Variation among desert topminnows in their susceptibility to attack by exotic parasites.
The susceptibility of sexually and asexually reproducing lineages of Poeciliopsis sp. fish to infection with the novel trematode Gyrodactylus turnbulli was evaluated. Two sexual species, P. lucida (outcrossed) and P. monacha (outcrossed) and two related clonal (nonrecombinant) lineages of Poeciliopsis sp., ML/VII and ML/VIII, that coexist naturally in Sonora, Mexico. P. monacha and ML/VIII were completely refractory to infection by G. turnbulli. P. lucida was relatively resistant and ML/VII was susceptible and infections led to increased mortality. Chance fixations of alleles that occurred during the hybrid origins of these clonal lineages were probably responsible for the differences in susceptibility.