Volume 36, Issue 2
  • ISSN: 0002-9637
  • E-ISSN: 1476-1645



Electrophoretically-detected allozyme variation is described in strains of (4 Philippine strains), (Laos), and an undescribed anthropophilic -like schistosome from Peninsular Malaysia. Results, together with those reported previously for 8 other strains (, China, Formosa, Japan, Philippines; , 2 substrains; Malaysian schistosome, 2 strains) permit a composite genetic characterization of 15 strains of Asian schistosomes at 9–18 presumptive loci. The proportion of polymorphic loci () and the mean heterozygosity per locus () were zero in all strains. Although this was expected for strains that had been in laboratory culture for up to 50 years, we expected to detect variation in strains based on 10–50 recently field-collected infected snails. We expected to be as variable as ( = 0.13 (0–0.33), = 0.04, 18 loci, 22 strains) as it is believed to reproduce sexually, has an evolutionary history of several million years, inhabits a wide geographic range, coevolved with a genetically variable intermediate snail host, and has a diversity of mammalian hosts.

No differences were detected between the 5 strains from Leyte and Luzon (Philippines), between the 3 strains, or between the 3 Malaysian schistosome strains; these groups and the remaining strains representing Mindoro (Philippines), China, Formosa, and Japan each have distinctive multilocus electromorphic patterns. Nei's genetic distances () were calculated to estimate interstrain and interspecific divergence. Interstrain genetic distances in averaged >0.3; much higher than those reported previously for ( = 0.06, (max) = 0.24). (Mindoro) was moderately differentiated from the Leyte-Luzon strains ( = 0.29, 12 loci). Estimates of the China-Philippine distance ( > 0.4, 11 loci) are high for conspecific populations and further studies of the still poorly characterized Chinese parasite may reveal that these are, in fact, separate species. is shown to be only distantly related to and the Malaysian schistosome ( > 1); the latter is closely related to, but genetically quite distinct from, ( = 0.61 ± 0.275, 11 loci) and warrants recognition as a new species. The medical significance of the isogenic nature of the Asian schistosome strains and their evolutionary divergence are discussed.


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