Systematics as a cornerstone of parasitology: overview and preface

What would it take to describe the global diversity of parasites?

How many parasites are there on Earth? Here, we use helminth parasites to highlight how little is known about parasite diversity, and how insufficient our current approach will be to describe the full scope of life on Earth. Using the largest database of host-parasite associations and one of the world's largest parasite collections, we estimate a global total of roughly 100 000-350 000 species of helminth endoparasites of vertebrates, of which 85-95% are unknown to science. The parasites of amphibians and reptiles remain the most poorly described, but the majority of undescribed species are probably parasites of birds and bony fish. Missing species are disproportionately likely to be smaller parasites of smaller hosts in undersampled countries. At current rates, it would take centuries to comprehensively sample, collect and name vertebrate helminths. While some have suggested that macroecology can work around existing data limitations, we argue that patterns described from a small, biased sample of diversity aren't necessarily reliable, especially as host-parasite networks are increasingly altered by global change. In the spirit of moonshots like the Human Genome Project and the Global Virome Project, we consider the idea of a Global Parasite Project: a global effort to transform parasitology and inventory parasite diversity at an unprecedented pace.

Molecular diversity and coalescent species delimitation of avian haemosporidian parasites in an endemic bird species of South America

Haemoproteus spp. and Plasmodium spp. are blood parasites that occur in birds worldwide. Identifying the species within this group is complex, especially in wild birds that present low parasitemia when captured, making morphological identification very difficult. Thus, the use of alternative tools to identify species may be useful in the elucidation of the distribution of parasites that circulate in bird populations. The objectives of this study were to determine the prevalence and parasitemia of the genera Plasmodium and Haemoproteus in Tachyphonus coronatus in the Atlantic Forest, Brazil, and to evaluate the molecular diversity, geographic distribution, and specificity of these parasites based on coalescent species delimitation methods. Microscopic analysis, PCR, cyt b gene sequencing, phylogenetic analysis and coalescent species delimitation using single-locus algorithms were performed (Poisson tree process (PTP) and multi-rate Poisson tree process (MPTP) methods). The analyses were performed in 117 avian host individuals. The prevalence was 55.5% for Plasmodium and 1.7% for Haemoproteus, with a mean parasitemia of 0.06%. Twenty-five Plasmodium and two Haemoproteus lineages were recovered. The MPTP method recovered seven different evolutionarily significant units (ESUs) of Plasmodium and one of Haemoproteus, whereas PTP presented fourteen ESUs of Plasmodium and one of Haemoproteus. The MPTP was more consistent with current taxonomy, while PTP overestimated the number of lineages. These ESUs are widely distributed and have already been found in 22 orders of birds that, all together, inhabit every continent, except Antarctica. The computational methods of species delimitation proved to be effective in cases where the classification of Haemosporida based just on morphology is insufficient.

A molecular phylogenetic appraisal of the acanthostomines Acanthostomum and Timoniella and their position within Cryptogonimidae (Trematoda: Opisthorchioidea)

The phylogenetic position of three taxa from two trematode genera, belonging to the subfamily Acanthostominae (Opisthorchioidea: Cryptogonimidae), were analysed using partial 28S ribosomal DNA (Domains 1-2) and internal transcribed spacers (ITS1-5.8S-ITS2). Bayesian inference and Maximum likelihood analyses of combined 28S rDNA and ITS1 + 5.8S + ITS2 sequences indicated the monophyly of the genus Acanthostomum (A. cf. americanum and A. burminis) and paraphyly of the Acanthostominae. These phylogenetic relationships were consistent in analyses of 28S alone and concatenated 28S + ITS1 + 5.8S + ITS2 sequences analyses. Based on molecular phylogenetic analyses, the subfamily Acanthostominae is therefore a paraphyletic taxon, in contrast with previous classifications based on morphological data. Phylogenetic patterns of host specificity inferred from adult stages of other cryptogonimid taxa are also well supported. However, analyses using additional genera and species are necessary to support the phylogenetic inferences from this study. Our molecular phylogenetic reconstruction linked two larval stages of A. cf. americanum cercariae and metacercariae. Here, we present the evolutionary and ecological implications of parasitic infections in freshwater and brackish environments.

The Importance of Fossils in Understanding the Evolution of Parasites and Their Vectors

Knowledge concerning the diversity of parasitism and its reach across our current understanding of the tree of life has benefitted considerably from novel molecular phylogenetic methods. However, the timing of events and the resolution of the nature of the intimate relationships between parasites and their hosts in deep time remain problematic. Despite its vagaries, the fossil record provides the only direct evidence of parasites and parasitism in the fossil record of extant and extinct lineages. Here, we demonstrate the potential of the fossil record and other lines of geological evidence to calibrate the origin and evolution of parasitism by combining different kinds of dating evidence with novel molecular clock methodologies. Other novel methods promise to provide additional evidence for the presence or the life habit of pathogens and their vectors, including the discovery and analysis of ancient DNA and other biomolecules, as well as computed tomographic methods.

Species limits in avian malaria parasites (Haemosporida): how to move forward in the molecular era

Delimiting species of malaria parasites (Haemosporida) has become increasingly problematic as new lineages of parasites are identified solely by molecular information, particularly mitochondrial cytochrome b sequence data. In this review, we highlight some of the issues, both historical and contemporary, that have hindered the development of objective criteria to diagnose, delimit and define species of haemosporidians. Defining species is not the focal interest of most researchers, most of whom merely wish to determine whether lineages identified in their samples match those of other researchers, and if so, where and in which host species. Rather than revisiting all the issues with respect to delimiting and naming species, we instead focus on finding a practical near-term resolution to the 'species problem' that utilizes the community's largest resource: mitochondrial cytochrome b DNA sequences. We recommend a standardized procedure to 'tag' these sequences, based on per cent sequence similarity, that will allow researchers to directly assess the novelty, known hosts and geographic distribution of avian malaria parasite lineages.

The Gyrodactylus (Monogenea, Gyrodactylidae) parasite fauna of freshwater sand gobies (Teleostei, Gobioidei) in their centre of endemism, with description of seven new species

While Gobioidei comprises showcases of (adaptive) radiation, the scientific interest they yielded did not ensure full understanding of goby biodiversity. Even in a well-studied region like Europe, wide knowledge gaps remain. Sand gobies represent one of the few clades whose monogenean parasites have been thoroughly studied. However, in the Balkans, part of the sand gobies' centre of endemism, these parasites were unstudied. We focus on Greek and Croatian freshwater gobies. From five sand goby species, the first parasites are reported, describing seven new Gyrodactylus species. Economidichthys pygmaeus harbours Gyrodactylus benedeni sp. n. and Gyrodactylus dorlodoti sp. n. Its congener E. trichonis hosts G. meelkopae sp. n. Knipowitschia milleri was found to host G. charon sp. n., K. thessala is infected by G. bios sp. n., and K. croatica by G. douglasadamsi sp. n. and G. hellemansi sp. n. Gyrodactylus bubyri was found on its type host K. caucasica. A diverse parasite fauna is expected for a region known for its biodiversity and endemism. The contribution of parasites to species richness in such hotspots is overlooked. The observed species richness per host is rather low compared to the better-studied eastern Atlantic sand gobies. Host vicariance is considered to mediate parasite specificity in this fauna. Some new flatworm species display unique morphological features, such as the remarkable size of the marginal hook sickle proper compared to its foot in the Economidichthys parasites, or a characteristically kinked marginal hook sickle in G. douglasadamsi sp. n. These features reflect their hosts' endemism in the Balkans.

Ultrastructural characteristics of the protonephridial terminal organ and associated ducts of adult specimens of the Aspidogastrea, Digenea and Monogenea, with comments on the relationships between these groups

Transmission electron microscopical observations were made on the protonephridial terminal organ and associated ducts of three adult trematodes, the aspidogastrean Aspidogaster limacoides Diesing, 1835 and the digeneans Azygia lucii (Müller, 1776) and Phyllodistomum angulatum Linstow, 1907, and the monogenean Ancyrocephalus paradoxus Creplin, 1839. Previously unreported ultrastructural details of the terminal organ of adult trematodes include multiple contact sites (septate junctions and zonulae adherentes) between the membranes of the terminal and adjacent canal cells. Septate junctions traverse the epithelial cytoplasm of the canal wall, and the same type of septate junctions are observed within the cytoplasmic cord at the level of the tip of the flame tuft in both longitudinal and oblique sections of all three trematode species studied. In the monopisthocotylean Ancyrocephalus paradoxus, the absence of any junctions in the cytoplasmic cord and the presence of septate junction within all of the protonephridial ducts are reported. On the basis of the small number of monogenean species in which these features have been studied, in relation to the size of the group, there seems to be a high diversity in some characters of the protonephridial terminal organ. The study confirms that the Aspidogastrea and Digenea possess the same morphology of their protonephridial terminal organ and, although this differs slightly from that of most members of the Monogenea so far studied, it supports previous views on the close relationship of these groups.