Abstract
Phenotypic polymorphism is a commonly observed phenomenon in nature, but extremely rare in free-living stages of parasites. We describe a unique case of somatic polymorphism in conspecific cercariae of the bird schistosome Trichobilharzia sp. “peregra”, in which two morphs, conspicuously different in their size, were released from a single Radix balthica snail. A detailed morphometric analysis that included multiple morphological parameters taken from 105 live and formalin-fixed cercariae isolated from several naturally infected snails provided reliable evidence for a division of all cercariae into two size groups that contained either large or small individuals. Large morph (total body length of 1368 and 1339 μm for live and formalin-fixed samples, respectively) differed significantly nearly in all morphological characteristics compared to small cercariae (total body length of 976 and 898 μm for live and formalin samples, respectively), regardless of the fixation method. Furthermore, we observed that small individuals represent the normal/commonly occurring phenotype in snail populations. The probable causes and consequences of generating an alternative, much larger phenotype in the parasite infrapopulation are discussed in the context of transmission ecology as possible benefits and disadvantages facilitating or preventing the successful completion of the life cycle.
Highlights
Phenotypic polymorphism is a commonly observed phenomenon in nature throughout the animal kingdom, arising from genetically based differentiation or phenotypic plasticity, or a combination of both (e.g., [1,2,3,4])
Molecular identification of the bird schistosome isolates sequenced from the single snail verified that both large and small morphs of cercariae belong to the same species Trichobilharzia franki haplotype “peregra” sensu Jouet et al [43]
Biometric evaluations of 105 cercariae (38 live and 67 formalin-fixed), including the two size-different morphs and other Trichobilharzia sp. “peregra” cercariae isolated from naturally infected Radix balthica (Linnaeus, 1758) (Gastropoda: Lymnaeidae) within the snail population sampled in September 2013 (Figure 1, Table 1), showed differences in cercarial sizes with respect to fixation method
Summary
Phenotypic polymorphism is a commonly observed phenomenon in nature throughout the animal kingdom, arising from genetically based differentiation or phenotypic plasticity, or a combination of both (e.g., [1,2,3,4]). It is traditionally defined as an intraspecific variability in observable characteristics of an organism’s trait, implying the co-occurrence of two or more distinct morphs of individuals (i.e., alternative phenotypes) in a population of the same species, inhabiting the same habitat at the same time [5,6]. Many examples of intraspecific polymorphism are found among uni- and multicellular parasites (e.g., [10,11,12,13,14]), which constitute a substantial proportion of global species diversity [15,16]
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