The J Lab
Group Leader: Dr Michael Jennions

Contact
School of Botany & Zoology
Australian National University,
Canberra, ACT 0200,
Australia
Email

Jean Drayton
                                                              

My honours project examined the genetic benefits of polyandry using the black field cricket Teleogryllus commodus. I used a quantitative genetic design (a diallel cross) to look for evidence of good genes and genetic incompatibility effects. My design also allowed me to generate inbred and outbred individuals and, as a side project, I examined the effect of inbreeding on various life history traits (such as egg hatching and lifespan) and on sexually selected traits (such as calling effort and advertisement calling structure). This side project was to lead into my PhD….

I took at year off in 2005 to work in the J-Lab. I decided to apply for a PhD at the end of 2005, and started early 2006.

Publications

Jennions MD, Drayton J, Brooks RC, Hunt J. 2007. Do female  black field crickets Teleogryllus commodus benefit from polyandry? Journal of Evolutionary Biology 20: 1469-1477 [PDF]

Drayton J, Hunt J, Brooks R, Jennions MD. 2007. Sounds different: inbreeding depression in sexually selected traits in the field cricket Teleogryllus bimaculatus. Journal of Evolutionary Biology 20: 1138-1147  [PDF]

My PhD (the short version)

 I am interested in the honesty of sexual signaling, and the reliability of such signals in indicating a male’s overall genetic quality and his immunocompetence. My study species is (still) the black field cricket Teleogryllus commodus.

My PhD (the long version)

Male sexually signaling may be honest because these traits are condition dependent in expression. The condition of a male may defined as the pool of resources that are available to him to allocate to production and maintenance of traits that enhance fitness. In general, males that are better able to accumulate resources (i.e. are in good condition) can allocate more to sexual signaling than those that are less efficient. Males can differ in condition due to external environmental factors (e.g. food availability, territory quality), biotic factors (e.g. pathogens) and/or because genotypes vary in how they affect resource acquisition and assimilation ability. Consequently, females choosing males that have invested more into condition-dependent sexual signals might gain genetic benefits for their offspring if some of the variation in signaling is due to additive genetic variation in condition.

The majority of experimental studies that have assessed the extent to which sexual traits are condition-dependent have used variation in environmental quality (e.g. by altering diet, or by adjusting parasite loads), rather than variation in genetic quality. The bias towards manipulating condition in this way is due to the comparative ease with which environmental quality can be adjusted. In contrast, it is harder to manipulate the genetic basis of differences in male condition because it is difficult to estimate genetic quality a priori or to create distinct categories of genetic quality. In the absence of detailed knowledge of the genes responsible for variation in condition one possible solution is to compare males that vary in their level of inbreeding. Sexual signals can then be compared between categories of males that should predictably vary in condition if it is subject to inbreeding depression. If sexual signals are condition dependent, and inbreeding affects the general ability of males to acquire and assimilate resources, then sexual signals will exhibit declines in trait values with inbreeding. Inbreeding depression in sexually selected traits might also be due to effects of changes in heterozygosity at loci that directly code for sexual traits.

If condition-dependence of sexual signals is commonplace then they should show strong inbreeding depression relative to other traits because they will capture the genetic effects of inbreeding at the many loci that affect the general ability of males to acquire and assimilate resources. However, to date few studies have examined in detail the effects of inbreeding on sexual traits involved in mate choice. So this is the first part of my PhD. Over the last year, I have used a simply breeding design to create 30 blocks of inbred and outbred families (each block containing 2 inbred and 2 outbred families), to examine the effect of inbreeding on various sexually selected traits (e.g. calling effort, the structure of the advertisement call, courtship song structure).

For the second part of my PhD, I have examined the role of immunity in male sexual signaling. The immunocompetence hypothesis proposes that sexual signaling is a costly and therefore honest signal of a male’s genetic resistance to parasite infections. The hormone testosterone stimulates the development of characters that enhance male mating success however, it also suppresses the immune system, thereby creating a trade off between increased susceptibility to parasites due to immune suppression and increased mating success due to sexual trait development. Males can optimize this trade off because testosterone-dependent signal intensity is a plastic response that can be modified depending on the competing demands of parasite infection and increased reproductive success. This trade off is mediated by a male’s genetic resistance to parasites such that males with good genes for resistance will pay less in terms of a parasite induced cost relative to a male with low genetic resistance for a given signal intensity. Consequently because sexual trait development and signaling is costly in terms of immunosupression, it sends an honest signal of a male’s ability to withstand increased susceptibility to parasite infection which is presumably a reflection of his good genes for parasite resistance.

The mechanistic refinement of the immunocompetence handicap hypothesis differs between vertebrates and insects, because unlike vertebrates, insects do not produce testosterone. In insects, juvenile hormone has a dualistic effect on sexually signaling and immune suppression and therefore may play an analogous role to testosterone in maintaining honesty of sexual signaling. Alternatively the trade off between sexual signaling and the immune system in insects may be driven by the allocation of limited resources (e.g. energy) to the competing demands of reproduction and immunity. Consequently, females choosing males with extravagant sexual signals may choose males that can afford to reduce their investment in immune function because they are resistant to disease and/or males that have abundant resources (ie in good condition) thereby allowing them to invest adequately in both ornamentation and their immune system.

It is possible to test for such trade offs between immunity and other traits by experimentally triggering an immune response and examining any adverse effects on the traits of interest. For example, injection with bacterial lipopolysacarides (LPS) has been used to examine trade offs between immunity and other life history traits (e.g. survival and fecundity).LPS’s are nonpathogenic surface molecules extracted from bacteria that elicit an immune response. Using LPS to stimulate an immune response, I have tested for a trade off between short range sexual signaling and immunity in male crickets T. commodus.

Finally, many studies of parasite mediated sexual selection have the a priori assumption that a large measurable immune response indicates high quality males. While this assumption may be correct in some systems, there are several reasons as to why this may not always be the case. Firstly, if mounting an immune response is energetically costly, selection might favour hosts that produce small quantities of highly effective immunological molecules. Secondly over-responding to an immune challenge may cause damage to self tissue. Thirdly, negative correlations between different aspects of the immune response (e.g. between cell-mediated immunity and lysozyme activity) may mean that an over response in one defense mechanism may come at a cost of under responding in another. To test the assumption that that a large magnitude response indicates high quality males I will manipulate both the environmental (diet) and genetic (level of inbreeding) condition of T. commodus males and measure two aspects of immunity, hemocyte loads and lysozyme activity in the hemolymph. Previous studies have shown that both restricted nutrition and inbreeding can reduce immune responses.
 

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