How do sex ratio distorting symbionts affect the evolution of their host?

The biology of animals is in part a function of the microbes they interact with. During digestion, for instance, food is broken down both by enzymes secreted by our digestive system and those secreted by the microbes that live within the gut. In many insects, microbe-host interactions are even more developed. Bacterial symbionts live inside the cells of the insect body, and these are passed from a female to her offspring via her eggs: heritable symbiosis.

We know a lot about how these symbionts affect the individual they infect. One particularly interesting impact is male-killing, where the bacterium passes from a female to her eggs, and kills those which develop as males. We know male-killing bacteria are common: they are present in many species, and, where they are present, can be present in the majority of individuals - this produces insect populations where males are rare. However, we know little about how these bacteria affect insect ecology or evolution. A variety of researchers believe these symbionts may drive changes in the way male and female insects are formed during development, sex determination. The hypothesis is simple - where symbionts target males only, natural selection counteracts this by favouring new ways of making a male that escape male-killing.

This study will examine this theory for a recent case of evolution of the blue moon butterfly to avoid the action of male-killing bacterium called Wolbachia. We have documented the spread of a mutation that rescues male blue moon butterflies from Wolbachia-induced death. This project will establish what this mutation is, whether it involves changes in a gene called 'doublesex', which defines male and female characteristics in insects.

A second aspect of male-killers is that they may drive very strong natural selection to rescue males. The intensity of selection is such that the changes that occur to rescue males may be otherwise deleterious. A second aim of the project is to establish if this is true, and whether the mutation (beyond rescuing males) degrades male and female function.

In completing this project, we will present the first direct test of the theory that the processes that make males/females different can be driven by microbes. This is an enigmatic link that would make clear the interdependence of insect and microbe evolution.

This system represents a spectacular case study of evolution in action, which involves both an attractive insect species and an intriguing parasite. This produces impact opportunities in the communication of science: because this is a study system in which we have observed natural selection, the research permits us to establish natural selection as an ongoing observable process, rather than a theory from which we can infer support, or as a long term process we cannot observe - a common misconception that raises skepticism over the process occurring at all. The importance of the study system in this communication is reflected in past work featuring as a front page story on Yahoo. The work also raises the topical issue of how 'male' and 'female' are defined, which is another aspect of biology very accessible to the public.

Who will benefit from this research?
a) Public sector/Third sector: Museum, Zoo and butterfly house educational facilities
b) Wider public
c) UK economic competitiveness

How will they benefit from this research?
a) Production of displays and lectures relevant to museum exhibitions on evolution, and to butterfly houses/zoos in which Hypolimnas butterflies are flying.
b) General public/society through increased science literacy: the key message for translation within this proposal is that evolution is an ongoing process rather than a purely historical phenomenon. This knowledge is required for assessment of impact of anthropogenic change in the medium term, which is required to evaluate policy and priorities with respect to this form of change.
c) UK economic competitiveness through training of researchers in leading genomic technologies used in biotech and health R&D.