Breeding systems in Echinococcus granulosus (Cestoda; Taeniidae): selfing or outcrossing?

We used the PCR-SSCP method followed by sequencing in order to assess the genetic variability of coding and noncoding parts of the genome of Echinococcus granulosus (Cestoda; Taeniidae) and to test whether or not the parasite populations are mainly self-fertilizing. For this, we analysed a sample of 110 E. granulosus metacestode isolates collected from different geographical regions (Southern Brazil, Europe and Australia) and from different intermediate hosts (ovine, bovine, human, macropod, swine and equine). Using appropriate controls, we were able to identify 4 strains in that sample (sheep, cattle, pig and horse strains). The high degree of genetic differentiation between strains, but not within, and the monomorphism found in most loci (EgAg4, EgActII, EgHbx2 and EgAg6-non-coding-EgAgB/1 and EgND1-coding) indicated that they are largely selfed. On the other hand, outcrossing was also shown to occur, since 5 potential hybrid genotypes between cattle and sheep strains were found in populations of Southern Brazil, but absent in other geographical areas. We suggest that both processes are adaptive. The article also reports, for the first time, the occurrence of the E. granulosus cattle strain in South America.