases superior hydraulic conduction. Surprising discoveries have been made recently regarding convergent evolution of one key feature of deciduous trees, namely autumn leaf colouration. It appears that red and yellow leaf colouration may be adaptive (possibly related to co-evolution with insects) and have evolved independently on many separate occasions in gymnosperms and woody angiosperms.

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This table lists all the Topics which are part of the Category "Plants & Algae"
Topic title Teaser text Availability
Solar powered animals n/a Unavailable
Parasitism in nematodes n/a Unavailable
Pheromone use in animals, fungi and plants n/a Unavailable
Endophytic fungi n/a Unavailable
Vestured pits in plant xylem n/a Unavailable
Tropical rainforest trees n/a Unavailable
Structure and function in plants: from leaves to petioles n/a Unavailable
Convergence in the Malva alliance (Malvaceae) n/a Unavailable
Lignin in seaweeds n/a Unavailable
Aerial rootlets for clinging n/a Unavailable
Parasitic flowering plants n/a Unavailable
Plant pollination syndromes n/a Unavailable
Carnivorous plants All plants are harmless? Well, not quite - at least not when you're an insect... Available
Carbon dioxide concentration in plants n/a Unavailable
Ant-mediated seed dispersal (myrmecochry) n/a Unavailable
Thermogenesis in plants and algae n/a Unavailable
Mangrove swamp ecology n/a Unavailable
Mycorrhizal fungus associations in plants n/a Unavailable
Zooxanthellae in corals and other animals n/a Unavailable
Growth forms in green algae n/a Unavailable
Flowering plants: angiosperms and gnetophytes n/a Unavailable
Agriculture in marine polychaete annelids Some polychaetes attach pieces of algae to their dwelling tube. Just for decoration? No, but for a much more substantial (and convergent) benefit... Available
Agriculture in dugongs When you think of grazing mammals, you might envisage large herds of antelopes roaming African savannahs. Did you know that there is an equivalent in the ocean, feeding on seagrass? Available
Agriculture in aquatic snails Termites and ants are famous for tending fungal gardens, but did you know that also a marine snail farms a fungus? And this is not the only example of agriculture in this group… Available
Agriculture in damselfish Don’t be tempted to think human agriculture is unique. On many coral rocks, there are very similar things going on… Available
Torus-margo pits in vascular plant xylem Torus-margo pits probably evolved once in the gymnosperms, after the split of more advanced gymnosperms from the cycads.�Surprisingly, eight genera from five families of angiosperms, which are characterised by highly effective xylem vessels, have also evolved torus-margo structures. Available
Reversion from xylem vessels to tracheids In three plant taxa that evolved in environments with frequent freeze-thaw cycles (Winteraceae, Trochodendraceae and cold desert Ephedra), vessel evolution has been reversed independently in favour of a return to a tracheid-based vascular system. Available
Xylem vessels in vascular plants Vessels are characteristic of the angiosperms, and yet they have evolved independently in several other groups, including the lycophyte Selaginella, horse-tail Equisetum and the enigmatic Gnetales. Available
Malodorous flowering plants Several groups of angiosperms have flower structures that produce foul odours to attract pollinating insects. This strategy is convergent, being found in species as distantly related as the 'Titan arum' Amorphophallus titanium (a monocot) and the 'Corpse flower' Rafflesia (a eudicot). Available
Muts proteins in plants and corals n/a Unavailable
Mitochondrial genome convergences Most likely, mitochondria have a single evolutionary origin, but that doesn't mean they are immune to convergence... Available
Lichens: fungal association with cyanobacteria and green algae n/a Unavailable
Explosive discharge in fungi and plants The very rapid release of reproductive bodies is perhaps most famous in the fungi, where several methods of flinging spores at high velocity have evolved independently. Available
Autumn leaf colouration Autumn colours are likely to be adaptive, as the 'default' is simply to remain green up to leaf fall, and both red and yellow leaf colouration have evolved independently on many occasions in gymnosperms and woody angiosperms. Available
Desert plants with succulent stems Fleshy, succulent stems have evolved in several distantly related desert plant families, including cacti, certain species of Euphorbia and two genera of the family Asclepiadaceae, Hoodia and Stapelia. Available
Desert plants with succulent leaves Perhaps the most striking case of convergence among leaf succulents occurs between Agave and its relatives Yucca and Hesperaloe in the Americas and Aloe and its relatives (e.g. Haworthia and Gasteria) in Africa. Available
Succulent desert plants Classic examples of convergence in desert plants include the so-called 'stem succulent' cacti in the Americas and cactus-like Euphorbia species in Africa and South Asia, and also the striking similarity between 'leaf succulent' Agave and Yucca of the Americas and Aloe and its close relatives in Africa. Available
SNARE protein receptors and the evolution of multicellularity There is an intriguing correlation with larger numbers of SNAREs and multicellularity, at least in plants and animals. Unavailable
Surface sculptures: from pollen to protists Amongst the most striking sculptured surfaces are those seen on pollen and also various protistans, notably the dinoflagellates and a group largely known from the fossil record that are called acritarchs. Unavailable
Dinoflagellate "nematocysts" Examples of convergence within the dinoflagellates range from the evolution of a camera-like eye to stinging 'nematocysts' reminiscent of those in jellyfish. Available
Latex in plants and fungi Latex is important in terms of defence not only because it typically gums-up attackers, notably insects, but often contains toxins. Unavailable
Mycorrhizal fungus associations with plants One of the most important of the fungal associations is the intimate link between the majority of plants and the so-called mycorrhizal fungi. Unavailable
Yeasts and yeast-like forms We find convergences not only within the true yeasts (which belong to the ascomycetes), but much more widely, indeed even amongst the algae! Unavailable
Protistan eye-spots and warnowiid dinoflagellate eyes Warnowiids propel themselves through the water, but unlike other dinoflagellates which are photosynthetic, they are hunters and the chloroplast is employed to make the lower part of the eye. Unavailable
Wire plants, moas and elephant birds Madagascar and New Zealand were once home to giant herbivorous birds. And the plants have not forgotten... � � Available
Innate and adaptive immune systems A vile cough, soaring temperature? When attacked by nasty microbes, our immune system comes in handy. Surprisingly (or not), plants have come up with a very similar solution to dealing with pathogens, but independently... � � Available
Pollen harvesting adaptations in bees (and honey-wasps) n/a Unavailable
Nitrogen-fixing bacteria in legumes One convergent avenue to obtaining nitrogen is to employ symbiotic bacteria that typically are found in root nodules, perhaps best known in the legumes. Unavailable
Functions of plant waxes Plant waxes are employed in contexts other than water retention, notably to deter insects or to provide a lethal glissade across which the prey of carnivorous pitcher plants slide to their doom. Unavailable
Evolution of fungicide resistance Just as with insecticides, we see both evolution in action and also striking instances of convergence where resistance is acquired independently Unavailable
Evolution of herbicide resistance Unfortunately, just as in insecticides, resistance rapidly develops and is not only an excellent example of evolution in action, but also is strikingly convergent. Unavailable
Gut fermentation in herbivorous animals Ever tried eating a newspaper? Don't. Plant cell walls contain cellulose, which is notoriously difficult to digest. Considering that all vertebrates lack the enzymes to attack this polysaccharide, how do so many of them manage to survive on a plant diet? Available
Carbonic anhydrase in vertebrates, plants, algae and bacteria Carbonic anhydrase is extremely convergent and may have evolved as many as six times. The most familiar variants are α, β and γ carbonic anhydrases. Available
Sap feeding and honey-dew production in insects Interestingly, it has now been shown that the saliva of the aphids has an analogue to the anti-coagulant properties of blood suckers, subverting the wound repair mechanism of the plant. Available
Chloroplast and mitochondrial plastid origins Not only are there intriguing parallels in the story of gene loss in chloroplasts and mitochondria, but there is also the re-invention of bacterial pathways, such as oxidation of quinols. Available
Agriculture: from ants to dugongs Human farmers tending their fields are a familiar sight. But don't forget about those fungus-farming termites or the fish with a garden of algae… Available
Mediterranean plant ecosystems Mediterranean climates are characterized by long hot, dry summers and short winters with variable rainfall. The plant communities have long been recognized to have a number of striking similarities, notably drought-evading annuals and ever-green shrubs forming dense scrub. Unavailable
Insecticide production: from plants to primates Application of insecticides, such as against mosquitoes, has been documented in several primates and birds. Unavailable
Developmental genetic pathways to convergence At first sight there is a fairly simple dichotomy between convergent features that have effectively the same genetic basis, and those where the same feature emerges but the underlying genetics are different. The former, however, is somewhat more complicated... Available