Rare but evolutionarily consequential outcrossing in a highly inbred zoonotic parasite

Recurrent self-mating can result in nearly clonal propagation of biological lineages, but even occasional outcrossing can serve to redistribute variation in future generations, providing cohesion among regional populations. The zoonotic parasite Trichinella spiralis has been suspected to undergo frequent inbreeding, resulting in genetically uniform larval cohorts which differ markedly from one another. Here, we explored the extent of inbreeding for this parasite by determining how genetic variation (at variable microsatellite markers) is distributed among 1379 larvae derived from 41 wild boars in Extremadura, Spain. In particular, we sought to determine how much of the genetic variation in this region's parasites occurs among the larvae of any given wild boar, and whether each derives from one, or more, parental lineages. We found strong evidence for inbreeding, resulting in genetically distinct parasite subpopulations among the parasites derived from many pairs of wild boar. Fully two-thirds of these parasite cohorts appear to derive from inbred parents; in 10% of the wild boars, parasites were so inbred as to become absolutely fixed in all of the assayed genetic loci. In spite of this, more than one pair of parents appear to have given rise to the infections in one-third of the sampled wild boars, resulting in mixed infections. These mixed infections should slow losses of heterozygosity and multi-locus polymorphism in any given parasite lineage. Such outcrossing should limit distinctions that would otherwise accumulate among transmission chains, thereby enforcing cohesion through the region's population in spite of its marked departure from panmixia. Conditions of transmission may differ in other regions, where such epidemiological features may engender different evolutionary outcomes.