Interactions between immunity and reproduction in the cricket, Gryllus texensis

Physiological trade-offs describe the negative relationship between two fitness traits of an individual, which occur based on competition between the two traits for energetic and other resources. A recent trend in the study of trade-offs involves the potential physiological trade-off between reproduction and immunity, which are both assumed to be energetically costly. This trade-off suggests that an increase in reproductive effort will result in a decrease in immune system activity, and conversely, an increase in immune system activity will result in a decrease in reproductive effort. While many studies of insect models suggest that both immunity and reproduction are costly, few studies directly investigate the relationship between the two. Evidence for this potential trade-off was examined here in the female cricket, Gryllus texensis . Four separate experiments were conducted. I predicted that: (1) infection of G. texensis with a live insect pathogen, Serratia marcescens, would lead to an adaptive increase in egg laying at the expense of immunity; (2) chronic immune system activation would lead to a decrease in the number and/or quality of eggs laid; (3) higher levels of reproductive activity would lead to decreases in immunocompetence, assayed by phenoloxidase (PO; an enzyme involved in insect immunity) activity of the hemolymph, and a functional test measuring survival following S. marcescens infection; and (4) higher levels of reproductive activity would lead to a reduction in somatic maintenance (including immunity) and result in reduced lifespan. I found no evidence to support these predictions, suggesting that no physiological trade-off exists between reproduction and immunity for this cricket. Additionally, I determined that PO provides a poor estimate for immunocompetence. While the lack of evidence for a trade-off could be explained in various ways, I suggest that under nutrient-rich conditions, reproduction may not be costly enough to reduce investment in somatic maintenance, including immunity. This possibility, along with the possible confound of PO activity measurements (a popular assay), forces a reexamination of previously published work within the field of insect ecological immunology.