Global patterns in the evolution of oak gallwasps

Gall inducers are highly specialized herbivores that hijack the development of their host plant, causing it to produce a novel growth called a gall. Gall structures are under inducer control and can be seen as the ‘extended phenotypes’ of inducer genes. Galls are induced by a wide range of organisms including bacteria, fungi, nematodes, mites and over 30,000 species of insects. The tissues these organisms induce in their host plant provide all of its food, while outer tissues protect it from external conditions and (in some cases) from attack by natural enemies (see the reference below by Stone and Schönrogge 2003). While some gall inducers are mutualists of their host plant (such as legume root nodule bacteria, and pollinating fig wasps), most are parasites.

This project focuses on oak gall wasps, which induce some of the most diverse and structurally complex galls (to get a general idea, do a Google image search for ’oak gall wasps’). Female gall wasps lay an egg in meristematic tissue, and substances secreted by the developing larva cause the development of gall tissues (see references by Hearn et al 2019 and Cambier et al 2019 for theories on how this might happen). Around 1000 oak gallwasp species have been described, primarily from temperate North America and Eurasia, but recent work has revealed dramatic new diversity on tropical oaks and their relatives in Southeast Asia. A striking feature of gall wasp evolution is that similar gall structures have evolved repeatedly in different branches of the gall wasp evolutionary tree - for an example, see the reference below by Nicholls et al (2016). Some of these traits - such as nectaries that recruit ant guards, and coatings of sticky glue or spines - protect the gallwasp against attack by their main natural enemies - parasitoid wasps (see Bailey et al 2009, below). All parasitoids attack these galls by drilling through gall tissues using their ovipositor, and laying an egg in or on the gall wasp larva within. Any gall trait under gall wasp control that reduces the risk of attack by a parasitoid should spread by natural selection. Convergent evolution of effective defences also makes adaptive sense. However, other aspects of gall structure - such as colour patterns - as yet have no inferred function. Another feature of oak gall wasps is that they have two alternating generations, each of which always induces a very different type of gall.

This PhD will address the following questions:
1. Which gall traits have evolved convergently in different gall wasp lineages, what are their functions?
2. How often do gallwasp lineages switch between alternative gall structures through evolutionary time? Are some switches more common than others? Are switches reversible? Have gall structures become more complex through evolutionary time?
3. Do we see similar patterns in gall evolution in separate continental gall wasp faunas?
4. Do the structures of sexual and asexual generation galls in each species evolve independently, or in parallel?

This project is for a student who would like to learn how to reconstruct the evolution of biological traits - particularly those mediating interactions between species. You will explore techniques for characterisation of gall structures, and identify characters whose evolution you wish to reconstruct. You will learn how to use a range of analytical methods (primarily in R) to address the research questions above. Your analysis will make use of an existing 6-gene phylogeny for 350 gall wasp species, representing over 500 gall structures. There will be opportunities to add further taxa to this phylogeny through working with molecular post-docs in the Stone lab and, if circumstances allow, fieldwork in the US and/or China. The project will benefit from working alongside a fully funded NERC research project on the evolution of oak gall wasp communities, which includes project staff with expertise in taxonomy, molecular phylogenetics, and analysis.

The supervisors for this project combine expertise in community ecology and the evolution of insect-plant associations (Graham Stone) and analysis of macroecological and macroevolutionary patterns (Ally Phillimore).

IF YOU WOULD LIKE TO APPLY FOR THIS PROJECT, PLEASE GET IN TOUCH! [Email Address Removed]

To find out more about what we do, visit our research web sites:
https://www.ed.ac.uk/profile/graham-stone
http://phillimore.bio.ed.ac.uk/AllyPhillimore
https://www.ceh.ac.uk/staff/karsten-sch%C3%B6nrogge

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