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

Abstract

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.

Genetic differences between Tunisian camel and sheep strains of the cestodeEchinococcus granulosusrevealed by SSCP

“…The present study demonstrates that PCR-linked SSCP provides a method to display variation between E. granulosus strains, which has significant implications for studying the population genetics of this parasite and for epidemiology and disease control. As it was previously described by Haag et al (1999) in sheep strain, the most polymorphic loci found were Ag4 with four alleles, followed by Ag6 with three alleles. The dromedary prevalence of hydatidosis in Tunisia is low (6,5 %: Lahmar et al, 2004) by the absence of alleles A2 and A3, and the presence of the A4 (Ag4 gene), also found by Kamenetzky et al (2002), and classical C1 (CO1 gene) alleles.…”

“…T o date, few studies using PCR-SSCP approaches in E. granulosus were carried out (Haag et al, 1998;Haag et al, 1999;Zhang et al,1999;Kamenetzky et al, 2002) S D vantage to be extremely sensitive to the temperature variations and to be very largely dependent on the size of fragments of DNA (between 100 and 500 bp) (Kain et al, 1996;Zhang et al,1999;Martins-Lopes et al, 2001). The present study demonstrates that PCR-linked SSCP provides a method to display variation between E. granulosus strains, which has significant implications for studying the population genetics of this parasite and for epidemiology and disease control.…”

“…One target is a mitochondrial DNA region coding for the cytochrome C oxidase subunit 1 (CO1) (Bowles et al, 1992a), which is used to differentiate the camel and sheep strains. Two other targets are non coding region 5' and 3' flanking regions of the gene Ag4 (a cytosolic malate dehydrogenase) and Ag6 (a calcium-binding protein) (Haag et al, 1999). Thirty five cycles of denaturation (94°C for 30 seconds), annealing (55°C for 30 seconds) and extension (72°C for 30 seconds) were carried out.…”

“…Nevertheless, the addition of glycerol is a useful supplement for increasing sensitivity in SSCP acrylamid gel. The recorded variations can be interpreted in terms of population genetics (homozygotes and heterozygotes frequencies) (Haag et al, 1999). In the present work, we intended to address the comparative genetic diversity and structuration of E. granulosus sheep and camel strains in Tunisia using SSCP.…”

“…This difference, based on a single gene, is not useful to study population genetic structure. Single-strand conformation polymorphism (SSCP) (Gasser et al, 1998;Haag et al, 1999;Zhang et al, 1999) have been used for the study of genetic variation between lines or strains of E. granulosus. The SSCP technique relies on the principle that the electrophoretic mobility of a single stranded DNA molecule in a non-denaturing gel is dependent on its struc-…”