Reply to Forni et al., “Multiple Selected Changes May Modulate the Molecular Interaction between Laverania RH5 and Primate Basigin”

We have previously shown that the human malaria parasite Plasmodium falciparum resulted from the cross-species transmission of a parasite (which we have termed Plasmodium praefalciparum) that infects wild-living gorillas (1). In a recent paper (2), we took issue with claims made by Forni et al. in 2015 (3) regarding the origin of P. falciparum. This paper was entitled “Positive selection underlies the species-specific binding of Plasmodium falciparum RH5 to human basigin,” with the final sentence of their abstract stating “Data herein provide an evolutionary explanation for speciesspecific binding of the PfRH5-BSG ligand receptor pair and support the hypothesis that positive selection at these genes drove the host shift leading to the emergence of P. falciparum as a human pathogen. ” These claims were the focus of our criticism, which is why we did not discuss their application of a “phylogenetics-population genetics method to search for sites that were positively selected” in BSG (basigin) genes: from that analysis, the authors reported that “no positively selected sites were found in the human lineage” (3), and so these analyses provided no evidence relevant to their claims. The data analyzed by Forni and colleagues (3) were insufficient to support the claim that positive selection at BSG and RH5 contributed to the emergence of the human parasite. For BSG, they presented (Table 1 in reference 3) evidence for two positively selected sites. However, the signal of selection was due to the inclusion of many monkey and prosimian sequences in their analysis. These species are irrelevant to our understanding of the recent evolution of P. falciparum, because Laverania (the subgenus comprised of P. falciparum and seven other described species [1, 4]) has only been found infecting African apes in the wild. One of the sites (codon 102) identified by Forni and colleagues is conserved among all apes analyzed and so is not pertinent to their claims. The other (codon 27) is conserved in humans, chimpanzees, and bonobos but differs in gorillas (2); again, it is hard to see how this site is relevant to the emergence of P. falciparum following a host jump from gorillas to humans. For RH5, Forni and colleagues (3) analyzed 13 sequences. However, 11 of these were near-identical alleles from strains of P. falciparum, one was the ortholog from Plasmodium reichenowi (a chimpanzee parasite), and the last was the sequence of a highly divergent paralog (RH2b) from P. falciparum. The relationship among these sequences is shown in Fig. 1A, which illustrates that Forni et al. were considering, in effect, only two sequences (those of P. falciparum and P. reichenowi), with no power to determine whether any differences between those two sequences arose on the branch leading to Published 22 May 2018 Citation Plenderleith LJ, Liu W, MacLean OA, Li Y, Loy DE, Sundararaman SA, BibolletRuche F, Learn GH, Hahn BH, Sharp PM. 2018. Reply to Forni et al., “Multiple selected changes may modulate the molecular interaction between Laverania RH5 and primate basigin. ” mBio 9:e00917-18. https:// doi.org/10.1128/mBio.00917-18. Editor Stephen P. Goff, Columbia University Copyright © 2018 Plenderleith et al. This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International license. Address correspondence to Paul M. Sharp, paul.sharp@ed.ac. uk. This is a response to a letter by Forni et al. (https://doi.org/10.1128/mBio.00476-18). AUTHOR REPLY