11: Blattodea: insect-microbe interactions
Blattodea: Insect-Microbe Interactions
Lecture Overview
Lecture 11 of Insect Biology 11:370:381
Focus on interactions between insects and microbes, particularly bacteria and fungi.
Microbes & Insects
Role of Bacteria
Endosymbiosis: An organism living within the body or cells of another, crucial for the development of multicellular organisms.
Establishes mutualistic relationships enhancing abilities of the host.
The symbiont transmission process enables insects to acquire their symbionts.
Insect Microbial Interactions
Heritable Bacterial Endosymbionts:
Common in insects, leading to extensive research in the past decade.
Divided into:
Obligate (Primary) Symbionts: Essential for host's survival.
Facultative (Secondary) Symbionts: Not essential, can be beneficial under certain conditions.
Facultative Bacterial Symbionts
Acquired through horizontal transfer from the environment or other hosts.
Pathogen Transmission: Occurs via bites from other organisms sharing microbes.
Effects range from mutualism to manipulating reproduction of the host.
Wolbachia:
Most common intracellular bacteria in arthropods, known as a reproductive parasite.
Wolbachia
Overview
Obligate intracellular insect parasite, possibly the most prevalent reproductive parasite globally.
Explored in population replacement programs to control insect populations.
Effects of Wolbachia on Insects
Effects vary significantly among different insect species:
Some species require Wolbachia for mating.
Helps hosts evade pathogens (e.g., prevents Plasmodium acquisition in Anopheles mosquitoes).
Wolbachia in Control Programs
Reproductive Effects:
Male Killing: Males die during larval development.
Feminization: Males develop as females or infertile pseudo-females.
Parthenogenesis: Females capable of reproducing without males.
Cytoplasmic Incompatibility: Males infected with one Wolbachia strain cannot mate successfully with females infected with a different strain.
Interferes with chromosome division in embryonic cells, leading to asynchrony in cell division.
Implementing Wolbachia in Control Programs
Insect populations are surveyed for Wolbachia presence.
Insects are cultivated with Wolbachia and released into the environment.
The introduction of Wolbachia into populations often leads to rapid spread and potential population crashes of target species.
Obligatory Bacterial Symbionts
Example: Pea Aphid and Buchnera
Buchnera bacteria provide essential amino acids that aphids cannot obtain from their diet.
Plant sap is low in amino acids, high in sugar and nitrogen.
The interaction provides a mutualistic relationship where the insect shelters the bacteria and reduces exposure to predators.
Characteristics of Obligatory Symbionts
Critical for the insect's survival, usually acquired through vertical transmission (from parent to offspring).
These symbionts cannot survive independently of their host, serving a nutritional function beneficial for insects feeding on impoverished or unbalanced diets, such as plant saps or cellulose.
Blattodea Classification
Includes cockroaches and termites; previously classified separately as Isoptera.
Recent molecular evidence indicates cockroaches and termites are closely related.
Cockroaches as Pests
Evolutionary Context
Cockroaches may evoke a stronger evolutionary aversion in humans than mosquitoes.
Among the 30 species associated with humans, only 4 are classified as pests.
Public Health Concerns
While cockroaches do not directly vector diseases, they can cause allergic reactions leading to asthma.
They can transport pathogens on their bodies, making them public health threats.
Their aversion to light contributes to infestations that may go unnoticed (up to 48% unnoticed infestations).
Resilience and Preferences
Cockroaches exhibit high resilience, surviving long periods without food (up to a month) and even extreme cold.
Pest species prefer dark, damp habitats.
General Characteristics of Cockroaches
Most cockroach species are not harmful and occupy diverse habitats, thriving in tropical and subtropical regions.
Outdoor species live in leaf litter, under bark, and embedded in wood, with few species considered at risk of extinction.
Biological Characteristics of Cockroaches
Omnivorous Diet: Capable of feeding on a wide variety of materials.
Symbiotic Gut Bacteria: Some cockroaches host bacteria enabling them to digest cellulose, with symbionts often playing a crucial role in their survival.
Social Behavior in Blattodea
General Observations
Social insects, often forming aggregations or groups; few exhibit parental behavior.
Pheromones directed behavior, allowing for odor-based communication.
Example: German cockroaches create fecal trails that help others locate food sources.
Termites and Social Structure
Termites, belonging to the order Blattodea, are classified as exclusively eusocial, representing the highest level of social organization.
Likely evolved through a subsocial pathway similar to cockroaches, with genetic relationships supporting eusocial characteristics.
Variation in Social Behavior
Social behaviors in Blattodea vary:
Cockroaches may gather and display parental care.
Cryptocercus: A genus exhibiting more complex behaviors, related to maternal care and hosting symbiotic wood-digesting bacteria.
Termite Feeding Ecology
Feeding Strategies
Termite feeding strategies are influenced by their internal gut fauna:
Lower Termites: Depend on symbionts to digest cellulose from wood.
Higher Termites: Lack the characteristic symbionts but have developed a specialized gut with diverse bacteria.
Cryptocercus and Evolutionary Relationships
The genus Cryptocercus feeds on wood, displaying subsocial behavior and possessing similar gut bacteria to termites.
Hypothesized as an evolutionary precursor to modern termites.
Diversity and Evolution of Termite Social Behavior
Termites are obligately eusocial, representing a monophyletic lineage.
Their close phylogenetic relationship to some cockroaches indicates reliance on gut microbiota for processing nutritionally poor food.
Parental care might be an evolutionary response fostering the development of eusociality, suggesting food resources play a significant role.
Insect and Fungus Interactions
Nature of Interactions
Interactions can be positive (mutualistic) or negative (parasitic) for insects.
Fungiculture
Mechanisms
Occurs when insects evolve to cultivate fungus for mutual benefit.
Leaf-Cutting Ants: Collect leaves to feed established fungus colonies, serving as their primary food source.
In some ant species, fungi are synonymous with obligate symbiosis, reflecting high levels of domestication.
Termites and Fungiculture
Termites engage in similar farming behavior, reliant on fungi for nutrition.
Termitomyces: A specific mushroom cultivated by termites, providing sustenance in their mutualism.
Parasitic Fungi
Infection Processes
Fungal spores can infect an insect's body cavity, proliferating until the insect dies, releasing spores back into the environment.
These fungi often manipulate the insect's behavior to enhance their reproductive success.
Entomopathogenic Fungi
Certain fungi can penetrate the exoskeleton directly, requiring no ingestion to infect the host.
Examples include Metarhizium and Beauveria, which have potential as eco-friendly alternatives to chemical insecticides.
Zombie Ants
Referring to fungi like Ophiocordyceps unilateralis that infect carpenter ants:
Alters behavior, compelling ants to abandon their nests for higher vegetation.
The manipulated ants secure themselves to specific leaves, the optimal height for fungal reproduction.
Diversity of Cordyceps
Thousands of Cordyceps species specifically target individual insect species, showcasing unique evolutionary adaptations.
Conclusion
Microbial interactions, be they beneficial or detrimental, play a vital role in the ecology of insects, influencing evolutionary pathways and behaviors across diverse species.