Sarcocystis neurona major surface antigen gene 1 (SAG1) shows evidence of having evolved under positive selection pressure

Abstract

The major surface antigen gene 1 (SAG1) is conserved among members of Sarcocystidae and may play an important role in parasite pathogenesis. Additionally, generation and selection of different antigenic variants of SAG1 has the potential for inclusion in a subunit vaccine or in the development of a diagnostic assay. In this study, patterns of nucleotide polymorphism were used to test the hypothesis that natural selection promotes diversity in different parts of SAG1 of Sarcocystis neurona. Nucleotide and amino acid sequence analysis of SAG1 from multiple S. neurona isolates identified two alleles. Sequences were identical intra-allele and highly divergent inter-alleles. Also, phylogenetic reconstruction showed sequences clustering into two clades. Tajima's and Fu and Li's neutrality tests indicated that selection is more likely to be acting on SAG1. Moreover, a sliding window analysis based on the ratio of silent substitutions to amino acid replacements provided strong evidence that two short segments in the central and 3' domain of SAG1 have been under positive selection in the divergence of the two alleles, suggesting that it may be important for the evasion of host immune responses and would be a suitable target for vaccine development.

Genetic Variation among Isolates of Sarcocystis neurona , the Agent of Protozoal Myeloencephalitis, as Revealed by Amplified Fragment Length Polymorphism Markers

“…Sporocysts were processed and introduced into cell cultures as described previously (24). In instances where this method failed, sporocysts were bioassayed in gamma interferon gene knockout mice as described previously (10). Parasite growth and tissue culture conditions, described elsewhere (11,21), were used to obtain viable clonal isolates in the asexual stage of the life cycle.…”

“…There have been a few studies that used molecular methods for the identification of S. neurona. These included use of random amplified polymorphic DNA (RAPD) (31), sequencing analysis of SSU rRNA (9) and SAG1 genes (10), and restriction analysis of PCR amplicons of a diagnostic marker (12,31). These methods differ in their discriminatory power, reproducibility, and ease of interpretation.…”

An update on Sarcocystis neurona infections in animals and equine protozoal myeloencephalitis (EPM)

“…The genes for these three SnSAG paralogues seem to be mutually exclusive to one another, since all strains of S. neurona that have been analyzed possess sequence for only SnSAG1 or SnSAG5 or SnSAG6 ; no strain has been found to possess more than one of these genes. Analysis of synonymous versus non-synonymous mutations in the SnSAG1 and SnSAG5 gene sequences suggests there may be evolutionary advantages to altering some regions of these surface proteins (Elsheikha and Mansfield, 2004b). …”

The roles of positive and negative selection in the molecular evolution of insect endosymbionts

“…This criterion has been used to identify diversifying selection genes whose products are involved in reproduction (Biermann, 1998;Mah et al, 2004;Metz et al, 1998;Swanson et al, 2004;Tsaur and Wu, 1997), surface antigens (Anisimova and Yang, 2004;Elsheikha and Mansfield, 2004;Tosh et al, 2003), toxicity (Riley et al, 2000;Zhu et al, 2004), among other functions. The prediction of reduced efficacy of positive selection in endosymbionts was challenged with the report of such selection on the groEL gene from Buchnera, the endosymbiont of aphids.…”