Chapter 2. Associations between insects and nonpathogenic microorganisms (I)

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1 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 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

3 (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.

4 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.

5 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

6 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 >

7 < 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

8 < 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 >

9 - 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 >

10 - 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>