Transcriptional signatures of clonally derived Toxoplasma tachyzoites reveal novel insights into the expression of a family of surface proteins

Abstract

Toxoplasma gondiihas numerous, large, paralogous gene families that are likely critical for supporting its unparalleled host range: nearly any nucleated cell in almost any warm-blooded animal. TheSRS(SAG1-related sequence) gene family encodes over 100 proteins, the most abundant of which are thought to be involved in parasite attachment and, based on their stage-specific expression, evading the host immune response. For most SRS proteins, however, little is understood about their function and expression profile. Single-parasite RNA-sequencing previously demonstrated that across an entire population of lab-grown tachyzoites, transcripts for over 70SRSgenes were detected in at least one parasite. In any one parasite, however, transcripts for an average of only 7SRSgenes were detected, two of which,SAG1andSAG2A, were extremely abundant and detected in virtually all. These data do not address whether this pattern of sporadicSRSgene expression is consistently inherited among the progeny of a given parasite or arises independently of lineage. We hypothesized that ifSRSexpression signatures are stably inherited by progeny, subclones isolated from a cloned parent would be more alike in their expression signatures than they are to the offspring of another clone. In this report, we compare transcriptomes of clonally derived parasites to determine the degree to which expression of the SRS family is stably inherited in individual parasites. Our data indicate that in RH tachyzoites,SRSgenes are variably expressed even between parasite samples subcloned from the same parent within approximately 10 parasite divisions (72 hours). This suggests that the pattern of sporadically expressedSRSgenes is highly variable and not driven by inheritance mechanisms, at least under our conditions.