Extensively variable surface antigens of Sarcocystis spp. infecting Brazilian marsupials in the genus Didelphis occur in myriad allelic combinations, suggesting sexual recombination has aided their diversification

摘要

Sarcocystis neurona and Sarcocystis falcatula are very similar species of Apicomplexan protozoa that use marsupials of the genus Didelphis as definitive hosts. These mammals can serve as definitive hosts not only for these two parasites, but for other Sarcocystis such as Sarcocystis speeri and Sarcocystis lindsayi. Sarcocystis shed by opossums (with the exception of S. neurona) can cause disease in a great variety of birds, being commonly associated with acute pulmonary sarcocystosis in zoos. S. neurona is the most commonly associated parasite with the equine protozoal myeloencephalitis in horses. Herein we assessed the variability of Sarcocystis spp. isolated from opossums of the state of Rio Grande do Sul, Brazil, by sequencing fragments of genes coding for glycosylphosphatidylinositol-anchored surface antigens (termed surface antigen or SAG), SAG2, SAG3 and SAG4. Two genetic groups were identified, one of them related to S. falcatula and the other related to S. neurona. Various allelic combinations of SAG2, SAG3 and SAG4 occur among S. falcatula related isolates and strong evidences suggest that such isolates may exchange high divergent alleles in possible sexual recombination processes. Regarding the group S. neurona-like (isolates G37 and G38), none of the individuals in this group share alleles with individuals of the other group. Comparing G37 and G38 strains and North American strains of S. neurona, four polymorphisms were identified at SAG-3, five at SAG-2 and three at SAG-4. Gene sequences of locus SAG-3 from isolates G37 and G38 differed from the other sequences by an insertion 81bp long. This insertion contains several dinucleotide repeats of AT, resembling a microsatellite locus and has already been detected in SAG3 sequences of S. neurona from North America. When aligned against North American strains of S. neurona, G37 and G38 isolates have a deletion of 8 nucleotides within this intron which indicate that S. neurona strains of South America are divergent from that of North America. From the results obtained so far, we have shown extensive variability in surface antigens coding sequences among Sarcocystis eliminated by mammals of the genus Didelphis spp. In addition, such divergent alleles may be exchanged in possible sexual recombination processes between different isolates of S. falcatula related isolate. The evolutionary relationships within S. falcatula related isolates will be best clarified after markers less subjected to selection pressures are analyzed in conjunction with surface antigen genes. These results may have a striking impact on the knowledge of the Sarcocystis species that infect opossums in Brazil and also in the epidemiology of the infections caused by these protozoans.