1 The evolution of heritable symbionts

Symbiotic microorganisms are extremely widespread in nature, having intimate and often obligatory associations with their`host' species. Despite the near ubiquity of arthropod symbionts, their study has been constrained by their fastidious nature and inability to be cultured out of the arthropod host. Recent advances in molecular biology have provided new tools for symbiosis research and stimulated new investigations of many symbiont systems that were described many years ago but were previously difficult to investigate in detail. The term 'symbiosis' in its most general (and original) sense refers to the i ntimate 'living together' of dissimilar organisms (de Bary 1879). Symbiont interactions with hosts have been traditionally classified as mutualistic (beneficial), parasitic (harmful) or commensal (neutral). However, assigning a symbiotic association to these groupings is often problematic. The associations between symbiont and host.are complex and can shift between the different states both over time and depending upon the particular phenotype being considered (Clay 1988; Saffo 1991). Furthermore, it is not entirely clear to what extent mutualistic relationships are reciprocal. Although the benefits to the host are often clear and can be established by experimental elimination of the symbiont, it is much less clear whether mutualistic symbionts truly benefit from the association. It has been argued that such symbionts may better be considered slaves of the host than mutualists (Maynard Smith and Szatlunary 1 995), although domesticated microorganisms may be a better analogy. Regardless of the relationship between symbiont and host (whether mutualistic, parasitic, or exploitative), there is considerable scope for reciprocal manipulation by both parties.