Managing the Emerging Risk of Trichinellosis in Organic and Free Range Pork
Location: Animal Parasitic Diseases
Title: Discernible but limited introgression has occurred where Trichinella nativa and the T6 genotype occur in sympatry
Submitted to: Infection, Genetics and Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 4, 2012
Publication Date: April 12, 2012
Citation: Dunams, D.B., Reichard, M.V., Torretti, L., Zarlenga, D.S., Rosenthal, B.M. 2012. Discernible but limited introgression has occurred where Trichinella nativa and the T6 genotype occur in sympatry. Infection, Genetics and Evolution. 12:530-538.
Interpretive Summary: Species of Trichinella are parasites transmitted by consumption of infected meat. Genetic markers have been helpful in distinguishing among otherwise similar species of Trichinella, which underscore distinct cycles of transmission. Such markers have, for example, been used as the sole basis for distinguishing two freeze-resistant lineages, one of which has been named Trichinella nativa and the other of which has not been formally named (and bears the designation T6 genotype). Here, we identified conflict between two kinds of markers employed to diagnose such parasites, and evaluated (for the first time) the extent of interbreeding that has taken place between these parasite lineages. The study concludes that in a vicinity where both types of parasites occur, their nuclear genomes generally remain distinct. However, we find that in this locale those parasites resembling T6 in other respects nonetheless descended from a T. nativa maternal ancestor. A broader geographic sampling of these parasites could establish, definitively, whether these comprise one or two biological species; doing so will have implications for the transmission of freeze-resistance in parasites in more temperate climes. This work will be of interest to parasite biologists, epidemiologists, and public health officials seeking to limit zoonotic risk by better understanding transmission patterns.
The genetic diversity within and among parasite populations provides clues to their evolutionary history. Here, we sought to determine whether mitochondrial and microsatellite variation could be used to evaluate the extent of differentiation, gene flow and historical reproductive isolation among the freeze resistant parasites Trichinella nativa and the Trichinella T6 genotype infecting wolverines (Gulo gulo) in Nunavut, Canada. To this end, we compared reference strains of each genotype to Trichinella isolates derived from the diaphragms of 39 wolverines from this locale. Results showed that certain individuals resembled T. nativa in their mitochondrial DNA, but resembled the T6 genotype when assayed at expansion segment V and the internal transcribed spacer of the nuclear rDNA. To adjudicate among these conflicting diagnoses, we further characterized each isolate at several nuclear microsatellite loci and again compared these to data from reference strains. Statistical assignment established that the nuclear genomes of most Nunavut isolates corresponded to those of the T6 genotype; however, two isolates corresponded to T. nativa, and one isolate exhibited equal similarity to both reference strains. Taken as a whole, the evidence suggests that these isolates derive from the T. nativa matrilineage, but that their nuclear genomes resemble individuals previously designated as T6. Assuming distinct lineages, this argues for cross-hybridization among these genotypes. Although introgression has occurred, recognizable genetic distinctions persist. One possibility is that selection disfavors the survival of hybrid offspring in most instances. Alternatively, the recent disappearance of glacial barriers might have increased contact, and therefore introgression. Broader geographic sampling will be required to determine the extent to which hybridization occurs beyond this particular geographic focus.