Escape from the Red Queen: An overlooked scenario in coevolutionary studies

Almost all eukaryotic organisms undergo sexual recombination at some stage of their life history. However, strictly asexual organisms should have higher per capita rate of reproduction compared with those that have sex, so the latter must convey some advantage which overrides the reproductive benefit of asexuality. For example, sexual reproduction and recombination may play an important role in allowing organisms to evolutionarily 'keep up' with parasites. Host-parasite coevolution can operate via negative frequency-dependent selection whereby parasite genotypes adapt to infect host genotypes as they become locally common. By producing more genetically diverse offspring with unique genotypes, sexual organisms have an advantage over asexual counterparts. Essentially, sexual hosts are more difficult for coevolving parasites to 'track' over time. This scenario has been named the "Red Queen hypothesis". It refers to a passage in Lewis Carroll's 'Through the Looking Glass' in which the Red Queen tells Alice: 'it takes all the running you can do, to keep in the same place'; this statement resembles the negative frequency-dependent dynamics of host-parasite coevolution. © 2011 The Authors. Oikos © 2011 Nordic Society Oikos

Tiny hitchhikers and parasites: a review of Australian heterostigmatic mites (Acari: Prostigmata) associated with insects, with description of three new species

Heterostigmatic mites (Heterostigmata) are highly diverse, have symbiotic associations with various insects and occur worldwide. Despite Australia's rich insect biodiversity, little is known about its heterostigmatic mites. In this study, we conducted a preliminary study on heterostigmatic mites associated with beetles in eastern Australia and found four species, of which three were new: Caraboacarus stammeri Krczal, 1959 (Caraboacaridae) and Eutarsopolipus orpheus Katlav & Seeman, sp. nov., (Podapolipidae) associated with Australian-endemic Gnathaphanus melbournensis (Castelnau, 1867) (Carabidae); Hoplocheylus australiensis Katlav & Seeman, sp. nov., (Tarsocheylidae) phoretic on Australian-endemic Mastachilus quaestionis (Kuwert, 1891) (Passalidae); and Pavania gazellatris Katlav & Seeman, sp. nov., (Dolichocybidae) phoretic on introduced Digitonthophagus gazella (Fabricius, 1787) (Scarabaeidae). The species of Caraboacaridae, Dolichocybidae and Tarsocheylidae are the first species records of these families in Australia. It is likely that two of the three new species are Australian endemics, while P. gazellatris may be a more recent arrival on introduced dung beetles, possibly arriving with imports of exotic adult dung beetles. Furthermore, we provide an overview of the heterostigmatic mite fauna of Australia that consists of less than 90 species belonging to 13 families, with representatives of all known seven superfamilies. These mites have so far been mostly reported on beetles but also on other insect orders (Blattodea, Diptera, Hemiptera, Hymenoptera, Orthoptera, Thysanoptera). Our findings of three new species and one new record based on the analysis of a relatively small number of specimens and beetle species suggest that more comprehensive studies of host insect taxa will reveal a substantially larger Australian diversity of heterostigmatic mites

Impacts of ocean acidification on multiplication and caste organisation of parasitic trematodes in their gastropod host

Ocean acidification is predicted to impact the structure and function of all marine ecosystems in this century. As focus turns towards possible impacts on interactions among marine organisms, its effects on the biology and transmission potential of marine parasites must be evaluated. In the present study, we investigate two marine trematode species (Philophthalmus sp. and Parorchis sp., both in the family Philophthalmidae) infecting two marine gastropods. These trematodes are unusual in that their asexually multiplying stages within snails display a division of labour, with two distinct castes, a large-bodied morph producing infective stages and a smaller morph playing a defensive role against other competing parasites. Using a potentiometric ocean acidification simulation system, we test the impacts of acidified seawater (7.8 and 7.6 pH) on the production of free-living infective stages (cercariae), the size and survival of encysted resting stages (metacercariae), and the within-host division of labour measured as the ratio between numbers of the two morphs. In general, low pH conditions caused an increase in cercarial production and a reduction in metacercarial survival. The ratio of the two castes within snail hosts tended to shift towards more of the smaller defensive morphs under low pH. However, the observed effects of reduced pH were species specific and not always unimodal. These results suggest that ocean acidification can affect the biology of marine parasites and may also impact transmission success and parasite abundance of some trematodes, with possible consequences for marine communities and ecosystems

Does timing matter? How priority effects influence the outcome of parasite interactions within hosts

In nature, hosts are exposed to an assemblage of parasite species that collectively form a complex community within the host. To date, however, our understanding of how within-host–parasite communities assemble and interact remains limited. Using a larval amphibian host (Pacific chorus frog, Pseudacris regilla) and two common trematode parasites (Ribeiroia ondatrae and Echinostoma trivolvis), we experimentally examined how the sequence of host exposure influenced parasite interactions within hosts. While there was no evidence that the parasites interacted when hosts were exposed to both parasites simultaneously, we detected evidence of both intraspecific and interspecific competition when exposures were temporally staggered. However, the strength and outcome of these priority effects depended on the sequence of addition, even after accounting for the fact that parasites added early in host development were more likely to encyst compared to parasites added later. Ribeiroia infection success was reduced by 14 % when Echinostoma was added prior to Ribeiroia, whereas no such effect was noted for Echinostoma when Ribeiroia was added first. Using a novel fluorescent-labeling technique that allowed us to track Ribeiroia infections from different exposure events, we also discovered that, similar to the interspecific interactions, early encysting parasites reduced the encystment success of later arriving parasites by 41 %, which could be mediated by host immune responses and/or competition for space. These results suggest that parasite identity interacts with host immune responses to mediate parasite interactions within the host, such that priority effects may play an important role in structuring parasite communities within hosts. This knowledge can be used to assess host–parasite interactions within natural communities in which environmental conditions can lead to heterogeneity in the timing and composition of host exposure to parasites