Restricted genetic diversity in the ubiquitous cattle parasite, Sarcocystis cruzi

Although parasites of the genus Sarcocystis have likely cycled between bovine herbivores and canine carnivores for tens of millions of years, humans may have profoundly influenced the ecology and evolution of those prevalent in domesticated dogs and cattle. To preliminarily assess the possibility of such anthropogenic effects, we surveyed genetic variation in conserved (18S small subunit) and variable (ITS-1) portions of ribosomal DNA from a large sample of Sarcocystis cruzi occurring in taurine beef cattle raised in the United States and Uruguay, and compared these data to available homologues, including those reported from zebu cattle, water buffalo, and bison. For additional context, we compared the apparent diversity of cattle parasites to that reported from congeneric parasites in other hosts. We find that the S. cruzi of taurine cattle, whether derived from the Americas or Asia, are devoid of variability in the sequenced portion (80%) of the small subunit rDNA. By contrast, geographically limited samples of related parasites in other hosts, including those of wildlife, are more variable. At the adjacent ITS-1 locus, allelic distribution patterns did not indicate any regional barriers to gene flow, suggesting that the parasite may have been introduced to the Americas via a common source such as domesticated dogs or cattle. Thus, human impact on this parasite's distribution and diversification would seem to have been great.