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  Alas, it remains unpublished.

Abstract: Parasitic infection can result from a trade-off between the amount of resources a host uses to build parasite resistance and those it uses engage in sexual activity. Factors that influence this trade-off may include both the sex of the host and its ploidy (chromosome number).

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.

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

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