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Monitoring haemosporidian parasites in birds is essential to comprehend the dynamics of avian malaria, a disease that significantly affects bird populations worldwide. This study concentrated on the prevalence and diversity of haemosporidian parasites in 198 specimens from two subspecies of the Eurasian bluethroat (Luscinia svecica), aiming to explore the genetic diversity and species richness of haemosporidian fauna across the host populations. By utilizing next-generation amplicon high-throughput sequencing (NGS), we observed a marked increase in the detection of haemosporidian diversity, revealing cryptic variants and species previously unidentified by Sanger sequencing. A high prevalence of Plasmodium was seen in all studied sites, accompanied by a less frequent Leucocytozoon infection in the red-spotted subspecies and minimal occurrence of Haemoproteus. Both previously known and new, low prevalence cryptic variants were detected, underscoring the complexity of haemosporidian infections in avian hosts. The use of species delimitation tools provided a detailed understanding of haemosporidian species diversity, their coexistence within hosts, and their phylogenetic relationships. Despite the varying ecological characteristics of the study sites, no significant difference in haemosporidian alpha diversity among populations was found. However, significant differences in beta diversity were identified, suggesting that habitat characteristics and geographic distance influence parasite distribution. These findings highlight the importance of advanced molecular techniques in revealing the hidden diversity of parasites, offering valuable insights into the ecology and evolution of haemosporidian infections. Given the threatened status of one of the host's populations, knowledge on local diversity of haemosporidian parasites also has implications for possible conservation strategies., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier Ltd.)

The origin and significance of host specificity are intriguing questions in parasitology. In the case of single-host versus multiple-host parasites, this topic integrates with the concept of the specialist/generalist trade-off. We use the model of sucking lice Polyplax serrata and rodent hosts Apodemus, to address these concepts. Polyplax serrata was shown to form a complex genetic structure, with a strictly specific S lineage living on Apodemus flavicollis, and a less specific N lineage on A. flavicollis and Apodemus sylvaticus. Moreover, the S lineage formed two mitochondrial clades with geographically exclusive distributions and a narrow hybrid zone, providing an opportunity to test the hypothesis that hybrids suffer a decrease in fitness. We sampled 451 individual lice from two host species at 103 localities. We used prevalences and intensities as proxies of fitness, which the parasites realize on their host. The S lineage, strictly specific to Apodemus flavicollis, reached significantly higher prevalences and intensities on its host compared with the N lineage. Conversely, the N lineage occurred with high prevalence and intensity on A. sylvaticus but tended to use also A. flavicollis when the louse populations became too dense. We discuss possible mechanisms behind this difference (particularly interspecific competition as a typical phenomenon in the specialist/generalist systems). We conclude that a parasite's "choice", not accessibility of the host or interspecific competition, is the main factor affecting the louse prevalences. We suggest that historical differences in geographic distribution of both lice and mice may provide a possible explanation for the observed life strategy differences. In contrast to the convincing picture in S and N lineage prevalences, we did not detect an expected drop in fitness in hybrids. We consider instability of the hybrid zone, or decline in abundance of the respective hosts, as possible explanations for this result., (Copyright © 2024 Australian Society for Parasitology. Published by Elsevier Ltd. All rights reserved.)

Competing Interests: Declaration of interests The authors declare no competing interests.

Competing Interests: Declaration of interests The authors declare no competing interests.

Parasites with wide host spectra provide opportunities to study the ecological parameters of speciation, as well as the process of the evolution of host specificity. The speciose and cosmopolitan louse genus Menacanthus comprises both multi-host and specialised species, allowing exploration of the ecological and historical factors affecting the evolution of parasites using a comparative approach. We used phylogenetic analysis to reconstruct evolutionary relationships in 14 species of Menacanthus based on the sequences of one mitochondrial and one nuclear gene. The results allowed us to validate species identification based on morphology, as well as to explore host distribution by assumed generalist and specialist species. Our analyses confirmed a narrow host use for several species, however in some cases, the supposed host specialists had a wider host spectrum than anticipated. In one case a host generalist (Menacanthus eurysternus) was clustered terminally on a clade almost exclusively containing host specialists. Such a clade topology indicates that the process of host specialisation may not be irreversible in parasite evolution. Finally, we compared patterns of population genetic structure, geographic distribution and host spectra between two selected species, M. eurysternus and Menacanthus camelinus, using haplotype networks. Menacanthus camelinus showed limited geographical distribution in combination with monoxenous host use, whereas M. eurysternus showed a global distribution and lack of host specificity. It is suggested that frequent host switching maintains gene flow between M. eurysternus populations on unrelated hosts in local populations. However, gene flow between geographically distant localities was restricted, suggesting that geography rather than host-specificity is the main factor defining the global genetic diversity of M. eurysternus., (Copyright © 2014 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved.)