Individual and population variation in cercariae of bird schistosomes of the Trichobilharzia ocellata species group as revealed with the polymerase chain reaction

Reduced infectivity in Himasthla elongata (Trematoda, Himasthlidae) cercariae with deviant photoreaction

Digenean larvae of hermaphroditic generation - cercariae - are known to be polymorphic at genetic and behavioural levels. Cercariae arise as a result of parthenogenetic reproduction of intramolluscan stages, and represent a clone if a snail was infected with a single miracidium. Here we investigated cercarial clones of Himasthla elongata - namely, the infectivity of cercariae with normal (negative) and deviant (positive) photoreaction. In our study, most H. elongata clones showed intraclonal variance in their response to light. The proportion of photopositive cercariae ranged between 0.2% and 60% in different H. elongata clones. Photopositive larvae demonstrated significantly reduced rates of encystment in Mytilus edulis haemolymph in vitro and in young mussels. We discuss the possible mechanisms behind intraclonal variations, such as non-specific genomic rearrangements.

Diversity of bird schistosomes in anseriform birds in Iceland based on egg measurements and egg morphology

Radix balthica is the only freshwater mollusc in Iceland that has been found to be the intermediate host of ocellate furcocercariae. Infections are common. On average, 4.8% out of almost 7,000 snails examined during 2003-2006 were found to shed cercariae in late summer and autumn. Search for adult bird schistosome worms, eggs and miracidia in the intestines and in the nasal cavity of 110 water birds of the orders Gaviiformes (one species), Podicipediformes (one species) and Anseriformes (nine species) revealed 39 (35.5%) infected hosts. Exclusively, anseriform birds were infected. Based on morphological features and measurements of eggs, seven distinct bird schistosome species were detected in four of the bird species. Allobilharzia visceralis was found in 26.7% of the whooper swans Cygnus cygnus. In Anas platyrhynchos, the nasal Trichobilharzia sp. I (prevalence 73.3%) and the visceral Trichobilharzia sp. III (66.7%) were detected. In Mergus serrator, the distinct egg types of Trichobilharzia sp. IV and Trichobilharzia sp. V were found in 83.3% of examined birds. In Anser anser, also two different egg types were found: large eggs, referred to as Trichobilharzia sp. II (prevalence 54.2%), and the small eggs of Dendritobilharzia sp. (8.3%). Egg size and shape comparisons indicate that Trichobilharzia sp. III and T. franki might be the same species.

Multiple near-identical genotypes of Schistosoma japonicum can occur in snails and have implications for population-genetic analyses

We genotyped (using 16 or 17 microsatellite loci) numerous adult Schistosoma japonicum raised in rabbits exposed to pooled cercariae from small numbers of naturally infected snails from several localities in China. As expected, duplicate multi-locus genotypes (MLGs) were found among these worms. Additionally, many more MLGs, often near-identical, were found than snails used as sources of cercariae. Explanations for these results include (i) genotyping errors, (ii) development within each infected snail of multiple sibling miracidia and (iii) somatic mutation producing genetically varied cercariae from a single miracidium. To control for genotyping errors we re-analysed samples from many individual worms, including repeating the initial PCR. Explanations invoking the development of multiple sibling miracidia within a single snail are not likely to be correct because almost all duplicate MLGs fell within same-sex clusters in a principal coordinates analysis. We would expect both sexes to be represented in a multi-miracidium infection. In addition, we exposed several snails to infection by a single miracidium. One such snail, via an experimentally infected mouse, yielded 48 adult worms. The presence of at least nine near-identical MLGs among these worms was confirmed by re-genotyping. We regard somatic mutation as the most likely explanation for our results. The implications of multiple MLGs for population-genetic studies in S. japonicum are discussed.