Spatial and genetic structure of directly-transmitted parasites reflects the distribution of their specific amphibian hosts

A biological invasion modifies the dynamics of a host-parasite arms race

By imposing novel selection pressures on both participants, biological invasions can modify evolutionary 'arms races' between hosts and parasites. A spatially replicated cross-infection experiment reveals strong spatial divergence in the ability of lungworms (Rhabdias pseudosphaerocephala) to infect invasive cane toads (Rhinella marina) in Australia. In areas colonized for longer than 20 years, toads are more resistant to infection by local strains of parasites than by allopatric strains. The situation reverses at the invasion front, where super-infective parasites have evolved. Invasion-induced shifts in genetic diversity and selective pressures may explain why hosts gain advantage over parasites in long-colonized areas, whereas parasites gain advantage at the invasion front.

Host defense or parasite cue: Skin secretions mediate interactions between amphibians and their parasites

Amphibian skin secretions (substances produced by the amphibian plus microbiota) plausibly act as a first line of defense against parasite/pathogen attack, but may also provide chemical cues for pathogens. To clarify the role of skin secretions in host-parasite interactions, we conducted experiments using cane toads (Rhinella marina) and their lungworms (Rhabdias pseudosphaerocephala) from the range-core and invasion-front of the introduced anurans' range in Australia. Depending on the geographical area, toad skin secretions can reduce the longevity and infection success of parasite larvae, or attract lungworm larvae and enhance their infection success. These striking differences between the two regions were due both to differential responses of the larvae, and differential effects of the skin secretions. Our data suggest that skin secretions play an important role in host-parasite interactions in anurans, and that the arms race between a host and parasite can rapidly generate spatial variation in critical features of that interaction.

Does host socio-spatial behavior lead to a fine-scale spatial genetic structure in its associated parasites?

Gastro-intestinal nematodes, especially Haemonchus contortus, are widespread pathogenic parasites of small ruminants. Studying their spatial genetic structure is as important as studying host genetic structure to fully understand host-parasite interactions and transmission patterns. For parasites having a simple life cycle (e.g., monoxenous parasites), gene flow and spatial genetic structure are expected to strongly rely on the socio-spatial behavior of their hosts. Based on five microsatellite loci, we tested this hypothesis for H. contortus sampled in a wild Mediterranean mouflon population (Ovis gmelini musimon × Ovis sp.) in which species- and environment-related characteristics have been found to generate socio-spatial units. We nevertheless found that their parasites had no spatial genetic structure, suggesting that mouflon behavior was not enough to limit parasite dispersal in this study area and/or that other ecological and biological factors were involved in this process, for example other hosts, the parasite life cycle, or the study area history.

The population genetics of parasitic nematodes of wild animals

Parasitic nematodes are highly diverse and common, infecting virtually all animal species, and the importance of their roles in natural ecosystems is increasingly becoming apparent. How genes flow within and among populations of these parasites - their population genetics - has profound implications for the epidemiology of host infection and disease, and for the response of parasite populations to selection pressures. The population genetics of nematode parasites of wild animals may have consequences for host conservation, or influence the risk of zoonotic disease. Host movement has long been recognised as an important determinant of parasitic nematode population genetic structure, and recent research has also highlighted the importance of nematode life histories, environmental conditions, and other aspects of host ecology. Commonly, factors influencing parasitic nematode population genetics have been studied in isolation, such that an integrated view of the drivers of population genetic structure of parasitic nematodes is still lacking. Here, we seek to provide a comprehensive, broad, and integrative picture of these factors in parasitic nematodes of wild animals that will be a useful resource for investigators studying non-model parasitic nematodes in natural ecosystems. Increasingly, new methods of analysing the population genetics of nematodes are becoming available, and we consider the opportunities that these afford in resolving hitherto inaccessible questions of the population genetics of these important animals.