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.