Introduction

One of the most predictable episodes of a female insect's life is the timing of mate-encounter and mating. In this chapter I propose that females are often subjected to predictable wounding during mating and that this wounding provides opportunity for environmental microbes to enter the female's haemocoel, thereby presenting immuno-logical costs. I argue that this combination of factors is likely to lead to reproduction being a period of heightened immunological activity that has resulted in specific immune defence mechanisms and management systems that function to minimize costs while maximizing immunological efficacy. If true, these phenomena may provide valuable insights into how organisms with mechanistically simple immune systems protect themselves against a complex pathogenic world, and may also provide logistic opportunities to better study immunity in the wild.

Parasites and pathogens are ubiquitous and major agents of selection, affecting host fitness by reducing longevity and/or reproductive output. Even if they do not kill their hosts, they often make them more susceptible to other forms of death. An organism's main form of defence against them is the immune system and, throughout this review, I use the concept of immunity developed by the field of ecological immunity (e.g. Rolff and Siva-Jothy, 2003; Siva-Jothy et al., 2005). Immune defence as envisaged by ecological immunity is broader than the physiological mechanisms examined by 'classical' immunologists: it encompasses all mechanisms that an organism uses to protect itself and so includes behavioural and anatomical traits, as well as the complex suite of systemic physiological mechanisms. In this chapter I consider the male-female interactions during reproductive encounters that are likely to affect overlooked patterns of female investment in immune function. Surprisingly little is known about how reproduction and immunity interact (but see Lawniczak et al, 2007), despite (a) the core position of this trade-off in evolutionary and ecological thinking, (b) its proposed role in sexual selection (e.g. Hamilton and Zuk, 1982; Folstad and Karter, 1992), and (c) the fact that immune genes are known to be activated by mating (Lawniczak and Begun, 2004; McGraw et al., 2004). I will concentrate on intersexual interactions and will not consider obligate sexually transmitted diseases (see review by Knell and Webberley, 2004). Rather I consider male traits that function to enhance male mating success but simultaneously increase the likelihood of wounding and therefore microbial transmission to females. The consequence is an increased exposure to 'opportunistic' microbes (e.g. bacteria, fungi, and fungal spores) that are present on the insect cuticle and in the immediate environment (see Figure 15.1). If females are wounded during mate encounters (which I propose is frequent and predictable across insect taxa), because of incidental damage caused by competition between males, courtship, copulation, and/or copulatory guarding then selection will favour the evolution of immune-management systems that offset these costs and operate in anticipation of these costs, or when they are incurred. My purpose is to stimulate research into this potentially rich but overlooked immuno-logical episode of an insect's life.

It is axiomatic that hosts encounter their parasites unpredictably. However, recent work on insects has uncovered some subtle immune regulation

Figure 15.1 An illustration of the natural microflora on the aedeagus of an insect (a bedbug, Cimex lectularius). The ablated structure was removed and held in sterile forceps and drawn in a zig-zag pattern across the sterile agar plate. The aedeagus is visible as the dark central dot in the bacterial colony at the bottom of the plate.

where hosts monitor their current level/likelihood of infection and adjust investment either in their offspring (e.g. Moret, 2006) or in later ontogenetic stages (e.g. Wilson and Reeson, 1998), suggesting that they are capable of 'predicting' periods of increased likelihood of infection. Alongside these phenomena are studies showing that insect immune genes are under circadian control (e.g. Lee and Edery, 2008). Whereas there are several interpretations for the functional significance of such oscillations, one simple explanation is that these cycles occur because there are diurnal changes in the likelihood of infection; that is, they function, at least in part, to balance costs and effectiveness in the context of the likelihood of infection. Insects may be very good at anticipating immune insult and modulating their immune-effector systems to optimize this balance.

I propose that insects may show well-developed abilities to 'anticipate' immune insult in relation to their environmental conditions/life-history stage and adjust investment in immunity accordingly. One aspect of all female insects' life cycle that is both predictable and, I suggest, associated with a high risk of infection is mating and its associated behavioural, anatomical, and physiological interactions. These encounters tend to be highly predictable for females because it is usually females that determine the temporal occurrence of mating. Insect mate-encounter sites are usually spatially predictable and males are often selected to concentrate their efforts defending and/or competing for resources that attract females, resulting in a highly male-biased operational sex ratio (the ratio of reproductively active males to reproduc-tively active females) at these sites (e.g. Thornhill and Alcock, 1983). Females can therefore avoid exposing themselves to males until they need to. However, a consequence of the male-biased operational sex ratio at these encounter sites is that females are then exposed to male traits that function to secure matings with females, and physically or physiologically 'subdue' them, often at a cost to their fitness. It is important to note that males will be subjected to similar selection, especially in species where fighting, and therefore wounding, is likely to lead to infection.

How To Bolster Your Immune System

How To Bolster Your Immune System

All Natural Immune Boosters Proven To Fight Infection, Disease And More. Discover A Natural, Safe Effective Way To Boost Your Immune System Using Ingredients From Your Kitchen Cupboard. The only common sense, no holds barred guide to hit the market today no gimmicks, no pills, just old fashioned common sense remedies to cure colds, influenza, viral infections and more.

Get My Free Audio Book


Post a comment