The Effect of Host Sex, Ploidy, & Sexual Activity on Parasite Infection

This was the thesis research that led to my MA in Biology at Indiana University.  I submitted it three times to journals (1 rejection because I couldn’t pay the publishing charges for open access and 2 for other annoying reasons).  Alas, it remains unpublished.

Potamopyrgus antipodarum under the dissecting scope

Experimental infections of the freshwater snail, Potamopyrgus antipodarum, were conducted in the lab with its common trematode parasite, Microphallus, to determine the presence or absence of a trade-off between sexual activity and parasite resistance. We also examined if host sex or ploidy influences susceptibility to these parasites across four treatments: isolated males, isolated females, males with females, and females with males.

A comparison of infected and uninfected Potamopyrgus antipodarum. This composite image illustrates the difference between an infected individual (top) and an uninfected individual (bottom) when the population has been artificially infected with its common parasite, Microphallus.

The bar graph shows the infection prevalence of Microphallus in P. antipodarum hosts of different ploidy. Only a subsample of individuals were tested for ploidy in order to comparatively illustrate the effect of ploidy on the variation in infection in the population. Shading of the bars indicates the different experimental treatments. Treatments consisted of either same sex or mixed sex conditions. Within the all female treatment, likelihood ratio chi-squared analysis revealed a significant relationship between infection status and ploidy. Overall, males tended to be more infected than females when the results were pooled across treatments. In the mixed treatments, the proportion of infected diploid females decreased while the proportion of infected triploid females increased. Male infection prevalence stayed relatively the same. Ploidy was not tested for the all male treatment.

The results indicated that 1) triploid females were underinfected compared to other classes, 2) males were more infected than females and 3) males differentially influenced the susceptibility of diploid and triploid females to trematode infection in the mixed treatments, presumably as a result of sexual activity.

The present study suggests that a cost of sexual activity between the sexes may not be borne equally among diploid and triploid females. These results demonstrate that the presence of males affects variation in susceptibility among sexual and parthenogenetic females with significant between-sex variation observed in the lab that has not been otherwise reported in the field. Because diploid females had a lower prevalence of infection in the treatments where males were present there does not appear to be an energetic cost of reproduction beyond the two-fold cost of producing male offspring.

Keywords: sex-biased infection, ploidy, host-parasite, trematode, Potamopyrgus antipodarum, trade-off, sexual activity, susceptibility