26 | Science Reporter | January 2019

Bizarre World of Plant-Animal

THE term ‘chimaera’ is used to refer to organisms with their bodies encompassing parts from

other organisms. As a figment of human imagination, one can find chimaeras in mythologies around the world.

Many humans have also claimed to possess plant-like photosynthetic attributes. The concept of breatharianism or “Inedia” (Latin for fasting) is based on the belief that human beings have plant-like essence within, and can live without food forever; all we need is water and sunlight. However, most of these claims have been invalidated by modern scientific research; breatharianism is now considered to be a form of pseudoscience.

There are, however, genuine cases of such plant-animal chimaeras one can find in nature. Most of these creatures often remain relatively unknown outside the arcane world of primary scientific research. So, let’s take a look at some interesting examples of chimaeras and other bizarre creatures that very well exist in nature.

EndosymbiontsHatena arenicola is one such extraordinary animal discovered in the year 2000 by two Japanese scientists. The scientists observed single-celled animals (flagellate eukaryotes) living on a beach in Japan with chloroplasts— the photosynthesising organelle. Animal cells usually do not contain chloroplasts — one of the principal distinguishing characteristics between animals and plants.

Further research on its life cycle revealed that these chloroplasts were indeed intact green algae Nephroselmis rotunda that the animal predated upon. After eating these microscopic plants, instead of digesting it, the animal kept it inside its sole cell. The algae supplied the food that it photosynthesised to its predator. The confined algae thus

became a feeding apparatus of the animal kidnapper and developed a life-long allegiance to its captor, a sort of Stockholm Syndrome!

Many single-celled algae like Nephroselmis have a region in their cell called eyespot, a heavily pigmented part that is attracted towards the light (a phenomenon called phototaxis). The eyespot of Nephroselmis enables the otherwise blind Hatena to detect the light and to swim towards it. In other words, the algal eyespot works like an eye for Hatena.

When the host animal cell divides into two, one daughter cell gets the whole algae, and keeps on living like a photosynthetic animal, while the other daughter cell, now devoid of the alga, lives like an animal until it ingests yet another alga. Scientists believe that this extraordinary situation in Hatena is due to ongoing “endosymbiosis.”

Chimaera

Endosymbionts: A) Hatena arenicola (Image credit: Scientific American). N indicates the nucleus of the host heterotroph, while S indicates the symbiont alga Nephroselmis rotunda. The arrow indicates the location of the eyespot. B) Paulinella chromatophora (Image credit: www.arcella.nl). C) Algae Oophila amblystomatis inside spotted salamander embryos (Image credit: Wikimedia).

D) Sea slug Elysia chlorotica (Image credit: Patrick Krug/MBL).

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FEATURE ARTICLE

Felix Bast

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Endosymbiosis (endo = inside, symbiosis = living together) is a prominent theory in cell biology first formulated by Russian botanist Konstantin Mereschkowski in 1905 explaining the origin of organelles –intracellular membrane–bound structures of eukaryotes (cells with true nucleus and organelles). The theory posits that eukaryotes originated when a prokaryote (bacteria) ingested yet another bacteria. The ingested bacteria, instead of getting digested, stayed there and evolved into an organelle that divides when its host cell divides.

Each cell of our body has an organelle (mitochondrion, the powerhouse of the cell), formed this way; the relic of a bacterial food that our bacterial ancestor billions of years ago ingested. Current estimates suggest that the eukaryotes originated this way approximately 2.7 billion years ago. Imagine swallowing a live fish, and the fish lives happily inside our stomach, and evolves to be one of our organs! Queer as it would seem, that is how the mitochondria originated.

Plants have another such organelle, the photosynthesising chloroplasts, in addition to mitochondria. Chloroplasts were once photosynthetic bacteria (blue-green algae), the food that an animal cell ingested approximately 2 billion years ago.

In a way, plants are more ‘accomplished’ living beings; they have chloroplasts and mitochondria, while animals have only mitochondria. Look at the world of plants around you; are these merely plants? Plants are indeed living examples of the animal-plant chimaera; a primitive animal that acquired photosynthetic ability from a blue-green alga (the closest living relative of this first algal food is marine picoplankton Synechococcus) that they ingested.

Paulinella chromatophora, an amoeboid animal, recently (100 million years ago, which is comparatively very recent in the history of life on this planet) did the exact same thing that the animal ancestor of plants did 2 billion years ago — it ate a cyanobacterium.

Instead of killing and ingesting it, the amoeba commanded the plant hostage to make food for it, what a clever tactic!

Imagine we also command the plants we eat; that would result in a photosynthetic human being. A human that needs no food; she can make the food within herself. All she needs is sunlight and water just like plants, the fantasy world of breatharians. Solar-powered human beings! The 2006 science fiction “Stuffing” by Jerry Oltion depicts such a photosynthetic human being (“Wings” series by Aprilynne Pike is yet another example).

With the advent of genetic engineering and genome editing tools like CRISPR-CAS9, scientists have already begun discussing the possibility of creating such a plant-animal hybrid, with animal cells having either the crucial solar energy capturing mechanisms or the complete organelles of chloroplasts for photosynthesis.

The story of the green sea slug (Elysia chlorotica) is even bizarre. These marine gastropod animals (mollusk) ingest intertidal green algae, Vaucheria litorea. Instead of killing and ingesting the whole algae, the animal’s gut absorbs the algal chloroplasts intact through a process called phagocytosis (“cell eating”). The intact chloroplasts thus absorbed become integrated into the animal cells, where they grow as a stolen chloroplast (“kleptoplast”). The algal prisoner now captures solar energy, diligently performs photosynthesis, and provides food for the slug.

The case of the marine ciliate unicellular animal, Myrionecta rubra, is very similar. These ciliates appropriate the intact chloroplasts from the algal food (a species of cryptomonad algae), and these become kleptoplasts lending a photosynthetic capability to the host. Mesodinium chamaeleon, yet another marine ciliate, eats plants (green and red algae), and the algal chloroplasts get integrated into the eukaryotic cell carrying out photosynthesis.

The strategy of acquisition of photosynthetic potential by selective phagocytosis of the algal chloroplast has

even been strategized by vertebrates. Consider an extraordinary amphibian, spotted salamander (Ambystoma maculatum). The salamander acquires Oophila amblystomatis, a green alga, right during its embryo phase. Nitrogen-rich embryonic fluid fosters algal growth; while algae provide oxygen and sugar to the developing embryo, the embryo provides carbon dioxide for the algal photosynthesis. As the embryo develops, the algae suffuse throughout the salamander’s body. However, unlike the embryo phase, the matured salamanders are not photosynthetic.

SymbiontsA relationship with algae has been reported for yet another amphibian, the American toad (Anaxyrus americanus). Tadpoles of this toad secrete inorganic nutrients that render green algae of the genus Chlorogonium attracted to it. The algae grow on the tadpoles as an appendage, providing oxygen and food to the developing frog. The tadpole provides nutrients and carbon dioxide for the growth of the algae.

The mutualism between tadpoles and algae is an example of symbiosis – long-term interaction of two organisms – and is different from endosymbiosis. The key difference is that in the case of endosymbiosis, one organism becomes an organelle within the cells of another organism, while in the case of symbiosis both the organisms are separate living entities. Animal-plant symbiosis is indeed a well-documented phenomenon.

Two well-known examples are corals and lichens. In the case of corals, cnidarian animals have a symbiotic relationship with dinoflagellate algae, Symbiodinium. In the case of lichens, fungi and yeast have a symbiotic relationship with green algae or cyanobacteria. The algal partners in both of these associations fix the carbon dioxide through photosynthesis and provide food to their partner. These associations were previously thought to be mutualistic, benefitting both the partners.

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While algae can very well live outside the partnership, its partners (cnidarian animal in the case of corals or fungi in the case of lichens) cannot. Ocean acidification, one of the major consequences of rising atmospheric carbon dioxide, has killed algal partners of cnidarians in a large number of coral reef systems across the world; the phenomenon is called coral bleaching. Once the algae are dead, the coral reef ecosystem irrevocably collapses and turns into nothing but white calcium carbonate rocks. The relationship is now considered to be commensalism (one is benefitted while the other is not harmed). Animal partners of these symbioses are thought to ‘domesticate’ the algae, just like we domesticate crops through agriculture.

Solar-powered AnimalsWe have seen so far that algae are the only plant group involved in all these animal-plant hybrids. However, there are some animals that can harvest solar energy without these algal partners; they don’t even need chloroplasts.

One example is the wasp species commonly found in India, Oriental Hornet (Vespa orientalis). These hornets have peculiar yellow bands in their exoskeleton; these bands contain the pigment xanthoperin. Like the pigment chlorophyll, xanthoperin traps sunlight and converts it into electricity. The voltage through biochemical intermediates including ATPs (Adenosine Triphosphate, the ubiquitous intracellular fuel), powers

the hornet’s locomotion. Stored electricity is also released as light during darkness.

Similar solar energy harvesting mechanism has also been reported for another insect species, pea aphid (Acyrthosiphon pisum). This aphid produces carotenoid pigments that absorb light and pass on energy to ATPs. ATP molecules power all the energy requirements of the organism. Technically, these processes cannot be called photosynthesis; photosynthesis implies fixation of carbon dioxide into sugars. However, here photons from the solar energy are directly used to power metabolism and other energy requirements of the organism, as in the case of a photovoltaic cell in the solar panel.

A. Leaf insect (Image credit: Wikimedia). B. Bee orchid (Image credit: Wikimedia). C. Wasp Lissopimpla excelsa pseudomating on orchid Cryptostylis (Image credit: Mitch Smith). D. Stick insect (Image credit: Wikimedia).

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So, Oriental hornets and pea aphids are “solar-powered” rather than photosynthetic; a bizarre phenomenon indeed, that for sure would inspire generations of scientists from biomimetics and genetic engineering, and science fiction writers.

Plant-like Animals and Animal-like PlantsOf course, there are several plant-like animals, such as sea anemones, northern corals, hydroids, bryozoans, tube worms, barnacles, sponges, sea squirts and so on. These sessile (immobile) marine animals would look like plants for a non-specialist diver.

Keeda jadi (“Insect Plant”), a pricey mountain product especially from Uttarakhand region and an ingredient in traditional medicine with supposedly aphrodisiac properties, is not exactly a plant. It is the product of entomopathogenic fungal (Ophiocordyceps sinensis) fruiting bodies parasitizing on ghost moth (Hepialus humuli) caterpillars, and contain a dangerous amount of heavy metal arsenic.

Shilajit is yet another mystic pseudoscience product from mountains, priced for its supposed aphrodisiac uses, like snake-oil. This tar-like substance is thought to be dried decomposed saps of succulent Euphorbia royliana plant. Evidence-based scientific research has invalidated its medicinal and aphrodisiac claims.

There are a number of insect species that remarkably resemble plants; examples include leaf insects of the family Phyllidae and stick insects of the order Phasmatodea.

There are several animal-like plants as well, especially insect-ingesting carnivorous plants. Examples include Drosera, Nepenthes, Heliamphora, Venus flytrap, Utricularia, etc. Several plant species mimic animal appearance and scent as an adaptation.

A well-known example is Cryptostylis, an Australian orchid. The orchid flowers remarkably mimic the females of its pollinators, the ichneumon orchid-dupe wasp (Lissopimpla excelsa). Male wasps mistake the orchid flowers for female wasps, copulate and waste copious amounts of sperm onto the flowers. The orchids are benefited such that the pseudocopulation – as it is called – ensures pollination.

An almost identical situation has also been discovered in the case of bee orchids (Ophyris apiphera. The orchid flowers resemble female bee pollinators in both shapes as well as scent. These orchids depend on male bees thus allured for pollination. Yet another orchid from the same genus, the fly orchid (Ophyris insectifera), has flowers shaped like flies. The flowers release a scent that mimics the female sexual pheromones of certain species of wasps and bees, its pollinators.

All these examples illustrate that

nature offers far more mysterious and delightful organisms to kindle human curiosity, although these legitimate chronicles remain little known in comparison to much more popular examples from mythology, pseudoscience, mysticism, obscurantism and science fiction.

What is more puzzling is that we know only the tip of the global biodiversity iceberg; it is estimated that the world has 1 trillion species, while we know only 2 million of it (0.02%). Photosynthetic slugs and solar-powered insects offer far more valuable scientific information than obscurantist religious mysticism.

Progresses in the fields of synthetic biology, biomimetics, and genetic engineering might lead us to the development of engineered solar-powered animals, even photosynthetic humans in the future. A candid appreciation of the hidden marvels of global biodiversity enables us to conserve nature and adopt a higher-level philosophy of life – citizens of the world and creatures of the earth.

Dr. Felix Bast is DST-INSPIRE Assistant Professor of Plant Sciences at Central University of Punjab, Bathinda. He is actively involved in science popularisation and communication. Address:Department of Plant Sciences, Central University of Punjab, Bathinda-151001; Email: felix.bast@gmail.com

Solar powered animals. A. Oriental Hornet (Image credit: www.asknature.com). B. Pea aphid (Image credit: Wikimedia).

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