Dr Julia Ferrari

Senior Lecturer


The role of bacterial symbionts in insects

The majority of insects are hosts to bacterial symbionts. Most of these symbionts have been discovered only recently, but it is becoming increasingly clear that many of these bacteria can protect their hosts from environmental challenges such as natural enemy attack or extreme temperatures. We use aphids as a model system to study these symbionts. Aphids are known to harbour at least seven species of facultative symbionts with up to four in the same individual.

We are particularly interested in how different species of symbionts interact with each other and their aphid host, and how these interactions alter the outcome for the host. These interactions within the host have the potential to affect not only the insect itself, but also the entire aphid population and other species that are part of the ecological network. We strive to understand these complex interactions as well as the mechanisms underlying them

Ecological speciation

We are also interested in how and why organisms diversify, one of the central questions in evolutionary biology. After a long lasting debate whether diversification can only occur when populations are physically separated, it has recently become clear that differentiation can evolve within a population in response to ecological factors. We are interested in the factors that promote specialisation to different environments or resources. Again, we use a plant-feeding insect, the pea aphid Acyrthosiphon pisum, as a model system for this work. The pea aphid feeds on various different plant species, but each individual is very specialised on the plant that it is found on in the wild.

We have shown that aphids that have been collected from different species of legume are strongly genetically differentiated. There is a gradient of differentiation between these populations, suggesting that some of the populations have gone further on the route to complete speciation. This means that the pea aphid is an unusual system that we can now use to study different stages of the divergence process within the same species.  Specialisation might be strongly influenced by the ability of the insect to recognise the host plant through smell and taste. We are currently investigating the role of chemosensory genes in the differentiation process.

Pea aphids on a broad bean leaf. 

Pea aphids on a broad bean leaf

The parasitoid Aphidius ervi attacking a pea aphid 

The parasitoid Aphidius ervi attacking a pea aphid.

Research Group

Dr Lee Henry Post-Doc Aphid secondary symbionts: a eukaryote horizontal gene pool (in Oxford).

Dr Margriet van Asch

Post-Doc Aphid secondary symbionts: from model system to crop pests (in Oxford).

Piotr Lukasik

Research Student Facultative symbiont in cereal aphids (in Oxford).

Ailsa McLean

Research Student Host plant specialisation in the pea aphid (in Oxford).

Michaela Nelson

Technician Chemosensory genes and the evolution of aphid host races.

Genevieve Phillips

Visiting Associate Facultative symbionts in pea aphids.

Teaching and Scholarship

teaching icon
‌As a teacher I try to use my experience in, and enthusiasm for, molecular science to help students learn the fundamentals of a subject and also to become skilled at extending their knowledge through independent research.

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‌As a structural biologist, my lecture material includes the interesting fundamentals of protein structure, how structure relates to function and the methods that can be used to determine the structure of proteins at high resolution. My teaching is inspired by my research, which uses biophysical methods to study proteins involved in bacterial infection. These proteins include those that enable Gram positive bacteria to adhere to human tissues and bacterial proteins and other polymers that enable the development of infections (biofilms) on the surfaces of medical devices.

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‌Subjects could include molecular biology, structural biology and biophysical techniques for studying protein structure and function.  Tutorials are an excellent opportunity for students to ask questions and discuss a subject in more detail. Student preparation for tutorials can include reading, essay writing or the preparation of a more formal short presentation. 

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‌Students taking a project in my lab will be working on a project related to our ongoing research. I offer both primarily lab based and computer based projects. Lab work often includes protein expression, purification and biophysical characterization of a bacterial protein domain. Computer based work could include assignment of heteronuclear nuclear magnetic resonance spectra of a protein domain. Students have many informal and formal opportunities to discuss their project with either their day-to-day supervisor in the lab or the project supervisor. The final year project is an excellent opportunity to experience what it is like to work in a research lab. 


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Dr Julia Ferrari

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2016 -  Senior Lecturer Department of Biology, University of York
2009 - 2016 Lecturer Department of Biology, University of York
2006 - 2009 Departmental Lecturer University of Oxford
2001 - 2006 Post-doc Centre for Population Biology, Imperial College London
1998 - 2001 PhD Imperial College London
1997 Diplom Biologie (MSc) University of Göttingen, Germany



My research focuses on ecological and evolutionary aspects of plant-herbivore-natural enemy interactions. I am particularly interested in the evolution of specialisation and ecological speciation in these systems.

Many insects are infected with bacterial symbionts which can have strong ecological effects on their hosts.  They can for example increase the resistance of insects to natural enemies such as pathogenic fungi.  I am investigating how the symbionts affect their hosts’ ecology.  I am particularly interested in how multiple partners in these systems interact and co-evolve.  Most of my current work uses the pea aphid, Acyrthosiphon pisum, as a model system.


We have shown that the pea aphid is a complex of host-adapted populations that show a gradient of differentiation between populations, both in their host use and genetically. These host specialists also differ in which bacterial symbionts they carry, suggesting that the bacteria help the aphid to feed on certain plants.


  • Chemosensory genes and the evolution of aphid host races (with R. Butlin, University of Sheffield, C. Smadja, Montpellier, and Jing-Jiang Zhou, Rothamsted Research) (Funding body: NERC)
  • Aphid secondary symbionts: a eukaryote horizontal gene pool (with C. Godfray and M. Maiden, University of Oxford) (Funding body: NERC)
  • Aphid secondary symbionts: from model system to crop pests (with C. Godfray, University of Oxford, and F. van Veen, University of Exeter), (Funding body: BBSRC)

Research group(s)


PhD student Thomas Wilkinson Plant-mediated interactions between above- and belowground symbiotic microbes
PhD student Megan Hasoon  Performance enhancing bacteria: the ecological benefits of insect symbiosis
Postdoctoral Research Associate (joint with R. Butlin, Sheffield) Isobel Eyres Candidate genes for host association in aphids


Available PhD research projects

Multiple beneficial symbionts – too much of a good thing? (2015-16)

All organisms face a multitude of environmental threats and have to adapt to overcome these challenges.  Many insects get help from an unexpected source – bacterial symbionts that live in their bodies.  These endosymbionts affect their host’s biology, from allowing the host insect to feed on unbalanced diets to protecting from extreme temperatures or natural enemies.  A newly acquired symbiont can rapidly spread through a population and alter the insects’ interactions with other species and the environment under suitable conditions. This can have undesirable consequences if the host insect is an agricultural pest.

Endosymbionts that confer ecologically important benefits are particularly well studied in aphids, where phenotypic effects are fairly well understood.  However, often multiple species of symbionts occur in the same aphid individual.  It is likely that these species compete for resources and the host might be overexploited as a consequence.  This project will elucidate how single and multiple infections of beneficial symbionts differ in their effect on the host’s ecology.  The student will have the opportunity to develop this project according to his/her interests; this might include investigating the competitive interactions within the aphid, assessing the effects on the wider ecological community, or a mechanistic understanding of the interactions.

Dr Julia Ferrari

Contact details

Dr Julia Ferrari
Senior Lecturer
Department of Biology (Area 2)
University of York
YO10 5DD

Tel: 01904 328550