NERC E4 Inferring patterns and process in the global evolution of oak cynipid galls

Interested individuals must follow the "how to apply" link on the Geosciences E4 Doctoral Training Partnership web page: http://www.ed.ac.uk/e4-dtp/how-to-apply

Summary

The project will use phylogeny-based approaches to analyse patterns and processes in the evolution of oak galls, diverse and dramatic examples of 'extended phenotypes' induced on plants by cynipid wasps.

Project background

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 are highly diagnostic of the gall-inducer, and can be seen as the ‘extended phenotypes’ of inducer genes. Galls are induced by a range of organisms from bacteria through fungi to 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 natural enemies. Some gall inducers are mutualists of their host plant (such as the bacteria inhabiting legume root nodules, and pollinating fig wasps), but most are thought to be parasites.

This project will focus on a group of insects – oak gall wasps – that induce some of the most diverse and structurally complex galls (see images, top, or 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. Some of these traits - such as nectaries that recruit ant guards and coatings of sticky glue or spines - protect the gall against attack by their main enemies - parasitoid wasps. Convergent evolution of effective defences 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 project will be the first to take a global look at the diversity of cynipid gall structures across America, Europe and Asia, using a DNA sequence phylogeny for over 350 species to infer patterns in their evolution.

Research questions

1. Which gall traits have evolved convergently in different gall wasp lineages? What is the apparent functional significance of such traits?
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?

Methodology

This project is for a student who would like to learn how to use a molecular phylogeny to reconstruct the evolution of biological traits - and particularly traits that mediate interactions between species. You will explore morphometric techniques for characterisation of gall structures, and identify characters (and sets of characters) whose evolution you wish to reconstruct. You will then use a range of analytical methods (primarily in R, for which training will be given) 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 if you wish, through working with molecular post-docs in the Stone lab and, if circumstances allow, fieldwork in the US and/or China. You will consider alternative hypotheses for the patterns you observe, bringing together insect community ecology and aspects of plant development. 

In Year 1 you will become familiar with the cynipid system and use a range of techniques to compare the diversity of gall structures in three continental gall wasp faunas (North America, Europe, and China), identifying traits that are shared, and structures that are regionally unique. If conditions allow, this year can involve fieldwork in California and/or China. You will also receive training in statistical modelling in R. 

In Year 2 you will use (and if required, extend) our existing gall wasp phylogeny in a range of advanced statistical methods (primarily in R) to focus on Research Questions 1 and 2. We will expect you to generate a research manuscript by the end of this year. 

In Year 3 the project will focus on analysis of relative rates of diversification, addressing Research Questions 3 and 4. Again, we will expect you to generate a second research manuscript by the end of this year. 

Training

A comprehensive training programme will be provided comprising both specialist scientific training and generic transferable and professional skills, including statistical analysis in R.

Specialist training will be provided in gall wasp biology and character traits, (if required) DNA extraction and Sanger sequencing of gene sequences, phylogeny reconstruction from sequence data, and R-based analyses of phylogenetic dynamics and character state evolution. 

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.

Requirements

1. A first class or good 2:1 undergraduate degree or MSc in ecology or evolution.
2. Strength in statistics, with experience in R desirable.
3. Experience of phylogenetic analysis is desirable - training will be given.
4. Interest in entomology is desirable - training will be given.
5. Enthusiasm for overseas fieldwork (we hope!) and willingness to work as part of an expedition team are desirable.