Insects represent the most abundant animal class, with over one million species, many of which rely on symbiotic microorganisms for nutrition and protection. These symbionts can be heritable or vertically transmitted, influencing host reproduction, disease transmission, and nutritional dynamics, as seen in aphids and termites. Additionally, symbiotic relationships, such as those involving Wolbachia and Buchnera, demonstrate the crucial roles symbionts play in the fitness and survival of their insect hosts.

2.  Insects are the most abundant class of animals living today, there are about
over one million spp. are known.
 20 % of insects support the symbiotic m’orgs.
 This symbiotic association provides nutritional advantages or protection.
 These symbionts found on the insect surface or in their digestive tracts.
 Endosymbionts – intracellular bacteria localized to specialized organs
within the insect.

3. • Microbial symbionts are acquired by the host from
Types of heritable symbionts:
• Heritable symbionts are obligate symbionts becoz they lack
free-living replicative stage.
• Bacteria requires host for replication.
An environmental reservoir
• Horizontal transmission (less
specificity)
Transferred directly from the parent
to the next generation
• Heritable or vertical transmission

4. Based
on
the
host
dependence
Primary
symbionts
Eg:
Buchnera
aphidicola
Required for
host
reproduction
Restricted to specialized region called
bacteriome.
Within the bacteriome the bacterial cells
reside in a specialized cells called
bacteriocytes
Secondary
symbionts
Eg:
Serratia
symbiotica
Not required for
host
reproduction
Not always present in every spp.
Not restricted to a particular tissues
Eg:pathogens invade different cell types & live
extracellularly within the insects hemolymph. 2
symbionts can invade bacteriocytes and Co-reside with
primary symbionts or Displace them
• Take nutritional advantage and protection from the
insect host
• some provide protection to the host against invasion
by pathogens or predators. Eg: In Drosophila
neotestacea – Spiroplasma protects the host from
parasitic nematode worm.

5. • Most heritable symbionts are transmitted maternally,
• These symbionts has the potential to manipulate the host reproductive
system and increasing the frequency of female progeny. The suppression of
male progeny helps in the expansion of infected individuals and also
increase the rate of spread through the insects
• Eg: Rickettsia infection in Whitefly population: 1% infected in 2000, 97%
infected in 2006.
• Wolbachia infection in Drosophila simulans spread within 3 years

6. • Used in pest management & the control of vector borne disease.
• Symbiotic Wolbachia (reproductive manipulators) – distributed among
insect spp. The sperm of infected male insect sterilize the uninfected
females. This mechanism is used to control elephantiasis by infecting the
Culex quinquefasciatus.
• Presence of symbionts decrease the transmission of disease
• Eg: Wolbachia infected Aedes aegypti mosquitoes less likely to transmit
the dengue virus.
• Sometimes presence of symbionts increase the disease transmission
• Eg: Hamiltonella bacteria (Enterbacteriaceae) infected whiteflies transmit

7. Nutrition significance of insect symbionts
• 1. Aphid – Buchnera aphidicola (γ proteobacterium)
Aphids feed on the carbohydrate rich but nutrient-poor sap of phloem
vessels in plants.
The symbionts provide the nutrients which is not acquired by their
primary diet
Insect harbours the bacteria Buchnera in their bacteriomes (cytoplasm)
This bacteria provides 9 essential amino acid to the host
If the insect is treated with antibiotics, the insect starves and dies
Likewise Buchnera lacks the enzyme needed for the synthesis of leucine
which was provided by the Aphid host

8. Aphids
Primary Symbiont
Buchnera Secondary
Symbiont
Provide nine
essential amino
acids
Provide enzyme for
leucine
biosynthesis
Posses 2 genes for
Tryptophan biosynthesis
Remaining genes is located in the
chromosome of secondary
endosymbiont

9. 2. Mealybugs (Planococcus citri) – Betaproteobacterium
These bugs have two stable bacterial symbionts
Candidatus Termblaya princeps (Betaproteobacterium)
Candidatus Moranella endobia (gammaproteobacterium)
These symbionts provide eesential amino acids to the host
Moranella bacterium lives inside the Tremblaya (bacterium within a bacterium
symbiosis)
Tremblaya lost their genome for tRNA synthetases which was supplied by the

10. • Termites has symbiotic association with Protists and bacteria that digest
lignocellulosic material.
• Like the rumen of herbivorous animals, insect gut also provides a
protective niche for microbial symbionts and in return, the insect gains
access to the nutrients provided by the symbionts.
• Microbial symbionts in termite decompose the greater part of cellulose
(74-99%) & hemicellulose (66-87%) ingested by the termites

11. • Termite gut consists of
• Foregut (crop & muscular gizzard)
• Tubular midgut (site of secretion of digestive
enzymes & absoption of nutrients).
• Large hindgut (consists of single chamber
called paunch) – harbours diverse microbial
community and it is the major site of nutrient
absorption.
• Hindgut communities
• 12 phyla of bacteria & few Archaea
• Treponema – predominant
• Phylum - Fibrobacteres

13. • The flagellated protozoa Trichonympha live in the gut of termites & wood
roaches
Termites
(lacks
cellulase)
Protozoa
(contains cellulase)
Cellulose
Acetate & other products
Metabolize
Oxidise
Eat wood
Carbon & energy
source