Degree Name

Master of Science (MS)

Semester of Degree Completion

2002

Thesis Director

Jeffrey R. Laursen

Abstract

Snails in Lymnaeidae serve as intermediate hosts in the transmission of many trematode species, including Fascioloides magna that is responsible for disease and death in domestic livestock in North America. Previous classifications of lymnaeid snails have relied primarily on morphological characters that exhibit high levels of homoplasy; thereby, impeding a sound assessment of relationships in this group. The present study provides a phylogenetic hypothesis for lymnaeid snails employing sequences from the internal transcribed spacer (ITS) region of nrDNA, and addresses the evolution of susceptibility to Fascioloides infection in lymnaeid snails. The final data set, comprising ten species of lymnaeid snails and one species of Physidae, included 1368 characters, of which 327 were parsimony-informative. Three major clades were recovered in neighbor-joining analyses that consisted of individuals of Stagnicola caperata, Fossaria s.s., and Stagnicola s.s. + F. Bakerlymnea sp. Stagnicola caperata, the main host of F. magna in Minnesota, revealed a closer alliance to Fossaria spp. than to other species of Stagnicola, suggesting that its placement in the stagnicoline sub-genus Hinkleyia is suspect. Members of Fossaria s.s., that have tricuspid first lateral teeth in the radula, were monophyletic to the exclusion of F. Bakerlymnea, a well-supported member of the stagnicoline clade. Therefore, our estimate of lymnaeid phylogeny supports the taxonomic scheme proposed by Baker (1911) that suggests members of Bakerlymnea be classified as stagnicolines based on their shared bicuspid dentition. Although a stagnicoline clade was strongly supported, there was low resolution of species within the clade. Log-determinent distances between species of Stagnicola s.s. were less than those observed between individuals of Stagnicola caperata, indicating that a region with higher rates of evolution is necessary to determine relationships in this group. Susceptibility to Fascioloides magna infection is widespread in North American lymnaeid snails based on experimental infections. However, an examination of naturally infected intermediate hosts suggests that host status may be due to high exposure rates that result from close interactions between intermediate and definitive hosts.

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