The conquest of a new habitat: a study of nutritional and sensory adaptations in Drosophila suzukii larvae

Project Information

ECONUTRISENS

Grant agreement ID: 838205

DOI

10.3030/838205

Project closed

EC signature date 18 March 2019

Start date 1 January 2020

End date 30 January 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 262 209,60

EU contribution

€ 262 209,60

Coordinated by

MAX-PLANCK-GESELLSCHAFT ZUR FORDERUNG DER WISSENSCHAFTEN EV
Germany

Periodic Reporting for period 1 - ECONUTRISENS (The conquest of a new habitat: a study of nutritional and sensory adaptations in Drosophila suzukii larvae)

Reporting period: 2020-01-01 to 2022-12-31

Summary of the context and overall objectives of the project

Animals need to feed to get substances to use as sources of energy and material to function, maintain their bodies, grow, and reproduce. These substances are nutrients such as proteins, carbohydrates, and lipids. Different food sources contain these nutrients in different proportions, which means that, depending on the specific needs of an organism, some foods are nutritionally better.
Animal have mechanisms to match their feeding choices to their needs. However, for some organisms, such as fruit-fly larvae, the choice is out of their control.
During their life cycle, Drosophila fruit flies go through two feeding stages, the larva and adult, which have very different objectives and needs. Larvae feed to grow and accumulate resources fast. They are on the clock to be able to enter metamorphosis and turn into adult flies. They feed where they are born, and cannot waste time or energy searching for food.
On the other hand, adult flies need nutrients to survive and reproduce. This means obtaining energy and material to find partners, fight, court, and couple, produce and lay eggs. By choosing where they lay the eggs, adults also choose what food the larvae will have to eat.
There are many species of Drosophlla flies. They live in a wide variety of habitats and have adapted to a wide variety of food sources and egg-laying substrates.
Adult D. melanogaster female flies prefer to feed and lay eggs on overripe fruits that are colonized by yeasts. These fruits are protein-rich, and a good feeding substrate for the larvae. In contrast, D. suzukii adult females prefer to lay eggs in ripening fruits that are still on the plants. They have a unique physical adaptation, an enlarged and serrated ovipositor, which allows them to pierce the skin of those fruits and insert the eggs inside. These fruits are tough, with high sugar and low protein content. D. melanogaster develops poorly with such food composition. Nonetheless, D. suzukii larvae develop without problem on this food.
How have the nutritional needs, behavior, and physiology of D. suzukii larvae adapted to the egg-laying preference of the adults?
With my project, I study these questions at different levels; the nutritional composition of host fruits, the larval behavior, and the larval physiology. How does the fruit change as larvae develop? How do larvae develop in it? Is there a relationship between larval behavior and their survival in this environment?
This project touches on important issues for society. First, D. suzukii flies attack berries, cherries, and grapes and produce damage to fruits that are still on the plant reducing productivity. Basic knowledge of the development, behavior, and physiology of the species is necessary to protect crops. Second, from a purely basic biology sense, this project will explore how an adaptation to a new niche, such as being able to use hard fruits to lay eggs on, influences many aspects of a species' biology.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The first steps of the project were to set up the methodology to study the interaction between larvae and fruits during the larval period. I decided to work with recently hatched first instar larvae and I placed them myself inside of the fruit. By bypassing the need for adult flies to lay the eggs in the fruit, I could choose the fruit and control the number of larvae per fruit, and their placement within it.
Fruit fly development from first instar larva to metamorphosis takes around 5-7 days depending on the species and the environmental conditions. Studying the development of larvae in their natural substrates meant maintaining fruits in an incubator for 7 days, a long time for a fruit outside of cold storage. I tested several berries and several methodologies. I found that most berries would become moldy, ruining the observations. The ones that lasted the most were blueberries. I succeeded to avoid molding by keeping them hanging in a mesh bag so that air circulates all around the fruit. Blueberries were perfect because they are a firm, sugary substrate in which D. suzukii can develop but D. melanogaster cannot, and they are available throughout the year.
I described the larval development of D. suzukii larvae in blueberries in detail. I found that as they develop their environment is not homogeneous in time. The blueberry starts out as a hard, protein-poor environment. As the larvae process it, the substrate softens and its protein content increases. This activity of the larvae in the fruits also stimulates fermentation. We could detect typical chemicals produced by fermentation a few days after the introduction of larvae in the fruit.
When the larvae reach the third instar, the feeding substrate has become rich in proteins. Enough even to allow D. melanogaster development. We observed that both D. melanogaster and D. suzukii sense the processed blueberry as a nutrient-rich substrate by releasing insulin-like peptides while they sense virgin blueberry as a poor substrate.
Within the fruit, D. suzuki activity usually grew out from their insertion point. Larvae did not tunnel through the whole fruit from the beginning. They seemed to remain always close to the already softened tissue.
These observations prompted us to test their behavior and preferences. We found that within a fruit first-instar larvae innately prefer the tissue already processed by other larvae. Furthermore, a mix of three chemical volatiles from larva-processed fruit applied to virgin fruit tissue is enough to attract the larvae to it. We also observed that they dig their way to a preferred substrate and choose the processed tissue instead of the virgin one.
In the course of the project, I presented these results at three international meetings. I am currently preparing a manuscript for publication. I have also taken part in dissemination activities, such as the "Long Night of the Sciences". I also published articles on my website as well as drawings and short posts on social media to educate about the use of Drosophila species to study biology, with great feedback from the community.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Previously, other researchers showed that contrary to D melanogaster, D. suzukii larvae do not discriminate feeding substrates by their protein content. We show here that even though that may be the case, larvae have an innate preference for substrates processed by other larvae. Larvae can recognize these substrates by the presence of fermentation chemicals. By choosing them, larvae indirectly choose substrates with higher protein content. This implies a positive feedback loop in which the activity of the larvae stimulates more activity, more fermentation, and faster conversion of the fruit into a better substrate to sustain development. These findings suggest that it would be interesting to investigate whether adult female flies might also choose to lay eggs in fruits where these chemicals are present. They also prompt us to further investigate what is the source of the fermentation, is it carried out by microorganisms exclusive to D. suzukii? Are they present on the fruit? Could they be exploited for pest management? These findings could thus inform future research on how to deter adult flies from laying eggs on commercially important fruits. Our results also shed light on how the tiny larvae cope with the harsh environment in which they are born. They teach us that we should look at the feeding substrates of flies as dynamic environments that change composition through time.

A Drosophila suzukii larva walking on a blueberry