Submitted to: Trichinellosis International Conference Proceedings
Publication Type: Abstract only
Publication Acceptance Date: 8/15/2004
Publication Date: 8/15/2004
Citation: Zarlenga, D.S., Pozio, E., La Rosa, G., Rosenthal, B.M., Hoberg, E.P. 2004. A new phylogenetic hypothesis for the genus trichinella. Trichinellosis International Conference Proceedings. Interpretive Summary: Speciation within the genus Trichinella remains a controversial issue, notwithstanding the problems associated with population differences and host specificity among the genotypes. Recent reports of outbreaks suggesting the avian species, T. pseudospiralis, as a pathogen in humans has prompted research on population differences and host specificities within the species that do not encapsulate within the muscles of infected animals. Herein, we examine 11 geographical isolates of T. pseudospiralis and 1 isolate of T. papuae with respect to host infectivity and numerous biochemical and molecular markers. Results demonstrated that; 1) T.papuae is clearly a distinct species from T. pseudospiralis; 2) several molecular characters can be used to delineate these species; and 3) geographical segregation exists between T. pseudospiralis populations from different zoogeographical regions. The identification and genetic characterization of zoogeographical strains of T. pseudospiralis is of great significance not only for their taxonomic and phylogenetic value, but also because of the well-documented potential for human infection by this species, and the importance of being able to delineate the geographical source of the acquired trichinellosis through molecular markers. These findings will therefore assist researchers in determining whether certain populations of this species are more infectious to humans than others and why swine shows natural protection against certain species.
Technical Abstract: We present the first robust phylogenetic hypothesis for the genus Trichinella based on variation in several genetic loci using a comprehensive sample of all ecologically and genetically recognized parasite forms. From its first description in 1835 through much of the following century, the genus Trichinella was considered monospecific; however, more recent biochemical, molecular, phenotypic, and crossbreeding experiments support a genus partitioned into at least 8 species. Three species of non-encapsulated forms, T. pseudospiralis, T. papuae and T. zimbabwensis, have recently been uniquely recognized on the basis of genotypic and phenotypic variation, raising the question of whether encapsulated and non-encapsulated genotypes may represent evolutionarily distinct assemblages. Our analysis of variation in nuclear and mitochondrial ribosomal DNA, and mitochondrial COXI DNA indicates strong support for an especially close relationship between the freeze-tolerant genotypes, T. nativa and T6, and between the two recently described non-encapsulated species, T. papuae and T. zimbabwensis. Furthermore, data are consistent with basal partitioning between the encapsulated and non-encapsulated clades. Our hypothesis provides a unique historical perspective on the biogeography and epizootiology of the group, indicating: 1) all members of the genus share a close evolutionary relationship to the exclusion of other known nematodes; 2) capsules evolved once in the history of the genus, and; 3) both the encapsulated and non-encapsulated clades contain species that are now cosmopolitan, as well as species inhabiting more geographically and ecologically restricted ranges. A history of geographic dissemination and host adaptation will be proposed and implications for taxonomic nomenclature discussed.