Summary similarities and differences

It is clear that Drosophila in natural populations is subject to attack from a range of parasites and pathogens. Of the macro-parasites, parasitoids are common and widespread, and D. melanogaster will often have to face parasitoid attack. Abundance data on mites are known from one species of Drosophila, but how often other species, especially D. mela-nogaster, are subject to mite attack is unclear at present. Nematode parasitism is relatively common in quinaria and testacea group species on mushrooms, but data on rates of parasitism on fermenting fruits are much more scarce; no records exist of D. mela-nogaster parasitized by nematodes in the field.

Although Drosophila in general, and D. mela-nogaster specifically, is susceptible to a range of microbial pathogens in the laboratory, knowledge is limited as to how often they are infected with microbial pathogens in the field. Wolbachia and viruses appear to be common and widespread, but little or nothing is known about rates of infection by entomopathogenic fungi, microsporidia, and bacteria in natural populations.

The fitness consequences of being attacked or infected by parasites or pathogens vary widely. At one end of the scale, infected individuals face either quick death (as in the case of pathogenic bacteria, although this is often linked to unnatural methods of inoculation) or, if they do not die soon after infection, genetic death (as in the case of para-sitoids and nematodes, who either kill their host before it becomes reproductively active or sterilize it). Reductions in a range of fitness parameters, including fecundity, longevity, and mating success, occur after parasitism by mites and infection by fungi and microsporidia; fitness effects of infection by Wolbachia appear to be variable. At the other end of the scale are viruses, which appear to have little or no negative effects on the fitness of their hosts (the sensitivity to anoxia reported in laboratory circumstances is unlikely to be important in the field).

Once an individual is attacked or infected, it launches an immune response. Only in the case of parasitoids do data exist to show that this actual resistance is costly (Table 12.1, first column), in that individuals that successfully encapsulate the parasitoid egg are poorer larval competitors, more likely to be attacked by a pupal parasitoid, and smaller adults (with negative knock-on effects on other fitness parameters). The nature and level of costs of actual resistance against mites, nematodes,

Table 12.1 Summary of fitness parameters identified in D. melanogasterassociated with costs of actual resistance and costs of ability to resist against a range of parasites and pathogens; see text for references.

Parasite/pathogen

Cost of actual resistance

Cost of ability to resist

Larval parasitoids

Larval competitive ability

Larval competitive ability

Susceptibility to pupal parasitoids

Adult size

Fecundity

Male mating success

Pupal parasitoids

?

Adult size

Mites

?

Larval competitive ability

Adult size

Fecundity

Nematodes

?

?

Fungi

?

Fecundity

Microsporidia

?

Larval competitive ability

Fecundity

Bacteria

?

Fecundity

Viruses

?

?

fungi, microsporidia, bacteria, and viruses are as of yet unknown.

Resources need to be spent on having a resistance mechanism, and ready to launch an immune response when parasitism or infection actually takes place. Table 12.1 (second column) summarizes the fitness traits identified as costs of the ability to resist. Interestingly, very similar costs are found in the ability to resist very different parasites/pathogens, which involve very different immunological pathways (and in one case is not even an immunological mechanism at all, but a behavioural one). Larval competitive ability is found as a cost of a resistance mechanism against parasitoids, microsporidia, and mites. A reduction in adult fecundity is a cost of the resistance mechanism against fungi, microsporidia, bacteria, and mites. These similarities suggest that re-allocation of resources at a very basic level plays a key part in the costs of resistance in D. melanogaster.

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