Phylogeography and Prevalence of Hemoparasites (Apicomplexa: Eucoccidiorida) in Galápagos Marine Iguanas, Amblyrhynchus cristatus (Reptilia: Iguanidae)

Abstract

Parasitism is among the most common forms of coexistence of organisms of different species. Hemoparasites live in the bloodstream of the host where they complete different life-cycle stages. Members of the phylum Apicomplexa constitute a large portion of all hemoparasites infecting reptiles and their parasite transmitting vectors, including arthropods. In this study, we carried out a survey and molecular identification of hemoparasites in blood samples of the iconic Galápagos marine iguana (Amblyrhynchus cristatus). Major island populations of marine iguanas were sampled to examine large-scale biogeographic patterns of parasite diversity and prevalence. Nested PCRs were used to amplify segments of the 18S rRNA-gene of hemoparasites. Furthermore, ticks attached to marine iguanas were collected and analyzed in the same way to assess their potential use as a non-invasive method for the detection of hemoparasites in vertebrate host species. PCR products were sequenced and a phylogenetic analysis was carried out showing the presence of two genetically distinct clusters of hemoparasites, one more commonly distributed than the other one, belonging to the genera Hepatozoon and/or Hemolivia (Apicomplexa: Eucoccidiorida). Overall, 25% of marine iguanas were infected by hemoparasites. However, infection rates varied strongly among particular island populations (from 3.45% to 50%). Although marine iguanas are an extremely mobile species that has colonized all islands in the Galápagos archipelago, parasite occurrence was not related to geographical distance, suggesting that dispersal behavior has a minor role in parasite transmission. On most islands, females tended to have higher infection rates than males, but this relationship was only significant on one island. Overall, ticks and marine iguanas had similar prevalence and diversity of parasites. However, the infection profiles of ticks and their corresponding hosts (marine iguanas) did not mirror one another, indicating that this method cannot be used reliably to assess marine iguana infection status. Interestingly, we found that hemoparasite prevalence in marine iguanas and ticks tended to be positively correlated across islands. Our results indicate that certain populations of marine iguanas may have special mechanisms and adaptations to cope with parasite infection. In addition, other factors such as vector density, anthropogenic-related activities or the immunological state of marine iguanas could potentially affect the striking variation in hemoparasite prevalence across island populations.

Keywords: biogeographic patterns; genetic diversity; hemoparasites; infection status assessment; marine iguanas; prevalence; ticks.

Figure 1

Map of the Galápagos Islands showing the different sampling sites of this study. Hemoparasite infection prevalence for marine iguanas (blood samples) in each site is shown. In the pie charts, black and white represent, respectively, the proportion of infected and non-infected marine iguanas (n = 390). Sampling sites are marked with circles. Colors filling the circles reflect prevalence (%) according to the scale.

Figure 2

Phylogenetic tree including unique (selected) sequences generated in this study together with GenBank retrieved sequences. Samples from this study are highlighted with blue (cluster 1) and red (cluster 2). Distribution for the two clusters is shown. Pie charts represent the proportion of the first 50 BLAST hits for Hepatozoon (black) and Hemolivia (white) for selected sequences (i.e., short sequences in cluster 2 are not shown). Short sequences (<242 bp) are marked with an asterisk *. Genbank numbers and genus/species name are shown only for retrieved sequences. For samples from this study, GenBank accession numbers and other metadata are shown in Supplementary File S2. Bootstrap values (higher than 50) are shown. The scale bare represents substitutions per site.

Figure 3

Prevalence of hemoparasites in tick samples (i.e., one or more ticks were collected for each “tick sample”, see Materials and Methods for clarification). (A) Percentage of infected and uninfected tick samples per each site. In brackets, the total number of tick samples collected. (B) Scatterplot showing prevalence values for marine iguanas (y-axis) and prevalence values for tick samples (x-axis). The size of the point is according to the sample size of tick samples. Most reliable estimates of hemoparasite prevalence in ticks—i.e., populations with ten or more samples—are marked with the name.