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NERC DTP BC218: Trading off chemical and physical defences: How do specialist caterpillars cope with stiff and sharp plant trichomes?

Supervisor: Dr Walter Federle, Co-supervisors: Dr Erika Pinheiro de Castro, Professor Chris Jiggins

Project summary:

Plants can protect themselves against herbivory by chemical and physical defences. Trichomes can act as physical defences by inhibiting insect walking and feeding, or entangling insects. Some stiff and sharp plant trichomes can injure or kill insects by piercing their cuticle or gut, but some herbivores have devolved counteradaptations to feed on plants with such trichomes.

This project will investigate the trade-off between chemical and physical defences in the evolutionary model system of Heliconius caterpillars feeding on Passiflora vines. Passiflora vines can fend off herbivores chemically via cyanogenic glucosides (CGs). Heliconius caterpillars are major herbivores sequestering CGs for their own defence. To escape Heliconius herbivory, some Passiflora species have diversified their CGs while others evolved sharp hooked trichomes as a physical defence. Unraveling how specialized Heliconius species have adapted to these physical/chemical challenges is key to understanding their arms-race with Passiflora.

What the student will be doing:

The project involves both laboratory and field work, and the detailed approach can be tailored to the student's specific interests:

1) The biomechanical properties of the plants’ trichomes, and their effects on caterpillars will be studied using force measurements, microscopy and modelling.

2) The effect of trichomes on specialist H. charithonia vs. non-specialist caterpillars will be studied by observing their behaviour, locomotion and growth rate.

3) Ultrastructural adaptations of caterpillars for handling hooked trichomes will be explored using electron microscopy.

4) The genetic mechanisms underlying caterpillar adaptations can be studied via RNA-seq transcriptomics.

5) The palatability of Passiflora leaves for specialist vs. non-specialist Heliconius caterpillars will be studied, and their CG composition will be quantified using LC-MS.

6) The distribution of chemical and physical defences will be mapped onto existing phylogenies of Passiflora and Heliconius.

7) To collect specimens and to study the variation in feeding performance between local populations of H. charithonia, field work will be conducted in Panama.

References:

1. Cardoso ZM. 2008. Herbivore handling of a plant's trichome: the case of Heliconius charithonia (L.) (Lepidoptera: Nymphalidae) and Passiflora lobata (Killip) Hutch. (Passifloraceae). Neotropical Entomology 37: 247-252.

2. de Castro ECP, Zagrobelny M, Zurano JP, Cardoso MZ, Feyereisen R, & Bak S. 2019. Sequestration and biosynthesis of cyanogenic glucosides in passion vine butterflies and consequences for the diversification of their host plants. Ecology and Evolution 9: 5079-5093.

3. de Castro ECP, Zagrobelny M, Cardoso ZM & Bak S. 2018. The arms race between heliconiine butterflies and Passiflora plants – new insights on an ancient subject. Biological Reviews 93: 555-573.

Specific qualifications desirable for this particular project:

The project will suit a biologist with an interest in field work and physical or mathematical approaches to biology.

Additional information:

Funding is available for this project via the NERC Cambridge Climate, Life and Earth (C-CLEAR) Doctoral Training Partnership – please refer to their website for more information: https://nercdtp.esc.cam.ac.uk/projects/BC218