Understanding host-parasite interactions between male-killing bacteria and ladybird and other invertebrate hosts

Ladybirds are economically important biocontrol insects, used to control aphids, scale insects and other species of crop pests. We have studied the effect of one group of ladybird endosymbionts, known as male-killers. These maternally inherited bacteria kill male hosts, and hence have a large reproductive cost to a female ladybird. Some ladybirds have evolved resistance to these bacteria. We know that these male-killing bacteria have been horizontally acquired from other ladybird or prey species. Ladybirds are also host to other endosymbionts, the effects of which are unknown.

The importance of the microbiota associated with other animals is increasingly being recognised. The majority of these organisms are not readily grown in lab cultures and such techniques therefore fail to provide useful levels of detail regarding the presence and function of such microorganisms. Fundamental questions about the host-parasite or commensal basis of these relationships are simply unstudied in many cases. Improvements in DNA sequence-based approaches have facilitated a step-change in the level of detail and understanding of these systems. Exciting recent discoveries include the considerable range of benefits provided to honey bees via their gut biota, including metabolic capabilities and protection from parasites. In addition, it is becoming clear that there are complex interactions between insect immune systems and resident microbiota, beneficial and otherwise, as demonstrated in studies on a diverse range of hosts including Drosophila, mosquito and aphid species (Douglas, 2014).

Inherited endosymbionts are well-established as causative agents of sex-ratio distortion in a wide variety of host organisms. To date the majority of research has focussed on identification of endosymbionts and evolutionary history and consequences of sex ratio bias. Ladybirds are considered a model system for the investigation of one particular group of sex-ratio distorting agents, namely the male-killers. Male-killers comprise a diverse group of bacteria. A combination of ecological and life-history factors make ladybirds particularly susceptible to invasion by male-killing bacteria (Majerus & Majerus, 2012).

Endosymbionts have other consequences for the survival of their hosts, for example they have been shown to affect dispersal behaviour in spiders (Goodacre et al., 2009) and susceptibility to insecticides in whitefly and spiders.

This project would use genomic and transcriptomic data to analyse gene expression in inherited endosymbionts in ladybird species to investigate the differential gene expression between infected and uninfected hosts and to identify genes and pathways involved in these interactions.