We first sequenced and characterised the complete mitochondrial genome of Toxocara apodeme, then studied the evolutionary relationship of the species within Toxocaridae. The complete mitochondrial genome was amplified using PCR with 14 specific primers. The mitogenome length was 14303 bp in size, including 12 PCGs (encoding 3,423 amino acids), 22 tRNAs, 2 rRNAs, and 2 NCRs, with 68.38% A+T contents. The mt genomes of T. apodemi had relatively compact structures with 11 intergenic spacers and 5 overlaps. Comparative analyses of the nucleotide sequences of complete mt genomes showed that T. apodemi had higher identities with T. canis than other congeners. A sliding window analysis of 12 PCGs among 5 Toxocara species indicated that nad4 had the highest sequence divergence, and cox1 was the least variable gene. Relative synonymous codon usage showed that UUG, ACU, CCU, CGU, and UCU most frequently occurred in the complete genomes of T. apodemi. The Ka/Ks ratio showed that all Toxocara mt genes were subject to purification selection. The largest genetic distance between T. apodemi and the other 4 congeneric species was found in nad2, and the smallest was found in cox2. Phylogenetic analyses based on the concatenated amino acid sequences of 12 PCGs demonstrated that T. apodemi formed a distinct branch and was always a sister taxon to other congeneric species. The present study determined the complete mt genome sequences of T. apodemi, which provide novel genetic markers for further studies of the taxonomy, population genetics, and systematics of the Toxocaridae nematodes.

Haemoparasites represent a diverse group of vector-borne parasites that infect a wide range of vertebrate hosts. In birds, haemoparasite infection rates may be associated with various ecological and life history traits, including habitat choice, colony size and migration distance. Here, we molecularly assessed the prevalence of 3 main haemoparasite genera (Plasmodium, Haemoproteus and Leucocytozoon) in 2 bird species with different habitat preferences and migratory behaviour: black-headed gulls (Chroicocephalus ridibundus) and common terns (Sterna hirundo). We found that gulls showed a much higher prevalence and diversity of Plasmodium or Haemoproteus (ca. 60% of individuals infected) than terns (zero prevalence). The prevalence of Leucocytozoon was low in both species (<3%). The differences in haemoparasite prevalences may be primarily driven by varying vector encounter rate resulting from different habitat preferences, as black-headed gulls mainly use vector-rich vegetated freshwater habitats, whereas common terns often use vector-poor coastal and brackish habitats. Since common terns migrate further than black-headed gulls, our results did not provide support for an association between haemoparasite prevalence and migratory distance. In gulls, we found a negative association between colony size and infection rates, suggestive of an ideal despotic distribution, and phylogenetic analyses of detected haemoparasite lineages provided evidence for higher host specificity in Haemoproteus than Plasmodium. Our results suggest that the preference for coastal areas and less vegetated habitats in terns may reduce haemoparasite infection rates compared to other larids, regardless of their migratory distance, emphasizing the role of ecological niches in parasite exposure.

Schistosomes, which live as parasitic adults in the blood vessels of mammals, birds and crocodiles, are unique among trematodes because they have separate sexes. Several studies have shown that the sex ratio is often biased towards males. Sexual dimorphism can be very important in several genera (Schistosoma, Heterobilharzia, Schistosomatium), as larger males grip smaller females in a gynecophoric canal. But not all schistomatid species have developed a gynecophoric canal. It is also striking that the number of testes varies widely, from a small number of testes reported (2–7) to a very high number (>500). We tested the hypothesis of trade-off investment by males: muscle (as estimated by the length of the gynecophoric canal) in order to sequester one or a few females, or testes in order to inseminate numerous females. We used recent comparative methods, which need phylogenetic information of the analysed species. We employed a morphological cladistic analysis of the Schistosomatidae and a construction of a phylogenetic supertree of Schistosoma based on available molecular information. Our phylogenetic analysis supports previous hypotheses on the origin and diversification of the schistosomes. The family seems to have originated as parasites of crocodiles (or possibly other related poikilotherms) and to have secondarily evolved within the Aves and then mammals. Our phylogenetic analyses show that (1) the possession of a gynecophoric canal is a plesiomorphic character, and several reductions of this character have occurred during the diversification of the Schistosomatidae; (2) a small number of testes is a plesiomorphic stage, and an increase of testes number has occurred several times. Finally, our comparative analysis suggests that there is a trade-off between investment in muscle or in number of testes (other variables being controlled for). Male competition for access to females seems to follow two ways: investment in testes in order to fertilize a high number of females or investment in muscle in order to hold and sequester one (Schistosoma) or a few females (Heterobilharzia). We show the existence of the trade-off between muscles and testes in connection with a male biased sex ratio. The development of dioecy cannot explain the trade-off between muscles and testes, which instead appears to be a result of sexual selection possibly driven by the male biased sex ratio.