Bedtime stories are our favorite part of the day. Instead of reading a book that night, I narrated my research work to my then-toddler. During the entire story time, she listened to me with little hums. When it was bedtime, while my brain was racing – did she follow the story? How do I improve my science communication skills? She broke the silence and said, “Just like you, fly mamma also knows how to protect her kids.” I sighed! I delighted in the realization that my research pitch lay in my toddler’s unconvoluted perspective.
Parental care is a fascinating biology. To foster and protect offspring, animals display various behaviors, from mammals nursing babies to birds building nests for their chicks, guarding against predators, teaching survival skills, etc. While some of these caring behaviors captivate our attention even in our day-to-day busy lives, others go unnoticed. For instance, fruit flies (Drosophila/vinegar flies) and their parasitoids that cohabit in our immediate surroundings. We might only pay attention to fruit flies when we want to control their infestation on our kitchen counters. We are unaware of the parental pressures on these flies, who must seek ways to protect their offspring from the parasitoid wasps, a major cause of insect mortality in the wild.
Parasitoid wasp lifecycle intertwined with Drosophila, Sadanandappa et al., 2024
Like all insects, the lifecycle of fruit flies and parasitoid wasps includes four stages: egg, larva, pupa, and adult. A fertilized egg goes through a series of developmental stages undergoing complete metamorphosis before adults emerge from the pupal case. However, unlike fruit flies, parasitoids feed on living hosts for a part of their development. Mated female wasps attack fruit fly offspring – larvae and pupae – and lay their eggs inside them (host). An immune response to such an assault is the primary defensive strategy. If the host immune system identifies the wasp egg and successfully responds against it, the fruit fly develops normally, killing the egg (a process called ‘melanotic encapsulation’). However, most of the time, the venom injected by the female parasitoid, along with its egg, suppresses the host’s immune response. The egg then develops, hatches, molts, and grows as an adult wasp emerging from a fruit fly pupal case by consuming the host from inside (1). While it sounds nasty, it is the life cycle of a parasitoid wasp, which obligately requires a host for their development and survival.
The developing stages of fruit flies are highly vulnerable to wasp infection due to their limited ability to escape the attack. Besides an immune response, larvae avoid parasitoids by crawling away from the media infested with wasps or exhibiting corkscrew rolling motion to escape the wasp grip during the attack. Additionally, adult flies display certain behaviors in the presence of a wasp, eventually reducing the risk of parasitoid infection in their offspring. For instance, they may avoid the wasp-invaded area, bury their eggs inside the food medium, or preferentially seek alcohol-rich food for egg laying, which is toxic for wasp egg development.
These behaviors are intriguing because unlike the juvenile stages, adult flies are not prone to parasitoid infection, leading to several interesting questions – how do parental flies recognize parasitoids? Do they distinguish between their threats? How are parental instincts encoded at the neuronal and molecular level that protect the progeny from parasitism?
To address these questions in the laboratory conditions, I presented female fruit flies (D. melanogaster) with their natural parasitoids – Leptopilinia boulardi that specifically lay eggs in developing larval stages, and as an experimental control, presented female flies with pupal parasitoids (Pachycrepoideus sp.) which infect the pupae. Female flies laid fewer eggs in the presence of larval wasps, but not to pupal parasitoids. Physiologically, adult females accomplish this reproductive change in two ways – first, they retain completely developed eggs, ready to be released after fertilization, and second, they increase cell death of the developing eggs in the ovary, leading to reduced egg-lay (2).
Though pupal parasitoids are a potential threat, why do fruit flies selectively respond to larval wasps? In the wild, fruit fly females encounter larval parasitoids more often than pupal wasps in their habitats. Consequently, adult flies plausibly evolved responses specific to larval parasitoids by detecting their presence through sensory systems such as olfaction (smell) and vision (sight). Removal of these sensory cues abolishes adult flies’ ability to respond to the wasp (2, 3, 4, and 5). Alternatively, fruit flies may react to pupal parasitoids differently other than egg lay decrement, which needs to be explored.
Surprisingly, I noticed something unusual! Female flies increased their egg-lay after prolonged cohabitation with pupal parasitoids. Combining behavioral assay with neurogenetics, immunohistochemistry, and quantitative analysis, I found that female fruit flies produced more eggs, and their ovaries were bulkier when cohabited with Pachycrepoideus wasp, but not larval parasitoids (6). Does that mean Drosophila adults try to outpace the reproductive potentials of parasitoids as a simple defensive tactic?
If correct, fruit flies outpacing strategy over Pachycrepoideus breeding appears costly and advantageous to wasps. Because egg production is metabolically expensive and even with increased progeny numbers, fruit flies are at risk of predation from different parasitoid wasps and other potential threats. However, experimental observations supported the idea that as a counter-offensive tactic, the parasitoid wasp-controlled fly reproduction to increase the number of hosts for itself.
How does Pachycrepoideus hijack the Drosophila reproductive system to produce more eggs? A simple answer is exploiting host innate signaling mechanisms involved in egg production (gametogenesis). Pachycrepoideus-specific olfactory and visual cues activate sensory signaling in fruit flies dwelling in the wasp-infested area. The parasitoid-activated multimodal sensory circuits recruit the signaling mechanisms that shape germline development and physiology. These pathways promote germ stem cell division and accelerate egg development in the ovary, increasing egg production.
Interestingly, there was no increase in egg production when the sensory inputs were lacking or when fruit flies cohabited with other wasp species. Shifting fruit flies from Pachycrepoideus-infested media to regular food reverses observed reproductive and physiological changes. This finding suggests that Pachycrepoideus evolved mechanisms to co-opt the host’s ability to stimulate germ stem cell proliferation and produce more offspring for their advantage.
These observations in fruit flies and their parasitoids, which serve as an ecologically relevant model, help us understand the evolutionary wicked intricacies in animal interactions. On the one hand, Drosophila adults recognize and differentiate between their threats and display selective behavioral and physiological responses. On the other hand, different wasp species developed distinct offensive strategies to maximize their survival. These defensive and counter-offensive tactics appear to be locked in a perpetual evolutionary arms race to beat another’s newly evolved modifications. Either way, both adults make the best selection for the fitness and survival of their offspring. Just like this mamma, as complimented by a toddler!
References:
- Sadanandappa MK, Sathyanarayana SH, Bosco G. (2023). Parasitoid Wasp Culturing and Assay to Study Parasitoid-induced Reproductive Modifications in Drosophila. Bio-protocol 13(01): e4582.
- Sadanandappa MK, Sathyanarayana SH, Kondo S, and Bosco G. (2021). Neuropeptide F signaling regulates parasitoid-specific germline development and egg-laying in Drosophila. PLOS Genet. 17:e1009456.
- Kacsoh BZ, Lynch ZR, Mortimer NT, and Schlenke TA. (2013). Fruit Flies Medicate Offspring After Seeing Parasites. Science. 339, 947–950. 10.1126/science.1229625.
- Ebrahim SAM, Talross GJS, and Carlson JR (2021). The sight of parasitoid wasps accelerates sexual behavior and upregulates a neuropeptide gene in Drosophila. Nat. Commun. 12(1): 2453.
- Pang L, Liu Z, Chen J, Dong Z, Zhou S, Zhang Q, Lu Y, Sheng Y, Chen X, and Huang J. (2022). Search performance and octopamine neuronal signaling mediate parasitoid induced changes in Drosophila oviposition behavior. Nat. Commun. 13, 4476. 10.1038/s41467-022-32203-5.
- Sadanandappa MK and Bosco G. (2024). Parasitoid cues modulate Drosophila germline development and stem cell proliferation. Cell Rep. 43, 113657.
Author-
Madhu, hailing from the small and remote village of Karnataka in Southern India, was captivated by the intricate interactions within nature early on. Inspired by Mendelian genetics and honed by Gordon Shepherd’s quote, “Nothing in neurobiology makes sense except in the light of behavior,” her research focuses on discovering the biological underpinnings of new and conserved behaviors. When not at the bench, she enjoys hiking (having completed the NH48 4000-footers, including Mount Washington, the most brutal and tallest in the Northeast), gardening, outreach activities, DIY projects, and reading, among other things.
email: madhumalaks@gmail.com
Editors-
Sumbul Jawed Khan and Roopsha Sengupta
Illustrator-
Cover image by Andreia Rocha. You can visit her website and follow her on instagram.
Inset image- Madhumala Sadanandappa
