VARIATION AMONG DESERT TOPMINNOWS IN THEIR SUSCEPTIBILITY TO ATTACK BY EXOTIC PARASITES

Exposure of native species to novel disease organisms is a major threa t posed by introduced exotic species. The genetic background of a nati ve population should affect its response to an exotic disease. We eval uate the susceptibility of sexually and asexually reproducing lineages of fish (genus Poeciliopsis) to infection by a novel trematode parasi te (Gyrodactylus turnbulli). The parasite commonly infects guppies (Po ecilia reticulata), which have obtained a worldwide distribution as a consequence of their value in the pet trade. We examined two sexual sp ecies and two related clonal (nonrecombinant) lineages of Poeciliopsis that coexist naturally in Sonora, Mexico. One sexual species (P. mona cha) and one clone (ML/VIII) were completely refractory to infection b y G. turnbulli. The other sexual species (P. lucida) was relatively re sistant. The second clone (ML/VII) was susceptible, however, and infec tions led to increased mortality. Chance fixations of alleles that occ urred during the hybrid origins of these clonal lineages probably were responsible for the differences in susceptibility between lineages. L ack of heritable genetic diversity within a clonal lineage would hampe r its ability to quickly evolve a suitable response to an exotic paras ite and would make it particularly vulnerable to extinction.