Evolution of ruminant Sarcocystis (Sporozoa) parasites based on small subunit rDNA sequences

We present an evolutionary analysis of 13 species of Sarcocystis, including 4 newly sequenced species with ruminants as their intermediate host, based on complete small subunit rDNA sequences. Those species with ruminants as their intermediate host form a well-supported clade, and there are at least two major clades within this group, one containing those species forming m icrocysts and with dogs as their definitive host and the other containing t hose species forming macrocysts and with cats as their definitive host. Tho se species with nonruminants as their intermediate host form the paraphylet ic sister group to these clades. Most of the species have considerable geno typic differences (differing in more than 100 nucleotide positions), except for S. buffalonis and S. hirsuta. There is a large suite of genotypic diff erences indicating that those species infecting ruminant and nonruminant ho sts have had very different evolutionary histories, and similarly for the f elid- and canid-infecting species. Furthermore, the rDNA sequences that rep resent the different structural regions of the rRNA molecule have very diff erent genotypic behavior within Sarcocystis. The evolution of these regions should be functionally constrained, and their differences can be explained in terms of the importance of the nucleotide sequences to their functions. (C) 1999 Academic Press.