Although not well studied, there are good theoretical reasons to believe that resistance mediated by secondary symbionts may have a rather different dynamic from resistance genes that are encoded by nuclear loci. In terms of theory, both represent traits whose positive effect on fitness is dependent on the presence of natural enemies. However, two factors may make secondary-symbiont-encoded resistance more likely to decrease in frequency in the absence of selection: transmission inefficiency and a difference in the cost of resistance.
First, we can compare cost-free nuclear and cyto-plasmically encoded resistance. Selection from pathogen threat will drive these genes up in frequency. For nuclear genes, a reduction in pathogen threat would make the resistance genes subject to random drift processes. In a large population, these will cause only small changes in frequency each generation, and an increase and decrease in frequency are equally probable. For a maternally inherited element, in contrast, there is likely to be progressive loss of infection over time through inefficient transmission. In order to be maintained in the absence of selection for its presence, each daughter of an infected female must inherit the infection. Whereas vertical transmission efficiency can be near perfect (e.g. Jiggins et al., 2002), imperfect vertical transmission would mean progressive loss of the symbiont over time. Thus, when pathogen threat recedes, the expectation is that the infection will decline in frequency.
Aside from this, a cost of possessing the factor would render it deleterious in the absence of natural enemy threat, or when the threat was low. A cost can, of course, occur for both nuclear genes and for maternally inherited bacteria. However, there are perhaps reasons to believe that costs will be greater and more certain for secondary sym-biont infections. First, these infections represent organisms that have a metabolic cost associated with their activity. This cost cannot be avoided by not expressing them: they must use ATP and nutrients to survive and to replicate. Second, as argued in the section below, they may also carry a cost associated with their interaction with the host innate immune system, or indeed other symbionts. Whereas all maternally inherited agents have common interest in the fitness of the female host, there may be unexpected interactions between bacteria. Secondary symbionts often cohabit with primary symbionts in the bacteriome, and co-infections with different symbionts can produce changes in density of the parties (Oliver et al., 2006).
These ideas suggest that resistance encoded through maternally inherited agents will generally decline in frequency in the absence of the natural enemy more rapidly than nuclear-encoded elements. Rapid reduction in frequency in the absence of natural enemies is observed when infected and uninfected clones compete in experimental population cages (Oliver et al., 2008), although fitness costs are not always evident in the absence of competition (Darby et al, 2003). In the field, this flux is perhaps reflected in the profound geographical variation in secondary symbiont prevalence (Tsuchida et al., 2002). The hypothesis is that, when unused, symbi-ont-encoded resistance is a trait that is lost.
This hypothesis awaits mathematical modelling, as well as empirical work to verify or refute the basic tenets (that resistance is more costly if symbiont-encoded), and to investigate its reality in the field. The view also needs to be extended to incorporate effects on the dynamics of the natural enemies against which the symbionts protect. The degree to which natural-enemy dynamics are driven by resistance, and by other factors external to resistance, will of course be important in this.
This field is in its infancy, and the above is a very simplified treatment of the population biology of the system. Secondary symbionts may provide resistance against a variety of enemies (e.g. Wolbachia in D. melanogaster provides resistance to more than one RNA virus). Further, they may evolve towards mutualism, as found for Wolbachia in D. simulans (Weeks et al, 2007), or be counteracted by adaptation on the part of the natural enemy, which would alter the dynamics of the element.
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