Abstract
BackgroundAdults and larvae of generalist parasites are exposed to diverse hosts and local environmental conditions throughout their life cycles, thus local adaptation is expected to occur through phenotypic plasticity and/or natural selection. We investigated how the combined effect of cryptic host specificity and local selective pressures could shape reproductive traits of a putative generalist parasite in the oceanic realm.MethodsThe LSU rDNA, ITS2 and the mt-COI of individuals of the digenean Pholeter gastrophilus (Kossack, 1910) Odhner, 1914 (Heterophyidae Leiper, 1909) from oceanic striped dolphins, Stenella coeruleoalba Meyen, and coastal bottlenose dolphins, Tursiops truncatus Montagu, in the western Mediterranean were used to elucidate whether worms were conspecific. Infection parameters were compared between both dolphin species. General Linear Mixed Models were used to analyse the influence of host species on four reproductive traits of P. gastrophilus: body size, maturity stage (non-gravid/gravid), egg size, and number of eggs in utero. AIC values were used to rank competing models, and p-values to assess the effect of specific predictors.ResultsEvidence indicated that worms collected from both dolphin species were conspecific. All worms collected were gravid and infection parameters did not differ between dolphin species. However, body size and egg size of individuals of P. gastrophilus were significantly larger in striped dolphins. The number of eggs in utero did not significantly differ between dolphin species but, for a given body size, worms in bottlenose dolphins harboured more eggs. A trade-off between egg size and egg number was found in worms from both dolphin species, with a higher slope in striped dolphins.ConclusionsApparently, striped dolphin is a more suitable host for P. gastrophilus, but reproductive investment seems to be adapted to the habitat where the life-cycle develops. Worms from striped dolphins likely face the problem of finding intermediate hosts in the oceanic realm and apparently invest into offspring size to enhance the early survival of larvae and the potential to multiply asexually within the first intermediate host. The small-sized worms from bottlenose dolphins would be adapted to reproduce early because of higher adult mortality, generating smaller and numerous eggs in a coastal habitat where chances of transmission are presumably higher.
Highlights
Adults and larvae of generalist parasites are exposed to diverse hosts and local environmental conditions throughout their life cycles, local adaptation is expected to occur through phenotypic plasticity and/or natural selection
Few studies have analysed how the combined effect of host species constraints, and local selective pressures on the life-cycle, could shape life history traits of a generalist parasite, e.g. [5]. We investigate this issue in a digenean infecting two cetacean species that occur in different habitats, i.e. coastal and oceanic
Comparison of pairwise divergence for each gene showed that the aligned portions of the sequences of all specimens of P. gastrophilus were identical
Summary
Adults and larvae of generalist parasites are exposed to diverse hosts and local environmental conditions throughout their life cycles, local adaptation is expected to occur through phenotypic plasticity and/or natural selection. Other studies have discovered more subtle effects on parasite’s life history traits (e.g. fecundity) of host species that, at first glance, seemed to be suitable for the parasite, e.g., [9,10,11]. It should be stressed, that some putative generalist parasites have later been re-interpreted, based on molecular evidence, as cryptic species complexes, each species being adapted to a different host [12, 13]
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