Predators, parasites, and pathogens all impose strong selection pressures on their hosts or prey, causing them to evolve better strategies to survive . This coevolution of pathogen virulence as well as host resistance depends on many factors, including the extent of genetic variation for traits influencing attack and defense, trade-offs between these traits and other components of fitness, and the specificity of the interaction between different genotypes in the natural enemy and in its prey or host.
Drosophila melanogaster through its peculiar interaction with wasps is a great model to study innate immunity and the co-evolving mechanisms of their survival combats.
This peculiar relationship that exists between Drosophila melanogaster (the fruit fly) and wasps is not a usual parasitic relationship but a combination of predatory and parasitic behavior – a rather lethal combination.
Because of this fundamental distinction, wasps are referred to as parasitoids, which is a feeding behavior intermediate between the parasite and predatory end of the behavioral continuum.
Wasps are parasitic only during their development phases, and adults are free-living, which is a type of parasitism known as Protelean parasitism.
The aim is to look at how Drosophila melanogaster deals with its larval endoparasitoids (e.g., Leptopilina species) and see how the combatants use different counter-strategies to protect their homeostatic mechanisms.
In these conflicts, either the hosts are consumed by the endophagous(feeding on within) juvenile stages of the parasitoid or the latter are destroyed by the host. The fate of the host depends on its ability to identify the endoparasitoid as nonself and to synthesize cytotoxic molecules that specifically target the foreign organism.
Conversely, the developmental fate of an endoparasitoid depends on its ability to suppress the host’s immune system.
Drosophila in the wild suffers massive mortality from the attacks of parasitoid wasps. As many as 80% of Drosophila larvae in natural environments may be killed by wasps that lay eggs in them. As many as 350,000 species of parasitoid wasps may inhabit the natural world, an indication of their enormous importance in ecology and evolution. The species they parasitize have evolved diverse defenses to protect themselves against these parasitoids. Some defenses operate at the cellular and behavioral level.
In immuno-competent larvae of Drosophila melanogaster, the cellular immune response against the eggs of endoparasitic wasps involves the proliferation and differentiation of larval blood cells and their participation in cellular encapsulation of the eggs of the parasitoid and in the generation of cytotoxic molecules. In Drosophila larvae, the most numerous blood cell type in circulation is the plasmatocyte. However, when an immune response is elicited against a foreign entity (e.g., parasitoid), plasmatocytes rapidly multiply and subsequently transform into large flattened cells called lamellocytes. Foreign organisms that are too large to be phagocytosed are surrounded by numerous lamellocytes and concealed within a multilayered capsule. Another type of blood cell that participates in immune responses is the crystal cell, which is characterized by dark, rectangular, paracrystalline inclusions that contain enzymes. The main role of crystal cells during an immune reaction is to come in contact with the foreign surface, lyse, and release enzymes that cause the formation of melanin and the synthesis of cytotoxic molecules during cellular encapsulation.
For this response to be effective, timing may be essential. The encapsulation should be completed in less than 48 hr (before parasitoid egg hatches), as the moving larva is likely to escape from the forming capsule.
As much as Drosophila is evolving its combat against Wasps, the wasps have also evolved various mechanisms to evade Drosophila’s immune system.
It is generally acknowledged that passive protection against cellular encapsulation is afforded to those endoparasites that either develop in host locations inaccessible to immune cells or possess molecular surfaces that the host fails to distinguish as nonself.
wasps introduce immune-suppressive substances (e.g., virus-like particles, polydnaviruses, proteins, or venom of maternal origin) into the host at the time of oviposition. Virus-like particles, presumably target-specific immunity cells, adversely affecting their ability to recognize nonself, to form melanotic capsules, and/or to synthesize cytotoxic molecules.
The venom affects the hemocytes, the lymph gland, or melanization. The venom contains soluble proteins and peculiar vesicles with unclear biogenesis, which are likely involved in parasitic success. These purified vesicles target the host lamellocytes, changing their shape from discoidal to bipolar, which prevents them to adhere and form a capsule, or inducing their lysis. Venomics analysis allowed identifying the main venom proteins like P40 and LbGAP associated with the vesicles.
Endoparasitoids that succumb to host encapsulation presumably lack or have diminished immune-suppressive capabilities.
Apart from these cellular combats between the two species, there comes another strategy evolved by Drosophila that is a behavioral response which is through the visual perception that is activated at the sight of wasps.
The sight of wasps induces the dramatic upregulation in the fly nervous system of a gene that encodes a 41-amino acid micro peptide. Mutational analysis reveals that the gene is essential for mating acceleration.
Exposed flies start mating more quickly and more in female flies, perhaps due to greater parental investment than males in their offspring. The mating acceleration is elicited by exposure to several wasp species that parasitize Drosophila, but not by other species, inviting a future investigation into the precise nature of the visual cues that drive this response.
Drosophila-parasitoid interaction paves the way to new concepts in insect immunity as well as parasitoid wasp strategies to overcome it and to decipher mechanisms ensuring parasitic success.
These distinctive associations between various Drosophila species and solitary endoparasitic wasps make for great experiments for studying the population dynamics of coevolving host resistance and parasitoid virulence.
By-Amulya
Nandini
Chhavi
Deepanshi
(B.Sc. Life Sciences, Miranda House, University of Delhi)
References:-
- Emily Vass, Anthony J. Nappi, Fruit Fly Immunity: The fruit fly provides a suitable experimental model for studying various aspects of the cellular and humoral mechanisms, genetics, signaling cascades, and cytotoxic molecules involved in insect innate immunity, BioScience, Volume 51, Issue 7, July 2001, Pages 529–535, https://doi.org/10.1641/0006-3568(2001)051[0529:FFI]2.0.CO;2
- Ebrahim, S.A.M., Talross, G.J.S. & Carlson, J.R. Sight of parasitoid wasps accelerates sexual behavior and upregulates a micro peptide gene in Drosophila. Nat Commun 12, 2453 (2021). https://doi.org/10.1038/s41467-021-22712-0
- Kim-Jo C, Gatti JL, Poirié M. Drosophila Cellular Immunity Against Parasitoid Wasps: A Complex and Time-Dependent Process. Front Physiol. 2019;10:603. Published 2019 May 15. doi:10.3389/fphys.2019.00603
- Kim-Jo, C., Gatti, J. L., & Poirié, M. (2019). Drosophila Cellular Immunity Against Parasitoid Wasps: A Complex and Time-Dependent Process. Frontiers in physiology, 10, 603. https://doi.org/10.3389/fphys.2019.00603
- Carton, Yves & Bouletreau, M. & Alphen, Jacques & van Lenteren, Joop. (1986). The Drosophila parasitic wasps.
mhbiosphere 