Insects and microorganisms symbiosis: the endosymbionts

Symbiotic relationships are an important motor for organisms’ diversification and evolution. The relationships insects have established with some endosymbiotic microorganisms (that is, those inhabiting the inner of their bodies) have provided them of a lot of surprising physiological and ecological adaptations. 

The value of the relationship between insects and their endosymbionts

The major cause for insects’ evolutive and adaptive success is their potential to stablish beneficial relationships with other life beings and, especially, with those microorganisms inhabiting their insides: the endosymbionts.

Some years ago, it was considered that the greatest contribution of endosymbiotic microorganisms to the physiology of insects was their role in feeding habits, which would explain, at least in part, the diversity of diets among insects. However, it has been shown that endosymbionts affect many other physiological traits.

Types of endosymbiosis in insects

Endosymbiotic microorganisms can be found inside the gut, in the spaces between cells and inside cells.

Generally, the more internal the endosymbiotic microorganisms are within the host’s body, the closer their relationship with the insect is. The four most common types of endosymbiosis in insects are explained below, from the most external and least close relationship to the most internal and closest one.

Gut microbes

Gut microbiota of insects is composed both of prokaryotes (unicellular, without nucleus, like bacteria and archaea) and eukaryotes (unicellular or pluricellular, with nucleus, like protozoans) that live outside the gut cells. They usually inhabit the hind part of insect’s gut (hindgut), either moving freely in its lumen or remaining attached to its walls. In some phytophagous insects, likes termites and cockroaches, the hindgut is a chamber without oxygen (anaerobic) where fermentation of cellulose and other complex sugars takes place.

 

Worker termite gut; the green part corresponds to the hindgut without oxygen. Figure belonging to the following paper: Brune, A. (2014). Symbiotic digestion of lignocellulose in termite guts. Nature Reviews Microbiology, 12(3), 168-180.

In termites, this anaerobic chamber contains facultative anaerobic prokaryotes (they can develop either with or without oxygen) and obligate anaerobic prokaryotes (they can only develop without oxygen), such as spirochetes and methanogens, which aid in digestion. In addition, in some worker termites, this chamber also contains protozoans that play a major role in the digestion of wood cellulose (Have you ever seen a piece of furniture pierced by termites?).

Unlike other endosymbionts, gut microbes are horizontally transmitted between insects; that is, insects don’t inherit gut microbes from their parents, but they should acquire them throughout their lives. In termites, acquisition of gut microbes takes place through a process called trophallaxis: the workers, which are the only able to feed by themselves, digest the food and transmit the resulting product mixed with gut microorganisms to the rest of the colony members through their mouthparts.

Trophollaxis. Picture by Shutterstock.

Moreover, microorganisms are removed during molting processes, so termites (and other insects performing trophollaxis) can acquire them again through trophollaxis.

Endoparasites

Parasites that live and/or develop inside an organism are known as endoparasites. They are also horizontally transmitted between insects.

Insects stablish fairly more relationships with pluricellular endoparasites than with microorganisms, being the pluricellular endoparasites the most harmful for insects in general terms; these are the cases of insect parasitoids (of which we talked in this post) and nematodes (able to transmit deathful bacteria to insects).

The most relevant endoparasitic relationship between insects and microorganisms, and the only one we are going to explain here, are vectors: the insect (or vector) serve as a container to the parasite until it reaches the definitive host. Parasites transported by vector usually are pathogenic protozoans harmful to vertebrates, like Trypanosoma (Chagas disease), Leishmania (leishmaniosis) or Plasmodium (Malaria).

Mosquito of the genus Anopheles, the major vector of the protozoan causing malaria worldwide: Plasmodium. Public domain image.

Extracellular and intracellular symbiosis

Unlike gut microbes and endoparasites, extracellular and intracellular endosymbionts are vertically transmitted generation after generation; that is, the insect inherits them from its parents

• Extracellular endosymbionts
  

Extracellular endosymbionts, which can be both prokaryotes and eukaryotes, can be found in different organs of the body (even in the intestine along with the gut microbes). In any case, they never penetrate inside the cells. However, some species can be found outside and inside cells.

Since many extracellular microorganisms can also be intracellular, the possibility that they are found, in an evolutionary sense, in a transition stage between gut microbes and intracellular endosymbionts has been discussed.

An interesting case of extracellular endosymbiosis takes place in some species of aphids of the tribe Cerataphidini. Generally, aphids stablish a close relationship with an intracellular endosymbiont bacteria (Buchnera), but in some species of the aforementioned tribe these bacteria are substituted by extracellular unicellular yeast-like fungi (YLS or ‘yeast-like symbiont’) which inhabit the cavities between organs and inside different adipose bodies. Like Buchnera in the rest of aphids, YLS would play a key role on aphid feeding habits, participating in the production of essential nutrients.

Ceratovacuna nekoashi (Cerataphidini). Link (CC 2.5)

It is suggested that YLS would have evolved from an entomopathogenic fungus (that is, harmful to insects) whose lineage would later have derived into beneficial endosymbiotic organisms.

• Intracellular endosymbionts
  

It is considered that at least 70% of insects has endosymbiotic microorganisms inside its cells. There exist two types of intracellular endosymbionts:

Mycetocyte symbionts or Blochmann bodies

Bacteriocytes or mycetocytes are specialized adipose cells containing endosymbionts which can be found in some groups of insects. These cells are vertically transmitted to the offspring and gathered together forming organs known as mycetomes o bacteriomes.

Blochmann bodies, or simply the endosymbionts inside mycetomes, are related to three groups of insects: Blattaria (cockroaches), some groups of heteropterans within Homoptera (cicadas, rust flies, aphids, etc.) and Curculionidae (curculionid beetles).

Buchnera aphidicola inside a mycetome of the aphid Acyrthosiphon pisum. The central element is the mycetome’s nucleus. Buchnera cells, which are round, are located packed in the citoplasm of the mycetome. Picture by J. White y N. Moran, University of Arizona (CC 2.5).

The most well studied case is the relationship between Buchnera and aphids. This intracellular bacterium recycles the uric acid and some other nitrogenous wastes produced by the aphid in order to produce the amino acid glutamine, which is then used by this same endosymbiont to produce other essential amino acids necessary for the aphid to develop. It is also considered that Buchnera produces vitamin B2 (riboflavin). This can explain why aphids have such a high reproductive rate and a big evolutive success despite having a diet rich in carbohydrates (which they obtain from plant’s sap) and poor in nitrogenous compounds.

It has been confirmed that Buchnera cells decrease in number when nutrients are scarce. This suggests that aphids use Buchnera cells as an alternative food source in difficult situations. So, aphids take more advantages from this relationship than Buchnera.

Guest endosymbionts

In this case, the guest (endosymbiont) alters some physiological traits of the insect to obtain some advantage.

Guest endosymbionts usually affect the sex ratio of insects (proportion of males and females in a population) as well as other reproductive traits. Guest endosymbionts that alter the sex ratio are known as sex-ratio distorters. Some guest microbes inhabiting the cytoplasm of insect’s cells are vertically transmitted to the offspring through ovules, so they need a higher proportion of female insects to guarantee their own perpetuity. To alter this proportion, they use different methods: male killing, induction of parthenogenesis, feminization or cytoplasm incompatibility, for which they usually induce changes at the genetic level.

One of the most well-studied cases is Wolbachia, an intracellular bacterium capable to induce a sex-ratio bias through almost every of the aforementioned methods.

Phenotypes resulting from insects infected with Wolbachia. Figure belonging to the following paper: Werren, J. H., Baldo, L. & Clark, M. E. 2008. Wolbachia: master manipulators of invertebrate biology. Nature Reviews Microbiology, 6(10), 741-751.

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Do you know any other relationship between microbes and insects? Leave your comments below!

References

• Bourtzis K.,  Miller T. A. (2003). Insect Symbiosis. CRC Press.
• Douglas, A.E. (1998). Nutritional interactions in insect-microbial symbioses: Aphids and their symbiotic bacteria Buchnera. Annual Review of Entomology, 43: 17–38.
• Vega F.E., Blackwell M. (2005). Insect-Fungal Associations: Ecology and Evolution. Oxford University Press, USA.

The cover image is a montage made by the author from two images: 1) bacterium vector (by Flaticon from www.flaticon.com) and 2) termite vector (obtained from www.allstatepest.com.au).

Plants and animals can also live in marriage

When we think about the life of plants it is difficult to imagine without interaction with the animals, as they establish different symbiotic relationships day after day. These symbiotic relationships include all the herbivores, or in the contradictory way, all the carnivorous plants. But there are many other super important interactions between plants and animals, such as the relationships that allow them to help each other and to live together. So, this time I want to present mutualism between plants and animals.

And, what is mutualism? it is the relationship established between two organisms in which both benefit from living together, i.e., the two get a reward when they live with the other. This relationship increase their biological effectiveness (fitness), so there is a tendency to live always together.

According to this definition, both pollination and seed dispersal by animals are cases of mutualism. Let’s see.

POLLINATION BY ANIMALS

Many plants are visited by animals seeking to feed on nectar, pollen or other sugars they produce in their flowers and, during this process, the animals carry pollen from one flower to others, allowing it reaches the stigma in a very effective way. Thus, the plant gets the benefit of fertilization with a lower cost of pollen production, which would be higher if it was dispersed through the air. And the animals, in exchange, obtain food. Therefore, a true relationship of mutualism is stablished between the two organisms.

 “Video:The Beauty of Pollination” – Super Soul Sunday – Oprah Winfrey Network (www.youtube.com)

The extreme mutualism occurs when the species evolve depending on the other organism, i.e., when there is coevolution. We define the coevolution such as these evolutionary adaptations that allow two or more organisms to establish a deep relationship of symbiosis, due that the evolutionary adaptations of one specie influence the evolutionary adaptations of another organism. For example, this occurs between various orchids and their pollinators, as is the well- known case of Darwin’s orchid. But there are many other plants that also have co-evolved with their pollinators, as a fig tree or cassava.

In no way, this should be confused with the trickery produced by some plants to their pollinators, that is, when they do not obtain any direct benefit. For example, some orchids can attract their pollinators through odours (pheromones) and their curious forms that resemble female pollinator, stimulating them to visit their flowers. The pollinators will be impregnated with pollen, which will be transported to other flowers due to the same trickery.

Bee orchid (Ophrys apifera) (Autnor: Bernard DUPONT, flickr).

SEED DISPERSAL BY ANIMALS

The origin of seed dispersal by animals probably had occurred thanks to a co-evolutionary process between animals and mechanisms of seed dispersal in which both plants and animals obtain a profit. The most probably is that this process began in the Carboniferous (~ 300MA), as it is believed that some plants like cycads developed a false fleshy fruits that could be consumed by primitive reptiles that would act as seed dispersers. This process could have intensified the diversification of flowering plants (angiosperms), small mammals and birds during the Cretaceous (65-12MA).

The mutualism can occur in two ways within the seed dispersal by animals.

The first case is carried out by animals that eat seeds or fruits. These seeds or some parts of the fruits (diaspores) are expelled without being damaged, by defecation or regurgitation, allowing the seed germination. In this case, diaspores are carriers of rewards or lures that result very attractive to animals. That is the reason why fruits are usually fleshy, sweet and often have bright colours or emit scents to attract them.

For example, the red-eyed wattle (Acacia cyclops) produces seeds with elaiosomes (a very nutritive substance usually made of lipids) that are bigger than the own seed. This suppose an elevated energy cost to the plant, because it doesn’t only have to produce seeds, as it has to generate the award too. But in return, the rose-breasted or galah cockatoo (Eolophus roseicapillus) transports their seeds in long distances. Because when the galah cockatoo eats elaiosomes, it also ingest seeds which will be transported by its flight until they are expelled elsewhere.

On the left,  Galah  cockatoo (Eolophus roseicapillus) (Autnor: Richard Fisher, flickr) ; On the right, red-eyed wattle’s seeds (black) with the elaiosome (pink) ( Acacia cyclops) (Autnor: Sydney Oats, flickr).

And the other type of seed dispersal by animals that establishes a mutualistic relationship occurs when the seeds or fruits are collected by the animal in times of abundance and then are buried as a food storage to be used when needed. As long as not all seed will be eaten, some will be able to germinate.

A squirrel that is recollecting som nuts (Author: William Murphy, flickr)

But this has not finished yet, since there are other curious and less well-known examples that have somehow made that both animals and plants can live together in a perfect “marriage.” Let’s see examples:

Azteca and Cecropia

Plants of the genus Cecropia live in tropical rain forests of Central and South America and they are very big fighters. The strategy that allow them to grow quickly and capture sunlight, avoiding competition with other plants, resides in the strong relationship they have with Azteca ants. Plants provide nests to the ants, since their stems are normally hollow and with separations, allowing ants to inhabit inside. Furthermore, these plants also produce Müllerian bodies, which are small but very nutritive substances rich in glycogen that ants can eat. In return, the ants protect Cecropia from vines and lianas, allowing them to success as a pioneer plants.

Ant Plants: Cecropia – Azteca Symbiosis (www.youtube.com)

Marcgravia and Bats

Few years ago, an interesting plant has been discovered in Cuba. This plant is pollinated by bats, and it has evolved giving rise to modified leaves that act as satellite dish for echolocation performed by these animals. That is, their shape allow bats to locate them quickly, so they can collect nectar more efficiently. And at the same time, bats also pollinate plants more efficiently, as these animals move very quickly each night to visit hundreds of flowers to feed.

Marcgravia (Author: Alex Popovkin, Bahia, Brazil, Flickr)

In general, we see that the life of plants depends largely on the life of animals, since they are connected in one way or another. All the interactions we have presented are part of an even larger set that make life a more complex and peculiar one, in which one’s life cannot be explained without the other’s life. For this reason, we can say that life of some animals and some plants resembles a marriage.

REFERENCES

• Notes from the Environmental Biology degree (Universitat Autònoma de Barcelona) and the Master’s degree in Biodiversity (Universitat de Barcelona).
• Bascompte, J. & Jordano, P. (2013) Mutualistic Networks (Chapter 1. Biodiversity and Plant-Animal Coevolution). Princeton University Press, pp 224.
• Dansereau, P. (1957): Biogeography: an Ecological Perspective. The Ronald Press, New York., pp. 394.
• Fenner M. & Thompson K. (2005). The Ecology of seeds. Cambridge: Cambridge University Press, 2005. pp. 250.
• Font Quer, P. (1953): Diccionario de Botánica. Editorial Labor, Barcelona.
• Izco, J., Barreno, E., Brugués, M., Costa, M., Devesa, J. A., Fernández, F., Gallardo, T., Llimona, X., Parada, C., Talavera, S. & Valdés, B. (2004) Botánica ªEdición. McGraw-Hill, pp. 906.
• Murray D. R. (2012). Seed dispersal. Academy Press. 322 pp.
• Tiffney B. (2004). Vertebrate dispersal of seed plants through time. Annual Review of Ecology, Evolution and Systematics. 35:1-29.
• Willis, K.J. & McElwain, J.C. (2014) The Evolution of Plants (second edition). Oxford University Press, pp. 424.
• National Geographic (2011). Bats Drawn to Plant via “Echo Beacon”. http://news.nationalgeographic.com/news/2011/07/110728-plants-bats-sonar-pollination-animals-environment/

Evolution for beginners 2: coevolution

After the success of Evolution for beginners, today we’ll continue  knowing the basics of biological evolution. Why  exist insects that seem orchids and vice versa? Why gazelles and cheetahs are almost equally fast? Why your dog understands you? In other words, what is coevolution?

WHAT IS COEVOLUTION?

We know that it is inevitable that living beings establish symbiotic relationships between them. Some depend on others to survive, and at the same time, on elements of their environtment as water, light or air. These mutual pressures between species make that evolve together, and as one evolve as a species, in turn it forces the other to evolve. Let’s see some examples:

POLLINATION

The most known process of coevolution is pollination. It was actually the first co-evolutionary study (1859) by Darwin, although he didn’t use that term. The first to use the word coevolution were Ehrlich and Raven (1964).

Insects existed long before the appearance of flowering plants, but their success was due to the discovery that nectar is a good reserve of energy. In turn, the plants found in the insects another way more effectively to carry pollen to another flower. Pollination by the wind (anemophily) requires more production of pollen and a good dose of luck to at least fertilize some flowers of the same species. Many plants have developed flowers that trap insects until they are covered with pollen and then set them free. These insects have hairs in their body to enable this process. In turn some animals have developed long appendages (beaks of hummingbirds, butterflies’ proboscis…) to access the nectar.

Darwin’s moth (Xantophan morganii praedicta). Photo by Minden Pictures/Superstock

It is the famous case of the Darwin’s moth (Xanthopan morganii praedicta) of which we have already talked about. Charles Darwin, studying orchid Christmas (Angraecum sesquipedale) saw that the nectar was 29 cm inside the flower. He sensed that there should exist an animal with a proboscis of this size. Eleven years later, Alfred Russell Wallace reported him that the Morgan’s sphinxs had proboscis over 20 cm long, and a time later they were found in the same area where Darwin had studied that species of orchid (Madagascar). In honor of both it was added “praedicta” to the scientific name.

There are also bee orchids that mimic female insects to ensure their pollination. To learn more about these orchids and the Christmas one, do not miss this post by Adriel.

The bat Anoura fistulata and its long tongue. Photo by Nathan Muchhala

But many plants not only depend on insects, also some birds (like humming birds) and mammals (such as bats) are essential to pollination. The record for the longest mammal tongue in the world is for a bat from Ecuador (Anoura fistulata); its tongue measures 8 cm (150% of the length of its body). It is the only who pollinates one plant called Centropogon nigricans, despite the existence of other species of bats in the same habitat of the plant. This raises the question of whether evolution is well defined, and occurs between pairs of species or it is diffuse due to the interaction of multiple species.

PREDATOR-PREY RELATIONSHIPS

The cheetah (Acinonyx jubatus) is the fastest vertebrate on land (up to 115 km/h). Thomson’s gazelle (Eudorcas thomsonii), the second (up to 80 km/h). Cheetahs have to be fast enough to catch a gazelle (but not all, at risk of disappearing themselves) and gazelles fast enough to escape almost once and reproduce. The fastest gaelles survive, so nature selects in turn faster cheetahs, which are who eat to survive. The pressure from predators is an important factor that determines the survival of a population and what strategies should follow the population to survive. Also, the predators will find solutions to possible new ways of life of their prey to succeed.

Cheetah hunting a Thomson’s gazelle in Kenya. Photo by Federico Veronesi

The same applies to other predator-prey relationships, parasite-host relationships, plants-herbivores, improving their speed or other survival strategies like poison, spikes…

HUMAN AND DOGS … AND BACTERIA

Our relationship with dogs since prehistoric times, it is also a case of coevolution. This allows, for example, to create bonds with just looking at them. If you want more information, we invite you to read this post where we talk about the issue of the evolution of dogs and humans in depth.

Another example is the relationship we have established with the bacteria in our digestive system, essential for our survival. Or with pathogens: they have co-evolved with our antibiotics, so using them indiscriminately has favored these species of bacteria to develop resistance to antibiotics.

THE IMPORTANCE OF COEVOLUTION

Coevolution is one of the main processes responsible for the great biodiversity of the Earth. According to Thompson, is responsible for the millions of species that exist instead of thousands.

The interactions that have been developed with coevolution are important for the conservation of species. In cases where evolution has been very close between two species, if one become extint will lead to the extinction of the other almost certainly. Humans constantly alter ecosystems and therefore biodiversity and evolution of species. Just declining one species, we are affecting many more. This is the case of the sea otter (Enhydra lutris), which feeds on sea urchins.

Sea otter (Enhydra lutris) eating sea urchins. Photo by Vancouver Aquarium

Being hunted for their fur, urchins increased number, devastated entire populations of algae (consumer of CO2, one of the responsible of global warming), seals who found refuge in the algae nonexistent now were more hunted by killer whales… the sea otter is therefore a key species for the balance of this ecosystem and the planet, as it has evolved along with urchins and algae.

Coevolutive relations between flowers and animals depend on the pollination of thousands of species, including many of agricultural interest, so we must not lose sight of the seriousness of the issue of the disappearance of a large number of bees and other insects in recent years. A complex case of coevolution that directly affects us is the reproduction of fig.

TO SUMMARIZE

As we have seen, coevolution is the evolutionary change through natural selection between two or more species that interact reciprocally.

It is needed:

• Specificity: the evolution of each feature of a species is due  to selective pressures of the feature of the other species.
• Reciprocity: features evolve together.
• Simultaneity: features evolve simultaneously.

REFERENCES

• Massa R (2007). Atlas ilustrado del origen de la vida: La evolución de las especies. Jaca Book spa, Milano.
• Muchhala N. Nectar bat stows huge tongue in its rib cage. Nature (2006) vol. 444 (7120) pp. 701-702
• Coevolución: patrones y procesos
• Celebrando a Darwin
• Los 10 mamíferos más rápidos de la tierra
• La coevolución y las enseñanzas de Darwin
• Coevolución
• Las nutrias marinas pueden salvar nuestro planeta

Symbiosis: relationships between living beings

Predation, parasitism, competition… all living beings, besides interacting with the environment, we relate to other living beings. What types of relationships in addition to those you know? Do you feel like to know them?

INTRODUCTION

The group of all living beings in an ecosystem is called biocenosis or community. The biocenosis is formed in turn by different populations, which would be the set of individuals of the same species occupying an area. For survival, it is imperative that relations between them are established, sometimes beneficial and sometimes harmful.

INTERESPECIFIC RELATIONSHIPS

They are those that occur between individuals of different species. This interaction it is called symbiosis. Symbiotic relationships can be beneficial to a species, both, or harmful to one of the two.

Detrimental to all the species involved:

• Competition: occurs when one or more resources are limiting (food, land, light, soil …). This relationship is very important in evolution, as it allows natural selection acts by promoting the survival and reproduction of the most successful species according to their physiology, behavior …

Rainforests are a clear example of competition between vegetals in the search for light. Kuranda rainforest, Australia. Photo by Mireia Querol

One species has benefits and the other is detrimented:

• Predation: occurs when one species (predator) feeds on another (prey). This is the case of cats, wolves, sharks …

Great white shark (Carcharodon carcharias) jumping to depretade a marine mamal, maybe a sea lion. Photo taken from HQ images.

• Parasitism: one species (parasite) lives at the expense of other (host) and causes it injury. Fleas, ticks, pathogenic bacteria… are the best known, but there are also vertebrate parasites, like the cuckoo that lay their eggs in the nests of other birds, which will raise their chicks (brood parasitism). Especially interesting are the “zombie parasites”, which modify the behavior of the host. Read this post to learn more!

  

  Reed warbler (Acrocephalus scirpaceus) feeding a cuckoo’s chick (Cuculus canorus). Photo by Harald Olsen

  Parasites that live inside the host’s body are called endoparasites (such as tapeworms), and those who live outside ectoparasites (lice). Parasitism is considered a special type of predation, where predator is smaller than prey, although in most cases does not cause the death of the host. When a parasite causes illness or death of the host, it is called pathogen.

  

  Cymothoa exigua is a parasite that replaces the tongue of fish with their own body. Picture by Marcello Di Francesco.

Kleptoparasitism is stealing food that other species has caught, harvested or prepared. This is the case of some raptors, whose name literally means “thief.” See in this video a case of kleptoparasitism on an owl:

Kleptoparasitism can also occur between individuals of the same species.

One species has benefits and the other is not affected:

• Commensalism: one species (commensal) uses the remains of food from another species, which does not benefit or harm. This is the case of bearded vultures. It is also commensalism the use as transportation from one species over another (phoresy), as barnacles attached to the body of whales. The inquilinism is a type of commensalism in which a species lives in or on another. This would apply to the woodpeckers and squirrels that nest in trees or barnacles living above mussels. Finally, metabiosis is the use of the remains of a species for protection (like hermit crabs) or to use them as tools.

  

  The woodpecker finch (Camarhynchus pallidus) uses cactus spines or small branches to remove invertebrates from the trees. Picture by Dusan Brinkhuizen.

  Both species have benefits:

• Mutualism: the two species cooperate or are benefited. This is the case of pollinating insects, which get nectar from the flower and the plant is pollinated. Clownfish and anemones would be another typical example, where clown fish gets protection and food scraps while keeps predators away and clean parasites of the sea anemonae. Mutualism can be optional (a species do not need each other to survive) or forced (the species can not live separately). This is the case of mycorrhizae, an association of fungi and roots of certain plants, lichens (mutualism of fungus and algae), leafcutter ants …

  

  Atta and Acromyrmex ants (leafcutter ants) establish mutualism with a fungus (Leucocoprinus gongylophorus), in which they gather leaves to provide nutrients to the fungus, and they feed on it. It is an obligate mutualism. Photo taken from Ants kalytta.

INTRAESPECIFIC RELATIONSHIPS

They are those that occur between individuals of the same species. They are most beneficial or collaborative:

• Familiars: grouped individuals have some sort of relationship. Some examples of species we have discussed in the blog are elephants, some primates, many birds, cetaceans … In such relationships there are different types of families.
• Gregariousness: groups are usually of many unrelated individuals over a permanent period or seasonal time. The most typical examples would be the flocks of migratory birds, migration of the monarch butterfly, herds of large herbivores like wildebeest, shoal of fish …

  

  Gregariousness of these zebras, along with their fur, allow them to confuse predators. Photo taken from Telegraph

• Colonies: groups of individuals that have been reproduced asexually and share common structures. The best known case is coral, which is sometimes referred to as the world’s largest living being (Australian Great Barrier Reef), but is actually a colony of polyps (and its calcareous skeletons), not single individual.
• Society: they are individuals who live together in an organized and hierarchical manner, where there is a division of tasks and they are usually physically different from each other according to their function in society. Typical examples are social insects such as ants, bees, termites …

Intraspecific relations of competition are:

• Territoriality: confrontation or competition for access to the territory, light, females, food… can cause direct clashes, as in the case of deer, and/or develop other strategies, such as marking odor (cats, bears…), vocalization …

  

  Tiger figthing for territory. Video caption by John Varty

• Cannibalism: predation of one individual over another of the same species.

And you, as a human, have you ever thought how do you relate with individuals of your species and other species?

REFERENCES

• Zompopas
• Relacions de la biocenosi
• Ecología de las comunidades
• Els ecosistemes (xtec)
• National Geographic

Communication among plants: allelopathy

As always have been said, plants are unable to speak. But, even if they don’t speak, this does not mean they do not communicate with each other. Relatively few years ago, during the period from 1930 to 1940, it was discovered that plants also transmit certain stimuli to others. But, what kind of communication exist among them? What are their words and how are pronounced? And what involves this interaction?

INTRODUCTION

In 1937, Molisch introduced the term allelopathy referring to the two Latin words “Allelon” and “Pathos”, which mean “another” and “suffering”, respectively. But, the actual meaning of the word was determined by Rice in 1984. Allelopathy now means any effect that a plant transmits to another directly or indirectly through production of different metabolism compounds, causing either a positive or negative effect on the other organism. These compounds are called allelochemicals.

The allelochemicals are released on the environment by plants. But, they are not directly aimed to the action site, thus it is a passive mechanism. To be effective, allelopathic interaction needs that these substances are distributed along the ground or the air and that they reach the other plant. Once inside the recipient plant, this one may have defense and degradation mechanisms of the compounds while avoiding the effect, or conversely, it will suffer a pathological effect.

Allelopathy (Adapted image of OpenClips)

ROUTES OF RELEASE

The release of allelochemicals can be 4 main ways:

• Leaching: the aerial part of the plant lets go substances by rain effect. Then, they can fall on other plants or on the ground. Therefore, it can be direct or indirect effect, depending on whether they falls on another plant or not. Although, in principle, it is considered indirect.
• Decomposition: the plants drop their leftovers on the ground, where they decomposed under the microorganisms action, which help the release of the compounds. The plant leftovers range from leaves to branches or roots. The substances found there may be inactive until coming into contact with moisture or microorganisms, or can be active and then be inactivated by the microorganisms activity or by being retained on the ground. So, it is an indirect way. The decomposition is very important because the most of allelochemicals are released this way.
• Volatilization: the substances are released by the stomata (structures that allow the exchange of gas and transpiration). These are volatile and water-soluble, thus can be absorbed by other plant’s stomata or be dissolved in water. Commonly, plants using these pathways occur in temperate and warm climates. It is considered a direct route.
• Exudation: the plants can also release allelochemicals directly by live roots. The exudation system depends especially of roots state, of the kind of roots and of their growing level (if they are growing or not).

The 4 main pathways of allelochemical releasing: volatilization (V), leaching (L), descomposition (D) and root exudation (E). (Adapted image of OpenClips)

REGULATORY FACTORS

Factors influencing the release of allelochemicals are normally abiotic, such as high radiation, low humidity, unsuitable pH, ultraviolet light, temperature, nutrient deficiency, pollution or contamination (including pesticides ). The higher is the stress caused by this factors to the plant, highest is the allelochemicals amount released from secondary metabolic routes.

• This is important for research and pharmacy: for generating relevant oils many plants are grown under stressful conditions, as it is thanks to the production of these secondary metabolites that they can survive.

Furthermore, biotic factors also take part, such as insects, herbivores or competition with other plant species. These activate the plant defenses and then the organism is stimulated to secrete bitter substances, or substances that harden the tissues, that are toxic or give off unpleasant odors, etc.

Finally, each plant has its own genome and this makes synthesize those or other substances. But, they are also determined by the phenology (life stages) and the development (if the size of the plant is bigger, it can release more allelochemicals).

ACTION MODE

The allelochemicals are very diverse and, therefore, it’s difficult to establish a general action model; since it depends on the compound type, the receiving plants and how it acts.

When we talk about how the allelochemicals can act at internal level, there is a large number of physiological parameters that can be affected. They have action on the cellular membrane, disrupt the activity of different enzymes or structural proteins or alter hormonal balance. They can also inhibit or reduce cellular respiration and chlorophyll synthesis, leading to a reduction in vitality, growth and overall development of the plant. Furthermore, these substances can also reduce seed germination or seedling development, or affect cell division, pollen germination, etc.

On the other hand, at external level, the allelochemicals may be related to the release or limitation of nutrients that are found in the soil. Others act on microorganisms, leading to a perturbation on the symbiotic relationships they establish. In addition, these substances have great importance into the generations succession, as they determine certain competition tendencies and also act on the habitat ecology. Even so, it is a successive competition, as they do not directly compete to obtain the main resources.

EXAMPLES

One of the best known allelochemicals is the juglone, produced by the Eastern black walnut (Juglans nigra). Juglone, once released to soil, can inhibit the other plants growth around the tree. This allows the issuing organism to get more resources, avoiding competition.

Eastern black walnut  (Juglans nigra) (Photo taken by Hans Braxmeier)

A very curious case is that of the acacias (Acacia). These plants synthesize a toxic alkaloid that migrates to the leaves when the body is attacked by a herbivore. This substance’s toxicity is high, because it damages with the contact and ingestion, becoming deadly even for large herbivores.In addition, this alkaloid is volatile and transferred by air to other nearby acacias, acting as an alarm. When the other acacias receive this signal, this component is segregated to leaves, making them darker. Even so, the effect is temporary. This makes animals like giraffes have to constantly move to eat a few leaves of each acacia, and always against the wind, to avoid toxicity.

Acacias (Acacia) (Photo taken by Sarangib)

REFERENCES

• A. Aguilella & F. Puche. 2004. Diccionari de botànica. Col·leció Educació. Material. Universitat de València: pp. 500.

• A. Macías, D. Marín, A. Oliveros-Bastidas, R.M. Varela, A.M. Simonet, C. Carrera & J.M.G. Molinillo. 2003. Alelopathy as a new strategy for sustainable ecosystems development. Biological Sciences in Space 17 (1).
• J. Ferguson, B. Rathinasabapathi & C. A. Chase. 2013. Allelopathy: How plants suppresss other plants. University of Florida, IFAS Extension HS944
• Notes of Phanerogamae, Applied Plant Physiology and Analisi of vegetation, Degree of Environmental Biology, UAB