Arxiu d'etiquetes: insect vectors

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).

Sleep tight, don’t let the bed bugs bite!

Have you ever felt uncomfortable when hearing this expression or feared to find your bed infested with bed bugs? Yes, bed bugs exist. However, good news is that not all insects known as ‘bugs’ sting nor live inside our bed sheets.

What bugs really are? Are all of them harmful? Where can we find them? Find out their diversity through this post, and give up thinking that bugs are dangerous!

Which insects are called ‘bugs’?

When talking about ‘bugs’, people are unconscious about the true diversity of these organisms. Bugs, and more exactly true bugs, belong to the Heteroptera suborder, which includes more than 40,000 species worldwide; in fact, they are the largest group of insects with simple metamorphosis. Their most ancient fossil, Paraknightia magnífica, which was found in Australia, has been dated from the late Permian (260-251 MA).

The Heteroptera belong to the Hemiptera order, inside which we can find other suborders which were formerly classified as a single suborder (‘Homoptera’). Some of the suborders once classified as ‘Homoptera’ include some well-known organisms, such as cicadas (Cicadidae) and aphids (Aphididae).

How can we recognize them?

Heteropterans appear in different forms and sizes. The tiniest specimens belong to the Anthocoridae, Microphysidae, Ceratocombidae, Dipsocoridae, Aepophilidae and Leptopodidae families, which are barely visible to the naked eye. Among the largest members there are some species of the Belostomatidae family, such as Lethocerus indicus (6.5-8 cm length). Despite this, they appear as a monophyletic group according to molecular data.

They show at least three synapomorphies:

  1. Piercing-sucking mouthparts, long, forming a stylet.

    Mouthparts of the predator Arilus cristatus (Reduviidae). Picture property of John Flannery on Flicker (CC 2.0).
  2. Paired odoriferous glands.
  3. Four-segmented antennae.

Furthermore, they have forewings (formally known as hemelytra) with both membranous and hardened portions, which gives its name to the group (Heteroptera, from the Ancient Greek ‘hetero’, different; ‘-pteron’, wings).

Pentatomidae. The proximal part of forewings is hardened, while the distal one is membranous. Picture property of Mick Talbot on Flickr (CC 2.0).

Ecology

Life cycle

Heteropterans undergo a simple metamorphosis, so youths or nymphs and adults almost show no differences and cohabit in the same habitat. After hatching, nymphs molt several times until reaching the last nymphal molt, known as imaginal molt, through which they reach adulthood.

Life cycle of heteropterans. Picture property of Encyclopedia Britannica, Inc. (link).

Adults differ from nymphs on having wings, a new disposition of odoriferous glands openings, a different number of tarsal (legs) and antennal segments, ocelli, ornaments (spines and glandular hairs), sexual traits on the terminal abdominal segments and sometimes a different coloration, besides a bigger size and a way harder tegument.

Nezara viridula nymph (Pentatomidae), still wingless. Picture property of S. Rae on Flickr (CC 2.0)

Communication and defense

Specimens of the same species emit volatile pheromones produced by their odoriferous glands as a way of communication. So, they can expel aggregation pheromones and sexual pheromones to gather in a point or to find a mate, respectively. In some species, it has also been documented the emission of sounds produced by stridulation, that is, producing sounds by rubbing together certain body parts.

Heteropterans develop passive and active defense mechanisms:

  • Among passive mechanisms, we can highlight the own body shapes (e. g., smooth and rounded structures which difficult their capture by predators), the inactivity as a way to go unnoticed by other organisms, and the crypsis or mimicry. Some examples of crypsis or mimicry are 1) color mimesis (homocromy) 2) shape mimesis (homotopy), through which they imitate structures of their environment, either plants or animals (e. g. ant-mimicry or myrmecomorphy) and 3) disruptive mimesis, that is, their outlines get blurred with the environment, so it gets difficult for predators to find them.
Leptoglossus occidentalis (Coreidae), with their wide tibiae that look like leaves. Picture property of Giancarlodessi (CC 3.0).
Myrmecoris gracilis (Miridae), a clear example of ant-mimicry or myrmecomorphy. Picture property of Michael F. Schönitzer (CC 4.0).
  • Some active mechanisms are 1) escaping, 2) biting, 3) the detachment of some appendices to confuse predators and 4) the emission of stink or irritating substances by their odoriferous glands, which in most of cases they acquire from plants they feed on. Others emit stridulating sounds.

Life forms and diversity

Even though most people know something about heteropterans due to the famous bed bugs, feeding on blood is far from being the only life form among true bugs.

  • Terrestrial

Most heteropterans inhabit terrestrial environments, either on plants or on the ground as phytophagous (they feed on vegetal fluids) or predators of other insects. There are also some terrestrial heteropterans that feed on roots or on fungi that develop under tree bark. Some examples of terrestrial phytophagous families are Pentatomidae and Coreidae. Among predators, which use their stylet to inoculate proteolytic agents inside their preys to dissolve their content and then suck it, there are a lot of members from Reduviidae family.

  • Aquatic and semiaquatic

Aquatic and semiaquatic forms have special adaptations to live in water, like hydrofuge hairpiles which repel the water. Most of them live in lakes and rivers, either on their surface (semiaquatic) or submerged.

Semiaquatic species usually have long legs and long antennae, which together with the hydrofuge hairpiles let them to stand on water. Water striders (Gerridae), which are very abundant in Europe, are a clear example of this life form.

Water striders (Gerris sp.). Picture property of Webrunner (CC 3.0)

Aquatic species usually have a pair of legs adapted to swim. A good example of this are the members of the family Notonectidae or backswimmers, which have the hind legs fringed for swimming.

Notonecta sp. (Notonectidae). Picture property of Jane Burton/Bruce Coleman Ltd. (link).

Despite living in water, aquatic heteropterans need surface air to breath, so they go out of water periodically. They present different strategies to absorb oxygen, such as swallowing air that goes directly to the respiratory or tracheal system through a siphon (Nepidae) or capturing air bubbles with their hydrofuge hairpiles (Nepidae). Other simply get covered of a tiny air layer using their hydrofuge hairpiles.

  • Hematophagous

Finally, there are heteropterans that feed on blood and live as bird and mammal parasites. This is the case of the Cimicidae family (e. g. Cimex lectularius, the bed bug) and some groups of Reduviidae, such as the members of the subfamily Triatominae, which are also known for being vectors of the Chagas disease in the center and south of America (being Triatoma infestans its main vector).

Cimex lectularius or bed bug nymph. Public domain.
Triatoma sp. (Triatominae). Picture property of Bramadi Arya (CC 4.0).

Scientific interest

  • They help to regulate some wood and crop pests, having an important role in integratative pest management. This is the case of some predator heteropterans from the Reduviidae, Anthocoridae, Miridae, Nabidae and Geocoridae families. However, some phytophagous heteropterans can act as pests too.
  • They have been an interesting scientific model for the study of insect physiology.
  • They are an important element on human diet in some countries, being Pentatomidae one of the most consumed families. Some aquatic heteropterans, such as Lethocerus sp. (Belostomatidae) are very appreciated as food in some Asiatic countries, like Vietnam and Thailand.
Lethocerus sp. Picture property of Judy Gallagher on Flickr (CC 2.0).
  • Some of them are disease vectors or a cause of discomfort. The most classic example is the bed bug (Cimex lectularius), which has become a frequent pest in temperate regions; some Cimidae are also a threat for free range chickens and other farm birds. In America, Triatominae are vectors of different diseases, being the most famous the Chagas disease (transmitted by a protozoan, Trypanosoma cruzi).

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All organisms on Earth are necessary for some reason: you only need to investigate about them. Even the true bugs!

References

Main picture property of Pavel Kirillov on Flickr, with license  Creative Commons 2.0. (link).