Systems-Based Analysis of the Sarcocystis neurona Genome Identifies Pathways That Contribute to a Heteroxenous Life Cycle

Blazejewski, Tomasz; Nursimulu, Nirvana; Pszenny, Viviana; Dangoudoubiyam, Sriveny; Namasivayam, Sivaranjani; Chiasson, Melissa A.; Chessman, Kyle; Tonkin, Michelle; Swapna, Lakshmipuram S.; Hung, Stacy S.; Bridgers, Joshua; Ricklefs, Stacy M.; Boulanger, Martin J.; Dubey, Jitender P.; Porcella, Stephen F.; Kissinger, Jessica C.; Howe, Daniel K.; Grigg, Michael E.; Parkinson, John

The sheer dominance of the fam2 domain proteins among the limited repertoire of SnSRS proteins suggests that they play a critical functional role in the life cycle. A recent study (43) found that TgSRS44 (CST1), a T. gondii SRS protein with 13 tandemly repeated fam2 domains, is an important structural constituent of the cyst wall, suggesting that the emergence of fam2 SnSRS domain-containing proteins in the common ancestor of S. neurona and T. gondii likely provided the parasite with the ability to form cysts, thereby extending its host range and promoting the transition to a heteroxenous (two-host) life cycle. Strains of S. neurona are known to exhibit important differences in the immunodominant SnSRS-encoding genes that they possess. SnSRS10 (SnSAG1) is encoded by an immunodominant gene present and expressed abundantly in some S. neurona isolates but absent from others (45) . While the type II SO SN1 strain sequenced encodes SnSRS10, it does not express it during merozoite growth (Fig. 5) , whereas SN3, another type II isolate, highly expresses this protein.