Many theories about the origin of the genus Schistosoma of African or Asian origin have been stated and developed with their respective arguments. Davis introduced the idea that the genus Schistosoma arose before the split of Gondwanaland more than 150 million years ago (Ma) and had begun exploiting pulmonate and pomatiopsid snails, whose fossil records suggest a Gondwanan origin. This theory implies that the dispersal of the genus Schistosoma was due to continental drift and that the Asian ancestor of the schistosomes was carried across to Asia when India separated from Africa and moved forward in Asia 70–148 Ma, generating the S. indicum and S. japonicum complex.20
The hypothesis “out of Asia” is the most commonly accepted hypothesis for the spread of schistosomiasis. This hypothesis suggests a migration followed by the dispersion of schistosomiasis from Asia to Africa.26 The Asian schistosome ancestor may have originally had a pomatiopsid or a pulmonate snail host. Snyder and Loker suggest that the schistosome colonized Africa approximately 15–20 Ma in the mid-Miocine. Following the invasion of the African continent, the parasites evolved to exploit pulmonate snails exclusively, thereby developing a more specialized host range.20
The phylogenetic study shows that the genus Schistosoma originates from Asia. This parasite has at least two descendants, invading the African continent independently. In Africa, these descendants easily radiated, parasitizing exclusively planorbid snails. Back in Asia, the parasites diversified into a group of species characterized by the absence of an egg spoke.26 The genus Schistosoma is split into six clades, which correlate with the different geographical distributions of the parasites. The genus forms the S. japonicum complex. It comprises S. japonicum, S. malayensis, S. mekongi, S. ovuncatum, and S. sinensium.20 Schistosoma japonicum and subsequently S. mekongi diverged from antecedents resembling S. sinensium. Schistosoma sinensium is a relative of S. ovuncatum. It occurred in northern India, northwest Thailand, and southern China.8
Schistosoma japonicum spreads eastward and establishes distinct groups in the central and eastern provinces of China along the Yangtze River.27 However, Yin et al. (2015) suggested that S. japonicum spread from the middle and lower reaches of the Yangtze River to the mountainous regions of China, to Japan, and then to Southeast Asia.28 Schistosomiasis japonica occurs in China, Indonesia, and the Philippines.29 Its intermediate hosts include O. lorelindoensis (formerly O. lindoensis) and O. robertsoni (formerly O. hupensis robertsoni).30
Phylogenetic tracking showed that S. malayensis originated from S. mekongi radiation in Cambodia. The split occurred at 2.5 Ma and approximately 3.8 Ma for S. japonicum.17 The divergence of the S. sinensium group in Central Asia is dated to 4.6 Ma.31 Phylogeography tracking shows that proto-S. malayensis entered Vietnam from Hunan via the Red River Valley. The Schistosoma malayensis clade entered Southeast Asia using a Vietnam-to-Cambodia route. Molecular dating suggests a radiation of S. mekongi into Cambodia and Laos at approximately 1.3 Ma. Schistosoma malayensis-mekongi diverged from S. sinensium during their migration from Hunan.17 Furthermore, Lawton et al.20 suggested that S. sinensium is a sibling of S. japonicum.20 It is also a sister group to other species: Schistosoma ovuncatum, S. mekongi, S. malayensis, and S. japonicum. Schistosoma ovuncatum is relatively close to S. sinensium.8 In Asia, three Schistosoma species cause human schistosomiasis: Schistosoma japonicum, S. malayensis, and S. mekongi.32 Schistosoma malayensis and S. mekongi differ only in biogeography, life cycle parameters, and intermediate hosts.31
According to the sequencing results, I found that S. japonicum AY157226.1 shares 99.45% identity with S. malayensis AY157227.1 and 98.77% identity with S. mekongi AF465928.1, suggesting that S. japonicum is closer to S. malayensis than S. mekongi. However, the sequence of S. malayensis AY157227.1 resulted in 100% identity with S. mekongi AF465928.1 and resulted in 99.45% identity with S. japonicum AY157226.1 (Table 1). In addition, according to pairwise alignments with dots for identities and CDS, S. malayensis AY157227.1 has no different genetic codes (bases) (100%) from S. mekongi AF465928.1, has ten different bases (99%) from S. japonicum AY157226.1, has 37 different bases from S. sinensium AY157225.1, and has five different bases from S. ovuncatum AF465929.1 and S. sinensium AF465930.1. This suggests that S. malayensis is the same as S. mekongi and is closer to S. mekongi than to S. japonicum. Schistosoma sinensium diverged from S. ovuncatum. Schitosoma japonicum diverged from S. sinensium. Schistosoma mekongi diverged from S. japonicum. Schistosoma malayensis diverged from S. mekongi (Table 1 and Fig. 1).
Two subfamilies of Asian schistosomes intermediate hosts comprise Pomatiopsinae and Triculinae.25 Triculinae radiate as aquatic snails in freshwater habitats in Southeast Asia and Southern China, namely Yunnan Province, China. It also occurs in Tibet.8 Pomatiopsinae spread throughout the world and have a variety of lifestyles ranging from aquatic to amphibian. Triculinae can include Neotricula aperta, Robertsiella spp., and Tricula spp. such as Tricula bollingi and Tricula hortensis.33 Pomatiopsinae belongs to the genus Oncomelania, which comprises O. hupensis,33 O. lorelindoensis, O. robertsoni, and O. quadrasi.2 Oncomelania occurs in Sulawesi, the Philippines, Taiwan, Japan, and China. It is an intermediate host of S. japonicum. Neotricula aperta is the intermediate host of S. mekongi. It is the only intermediate host for S. mekongi. Attwood and Upatham34 suggested that there are three strains of N. aperta: α, β, and γ. Only the γ-strain is epidemiologically significant.34 Robertsiella spp. is an intermediate host of S. malayensis. Tricula bollingi and Tricula hortensis are intermediate hosts of S. sinensium.8 In addition, Tricula spp. can also be an intermediate host of S. ovuncatum.
According to the sequence results of the COI gene, I found that O. robertsoni KR002675.1 shared 86.12% identity with Cambodia/Laos N. aperta AF531541.1. It has the highest percentage among Triculinae. The second place is Robertsiella sp. Malaysia, with an identity similarity of 85.88%. The third most common species was Tricula bollingi, with an identity similarity of 85.28% (Table 2). According to alignments pairwise with dots for identities and CDS, O. robertsoni KR002675.1 has 83 different bases (86%) from N. aperta AF531541.1, 84 different bases (86%) from Robertsiella sp AF531550.1, and 88 different bases (85%) from T. bollingi AF531553.1. Furthermore, the sequence results show that N. aperta shares 85.95% identity with O. robertsoni LC2768.1, 85.76% identity with O. robertsoni EU079378.1, 85.13% identity with O. hupensis hupensis JF284688.1, 84.44% identity with O. hupensis hupensis JF84690.1 and O. hupensis tangi DQ1796.1, and 84.62% identity with O. hupensis GU367391.1. These findings suggest that O. robertsoni is closer to N. aperta than to Robersiella sp. and Tricula bollingi. This suggests that O. robertsoni (Pomatiopinae) came from the same ancestor (Pomatiopsidae) as N. aperta, Robertsiella sp., and Tricula bollingi (Triculinae).
Nelwan2 suggests that the genus Oncomelania comprises five species: Oncomelania hupensis, O. lorelindoensis, O. minima, O. robertsoni, and O. quadrasi. However, the author also points out that GenBank data for O. lorelindoensis (formerly O. hupensis lindoensis) are not yet available.2 Oncomelania hupensis lindoensis (O. lorelindoensis) has a distant relationship as a subspecies of O. hupensis. It shares 86.10% identity with O. hupensis hupensis.35 In addition, O. lorelindoensis and O. quadrasi have a 6.2 pairwise difference in the percentage of the 12S rRNA gene.36 This suggests that O. lorelindoensis and O. quadrasi have distant relatedness. If the percentage of identity is high, it should be O. quadrasi, a subspecies of O. lorelindoensis. Liu et al.33 suggested that proto-Oncomelania originated from Northwest Australia, which today forms parts of Borneo and eastern Indonesia,33 including Sulawesi. This suggests that Sulawesi proto-Oncomelania, i.e., proto-Oncomelania lorelindoensis, is older than the Philippines proto-Oncomelania, i.e., proto-Oncomelania quadrasi. Other proto-Oncomelania comprise proto-Oncomelania hupensis, proto-Oncomelania minima, and proto-Oncomelania robertsoni, due to each species of Oncomelania rising from its antecedent form.
In China, there are two species of Oncomelania: Oncomelania hupensis and O. robertsoni. Oncomelania hupensis has subspecies such as O. hupensis hupensis and O. hupensis tangi.2 To make sure of this, I performed NCBI BLAST on O. hupensis NC_013073. I took eleven Oncomelania from the sequence results (Table 3). These results show that O. hupensis NC_013073 shares 98.00% identity with O. hupensis hupensis JF284692.1, shares 97.80% identity with O. hupensis hupensis JF284688.1 and shares 97.68% identity with O. hupensis hupensis JF284690.1. In addition, the sequence of O. hupensis NC_013073 shares 95.44% identity with O. hupensis nosophora LC276225.1 and 95.37% identity with O. hupensis nosophora LC276226.1. This suggests that O. hupensis hupensis (JF284692.1, JF284688.1, and JF284690.1) and O. hupensis nosophora (LC276225.1 and LC276226.1) are closely related to O. hupensis NC_013073.1. This confirms that they are all subspecies of O. hupensis, as suggested by Nelwan.2 Moreover, the sequence results of O. hupensis NC_013073 share 90.06% identity with O. robertsoni JF284697.1, 89.50% identity with O. quadrasi JF284698.1. According to Pairwise with dots for identities and CDS, O. hupensis NC_013073.1 has 130 different amino acids from O. quadrasi JF284698.1. It has 1322 different bases from O. quadrasi JF284698.1 (89%). Oncomelania robertsoni EU079378.1 has 173 different amino acids from O. hupensis NC_013073.1. It has 1569 different bases from O. hupensis (13636/15205 = 89.63%). Oncomelania robertsoni EU079378.1 has 170 different amino acids from O. quadrasi LC276227.1. It has 1675 different bases from O. quadrasi (89%). These findings suggest that O. robertsoni and O. quadrasi are distantly related to O. hupensis.
Nelwan2 suggested that O. robertsoni and O. quadrasi are full species in the genus Oncomelania. In addition, O. lorelindoensis is more closely related to O. robertsoni than O. hupensis. In the current study, I found that O. robertsoni EU079378 shares 94.04% identity with O. robertsoni LC276228.1 and 93.38% identity with O. robertsoni JF284697.1. Oncomelania hupensis has two subspecies: Oncomelania hupensis hupensis and O. hupensis nosophora (Table 3 and Fig. 3). Moreover, O. hupensis, O. robertsoni, and O. quadrasi are in different groups in a tree-view slanted cladogram (Fig. 3). This finding supports the fact that O. hupensis, O. robertsoni, and O. quadrasi are separate species. This supports the fact that the genus Oncomelania comprises five species: Oncomelania hupensis, O. lorelindoensis, O. minima, O. robertsoni, and O. quadrasi. In addition, these findings also support that O. hupensis comprises five subspecies: O. hupensis hupensis, O. hupensis chiui, O. hupensis formosana, O. hupensis nosophora, and O. hupensis tangi.2
Sequences of O. hupensis COI and O. quadrasi COI against all O. robertsoni of the Sichuan Plain (SCB) 24 confirm that O. quadrasi and O. robertsoni are distinct from O. hupensis. For example, the sequence of O. quadrasi DQ112287.1 shares 87.77% identity with O. robertsoni DQ212800.1 (S1 Table), and the sequence of O. hupensis GU367391.1 shares 88.99% similarity with O. robertsoni DQ212802.1 (S2 Table). The sequence of O. hupenis GU367391.1 shares an identity of 84.48% with O. quadrasi DQ112287.1 (Nelwan, 2022).2 This supports the fact that O. quadrasi and O. robertsoni did not drive from O. hupensis because they have a distant relationship. In addition, it also supports the fact that O. hupensis, O. lorelindoensis, O. minima, O. robertsoni, and O. quadrasi are derived from their antecedent forms. Thus, the genus Oncomelania is derived from its antecedent forms. Moreover, these findings support the fact that, except for O. hupensis, Oncomelania developed in its own territory. Oncomelania robertsoni was developed in China. Oncomelania lorelindoensis was developed in Sulawesi (Indonesia). Oncomelania minima was developed in Japan. Oncomelania quadrasi developed in the Philippines. Oncomelania hupensis gave rise to O. hupensis chiui (Taiwan), O. hupensis formosona (Taiwan), O. hupensis hupensis (China), O. hupensis nosophora (Japan), and O. hupensis tangi (China).
Sequence results did not support the idea that O. robertsoni has subspecies. Oncomelania robertsoni KR002675.1 of the Sichuan Plain (SCB) 24 shared 91.20% identity with O. robertsoni AF531547.1 (S3 Table). The sequence of O. robertsoni from the Sichuan Anning River Valley (SAV) 24 showed that O. robertsoni AF113391.1, for example, shared 92.16% identity with O. robertsoni EU079378.1, and 91.07% identity with O. robertoni DQ212844.1 (S4 Table). The sequence of O. robertsoni AF531547.1 in the SCB shared 97.63% identity with O. robertsoni DQ212846.1 (SAV) and shared 95.55% identity with O. robertsoni AF253074.1 (YEB) (S5 Table). These results show that there are four O. robertsoni clades in China. Oncomelania robertsoni SAV, SCB, and YEB is a novel clade of O. robertsoni.
This study has advantages and limitations. Advantages can include the easy use and reliability of the BLAST technique. For example, the sequence of O. hupensis NC_013073.1 using NCBI BLAST can produce other data, such as O. quadrasi LC276227.1 and O. robertsoni LC276228.1. Another example, O. robertsoni KR002675.1 in Table 1 is new data. In the NCBI BLAST results, the O. robertsoni group would occupy a separate group from other species in the slanted cladogram (see Fig. 3). Thus, it does not require new data collection. NCBI BLAST is as reliable as ClustalW, MUSCLE, PhyML and RaxML. The limitations are that, in some cases, GenBank accession numbers are not accurate. For example, O. hupensis hupensis JF284697 should be O. robertsoni JF284697. Oncomelania hupensis hupensis JF284698 should be O. quadrasi JF284698. There were no samples of parasites and intermediate hosts taken to obtain partial DNA sequences using the popular Former and Palumbi primers to compare data from previous studies, for example, although it does not affect the results of this study. In addition, data on O. hupensis lindoensis (O. lorelindoensis) are not yet available at NCBI. In this study, I used only GenBank data from O. quadrasi to assess Southeast Asian Oncomelania as an intact species. However, this does not mean that O. lorelindoensis is not a full species of the genus Oncomelania. This is due to the proto-Oncomelania coming from eastern Indonesia, namely Sulawesi. Then, off to the Philippines, Japan, and China.