Sarcocystis kani, J & akel & Raisch & Richter & Wirth & Birenbaum & Ginting & Khoprasert & Mackenstedt & Wassermann, 2023

J, Thomas, akel, Raisch, Lisa, Richter, Sarah, Wirth, Mareike, Birenbaum, Damaris, Ginting, Sulaiman, Khoprasert, Yuvaluk, Mackenstedt, Ute & Wassermann, Marion, 2023, Morphological and molecular phylogenetic characterization of Sarcocystis kani sp. nov. and other novel, closely related Sarcocystis spp. infecting small mammals and colubrid snakes in Asia, International Journal for Parasitology: Parasites and Wildlife 22, pp. 184-198 : 194-195

4.2. Potential recent origin of Sarcocystis kani and related species in Asia

We demonstrate in this study that a clade of Sarcocystis spp. of colubrid snakes and small mammals in Asia exhibits a low level of genetic divergence, so low that one could speak of molecularly cryptic taxa. This was especially evident from cox1 gene sequence comparisons, where three morphologically distinct species of the S. zuoi -complex, including S. kani , shared identical sequences. Although the sequences are incomplete, they include the barcode area which has been selected as global marker for species discrimination ( Hebert et al., 2003; Pentinsaari et al., 2016). The close relatedness of these Sarcocystis spp. is also apparent in the short branch lengths of the 18S rRNA gene tree (and contrasted by relatively longer branches of species of lineage S2). This is probably the reason why various Sarcocystis -isolates from colubrid snakes in Asia were previously considered monospecific (collectively addressed as S. zuoi ), especially, if phylogenies were based on the highly conserved parts of the 18S rRNA gene (Ortega P´erez et al., 2020). Thus, given the high degree of genetic similarity, what could be the potential evolutionary drivers that have led to morphological disparity and differences in host preference among these species?

Because mitochondrial DNA shows a relatively rapid rate of mutation, which makes it suitable as marker for more recent evolutionary history of natural populations ( Brown et al., 1979; Norman et al., 2014), we interpret low sequence variability at the cox1 locus as an indicator for a relatively recent origin of these Sarcocystis spp. Perhaps, the Sarcocystis spp. of rat snakes from Asia ( Elaphe spp. and other closely related genera of the Colubridae ) constitute an example of adaptive radiation ( Schluter, 2000) that followed the radiations of their hosts, exploiting new ecological opportunities for speciation. Indeed, the evolutionary mechanism involved in speciation of Apicomplexan parasites and other pathogens, especially in the case of closely related taxa (Woolhouse et al., 2005), have been suggested host switching or adaptive processes rather than co-speciation ( Kvicerova and Hypsa, 2013; Santiago-Alarcon et al., 2014). However, ˇSlapeta et al. (2003) provided evidence for co-speciation events between pythonid and viperid snakes and their associated Sarcocystis spp. based on the 18S rRNA tree, proposing coevolution of Sarcocystis spp. with snake-rodent life cycle with their definitive hosts. The latter scenario is not necessarily in conflict with potential adaptive evolutionary processes among the Sarcocystis spp. of colubrid snakes, since Sarcocystis spp. in pythons (e.g., S. singaporensis ), African/Asian vipers (e.g., S. atheridis , Sarcocystis isolated from

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Pseudocerastes ), and from other colubrid snakes (e.g., S. pantherophisi ) are apparently phylogenetic older taxa that exhibited considerably longer branch lengths in phylogenetic trees.

Regarding intermediate hosts, the occurrence of Sarcocystis in omnivorous treeshrews ( S. scandentiborneensis ) and insectivorous shrews ( S. attenuati ) is uncommon if one considers that most of the Sarcocystis spp. of lineage S1 (such as S. kani ) use murid rodents as intermediate host (Wassermann et al., 2017). This could be due to undersampling of non-murid small mammal species in the past. The 18S rRNA phylogenetic tree hints to another potential host switching scenario: S. scandentiborneensis and S. attenuati share a direct common ancestor that could have initiated switch from a murid to non-murid mammalian host. Since treeshrews and shrews are believed phylogenetically older mammals ( Roberts et al., 2011; three studies on shrews in Kumar et al., 2017, last accessed March 12, 2023) than murid Rattus spp. (Verneau et al., 1998; Song et al., 2014), such a host switch could have occurred more recently which would be congruent with the absence of sequence variability at the cox1 locus and a relatively low sequence divergence of the 18S rRNA of the these Sarcocystis species. The opposite scenario, origin in phylogenetically older treeshrews or shrews and host switch to murids, is difficult to reconcile with a low level of genetic divergence. A similar discussion of whether the origin of hantaviruses is either in murid rodents or in shrews and other non-rodent mammals has been resolved by the fact that phylogenetically older, non-rodent borne hantaviruses have been detected in Asia ( Henttonen et al., 2008, Bennett et al., 2014). This supports our reasoning that if Sarcocystis were originated from shrews or tree shrews, one would expect a higher level of genetic diversity indicating that they are phylogenetically older than those from rodents; which is not the case. Given the high diversity of murid rodents of the genus Rattus and related genera which diversified in Asia within the last 5–6 million years (Verneau et al., 1998; Song et al., 2014), this would have provided ample opportunities for dispersal and speciation of Sarcocystis .