| Summary: | <p>Abstract</p> <p>Background</p> <p>The eel parasitic nematodes <it>Anguillicola crassus</it> (originating from Asia) and <it>Anguillicola novaezelandiae</it> (originating from New Zealand) were both introduced to Europe, but occurred in sympatry only in Lake Bracciano in Italy, where they both infected the European eel (<it>Anguilla anguilla</it>). <it>A. novaezelandiae</it> was introduced to the lake in 1975 and disappeared soon after <it>A. crassus</it> was also found there in 1993. We tested the hypothesis if hybridization of the two species might be an explanation for the findings at Lake Bracciano.</p> <p>Findings</p> <p>After laboratory infection of one European eel with 10 third stage larvae of each parasite, two living female and 4 male adults of each species were found to co-occur in the swim bladder after 222 days post exposure. In 9 out of 17 eggs, isolated in total from uteri of the two <it>A. novaezelandiae</it> females, alleles were detected by microsatellite analysis that are characteristic for <it>A. crassus</it>, suggesting the hybrid origin of these eggs. In contrast, none of the eggs isolated from <it>A. crassus</it> females possessed alleles different from those found in <it>A. crassus</it> adults, but it was revealed that one female can be inseminated by several males.</p> <p>Conclusion</p> <p>Our results show that <it>A. crassus</it> and <it>A. novaezelandiae</it> can co-infect a single eel and can mature together in the same swim bladder. We also provide evidence for the possibility of hybridization of <it>A. crassus</it> males with <it>A. novaezelandiae</it> females. Therefore, hybridization might be an explanation for the disappearance of <it>A. novaezelandiae</it> from Lake Bracciano.</p>
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