Variation among Desert Topminnows in Their Susceptibility to Attack by Exotic Parasites

Exposure of native species to novel disease organisms is a major threat posed by introduced exotic species. The genetic background of a native population should affect its response to an exotic disease. We evaluate the susceptibility of sexually and asexually reproducing lineages of fish (genus Poeciliopsis ) to infection by a novel trematode parasite ( Gyrodactylus turnbulli ). The parasite commonly infects guppies ( Poecilia reticulata ), which have obtained a worldwide distribution as a consequence of their value in the pet trade. We examined two sexual species and two related clonal (nonrecombinant) lineages of Poeciliopsis that coexist naturally in Sonora, Mexico. One sexual species ( P. monacha ) and one clone ( ML/VIII ) were completely refractory to infection by G. turnbulli . The other sexual species ( P. lucida ) was relatively resistant. The second clone ( ML/VII ) was susceptible, however, and infections led to increased mortality. Chance fixations of alleles that occurred during the hybrid origins of these clonal lineages probably were responsible for the differences in susceptibility between lineages. Lack of heritable genetic diversity within a clonal lineage would hamper its ability to quickly evolve a suitable response to an exotic parasite and would make it particularly vulnerable to extinction.