Chapter 2. Associations between insects and nonpathogenic

microorganisms (I)

The diversity and ubiquitousness of insects provide ample opportunities for them to

come into contact with microorganisms

1. Symbiosis ("symbio" living together + "sis" the act of)

- first used by De Bary (1879) for the living together of dissimilar organisms

regardless of the result of such an association (evolutionarily diverse

groups of organisms live in close association with one another)

Host Symbiont

Parasitism ― ＋Commensalism 0 ＋Mutualism ＋ ＋

● Symbiosis in evolution

- Modern synthesis on evolution: natural selection acting on mutations could

generate new adaptation and new species

- Evolution proceeds through cooperation, not competition, as in plastids

(chloroplast) from cyanobacteria and mitochondria from alpha-proteobacterial

ancestor

- Symbiosis is major driving force behind evolution

: Lynn Margulis (Univ. of Massachusetts, Dept. of Geosciences)

2. Mutualism

- Mutualism occurs when;

a) there is a close and intimate association between the host and symbiont,

and usually neither can survive without the other

b) the symbiont causes no apparent injury or harm to the host, and the

changes or effects that it causes are not pathological

c) the symbiont is transmitted by an elaborate and complicated manner from

generation to generation of the host

d) the host rigidly controls the numbers of the symbiont so that it is not

overcome by their numbers

e) the symbiont provides nutrition or other beneficial products to the host and,

in return, it obtains protection, food, lodging, and a means of distribution

(1) Mutualistic association between animal and microbe

< The host: Euprymna scolopes, Hawaiian bobtail squid >

< The symbiont: Vibrio fischeri, bacteria >

< Light organ in the center of mantle cavity of squid >

※ Vibrio fischeri is non-pathogenic symbiotic bacteria with bioluminescent

properties found globally in the marine environments

☞ Other Vibrio species, such as Vibrio parahaemolyticus, causal agent of

food poisoning and Vibrio vulnificus, causal agent of vibrio septicemia

◇ Euprymna scolopes is a small bobtail squid, occurring in the marine coastal

environment of the Hawaiian archipelago. Whereas most other squid are

always out and about, the bobtail squid buries itself in the sand during its

daylight quiescent period. Then, at night, individuals of this species emerge

from the sand to forage on prey, such as shrimp and worms

◇ To camouflage themselves while swimming, these squid form a relationship

with the luminous bacterium Vibrio fischeri. The light of their symbiotic

partner is emitted out of the animal’s ventral surface, and mimics

downwelling moonlight and starlight, so that the squid does not cast a

shadow. In this way, potential predators on the sea floor cannot see the

bobtail squid as easily.

◇ The bacteria occur in the seawater at about 20,000 cells per milliliter. The

newly hatched juvenile squid draws the symbiont-containing water into its

mantle cavity as it ventilates its gills in respiration.

◇ Full colonization of the crypts by the symbionts sends a developmental signal

to the host that results in the regression of the superficial epithelium that

facilitated the initial colonization process. This regression involves

programmed cell death (apoptosis) of these epithelial cells and their

eventual sloughing.

◇ Each day at dawn, as it begins its quiescent period in the sand, the animal

vents 70-80% of the symbiont population into the surrounding seawater.

The bacteria that remain behind in the crypts grow to fill the organ, so

that, by dusk, the light organ again has a full complement of symbionts.

3. Parasitism

- Parasitism is a type of symbiotic relationship between two different organisms

where one organism, the parasite, takes from the host, sometimes for a

prolonged time.

- In general, parasites show a high degree of specialization for their mode of life,

and reproduce more quickly and in greater numbers than their hosts.

- Parasitism is differentiated from parasitoidism, a relationship in which the host is

always killed by the parasite

1) Wolbachia in arthropods and nematodes

(1) Pathogenic/mutualistic to insects (depends on sex)

; pathogenic to males, beneficial to females

- one of the most common parasitic microbes (infect up to 70% of insects)

- intracellular bacteria

- does not infect vertebrates including humans

- vertical (thru oocyte) and horizontal transmission

- male killing: males are killed during larval development, which increase the

rate of born females

• infected females could live longer, produce more eggs, and have higher

hatching rates than its noninfected cousins

➜ facilitating transmission from mother to offspring

- feminization of genetic male: infected males develop as females (distort sex

ratio)

- parthenogenesis: produce offspring without males with the help of

Wolbachia • several species are so dependent on Wolbachia that they are unable to

reproduce effectively without the bacteria in their bodies

- reproductive (cytoplasmic) incompatibility (inability of Wolbachia infected

males to successfully reproduce with uninfected females or infected with

another Wolbachia strain) ➜ produce few or no offsprings

- Wolbachia has been linked to viral resistance in Drosophila melanogaster and mosquito species

• in the common house mosquito, higher levels of Wolbachia density were

correlated with more insecticide resistance

(2) Mutualistic to filarial nematodes

- obligate mutualist to filarial nematodes (Onchocerca volvulus), causative

agent for river blindness, transmitted by black fly (Simulium damnosum

complex)

< Human onchocerciasis (=river blindness) and causative agent, Onchocerca volvulus filarial nematode, and black fly vector >

< Wolbachia within an insect cell >

2) Entomopathogenic nematode-symbiotic bacteria-insect hosts

- The nature of their relationships

* Mutualistic: EPN and bacteria

* Pathogenic: EPN-bacteria complex to insect hosts

- Entomopathogenic nematodes and symbiotic bacteria

* Steinernematidae

→ symbiotic bacteria: Xenorhabdus spp.

* Heterorhabditidae

→ symbiotic bacteria: Photorhabdus spp.

< Steinernema carpocapsae IJ >

< Steinernema feltiae IJ and adult male >

- Only IJ contained symbiotic bacteria in the vesicle

* nematodes provide bacteria with;

▷ secure shelter

▷ nutritions

▷ transportation

* bacteria provide nematodes with;

▷ toxin

▷ antibiotics

▷ food

- Mode of action

* IJs containing symbiotic bacteria seek susceptible insect host

; foraging strategies (ambushing, active cruising)

* IJs enter the insect host via natural openings (e.g, mouth, anus, spiracles)

toward hemocoel

* IJs exit from non-feeding stage and reinitiate development

* Xenorhabdus bacteria are multiplied in the vesicle and released into

hemocoel thru intestine

* Released bacteria proliferate in the hemolymph

* Bacteria produce virulent factors (toxins, antibiotics)

* IJs resume developmental cycles (egg → J1 → J2 → J3 → J4 → adult)

* Insect host died after 48h

* IJs re-colonized by few cells of symbiotic bacterium exit the insect cadaver

to search for a new host insect

< IJs and their host, Galleria mellonella larvae >

< IJ with symbiotic bacteria before/after being exposed to hemolymph>

< Consumed host insect by Steinernema jollieti >

< IJs exit the insect cadaver to search for a new host insect >

- Without symbiotic bacteria nematodes (not all of them) cannot kill the insect

hosts

- Without nematodes symbiotic bacteria cannot invade the host insect's hemocoel

and cannot survive in the soil

4. Commensalism

- Commensalism is a type of relationship between two organisms where one

benefits and the other is not significantly harmed or benefited

(1) Phoresy

- One animal attaching to another animal for transportation only

< Harvestmen (daddy longlegs) and beetle with phoretic mites >

(2) Inquilinism

- An organism shares a home of another

- Staphylinid beetle and European ant

< Atemeles pubicollis > < Formica rufa >

- Staphylinid larva produces a glandular secretion that induces brood-tending

ants to groom the alien

- Food is obtained by adoption of the begging posture of an ant larva, in

which the larva rears up and contacts the adult ant's mouthparts,

provoking a release of regurgitated food

< Staphylinid beetle tending by ant worker >

<Rove beetle defensive behavior>