Abstract
Trophically-transmitted parasites are regularly exposed to potential new hosts through food web interactions. Successful colonization, or switching, to novel hosts, occur readily when ‘donor’ and ‘target’ hosts are phylogenetically related, whereas switching between distantly related hosts is rare and may result from stochastic factors (i.e. rare favourable mutations). This study investigates a host-switching event between a marine acanthocephalan specific to pinnipeds that is apparently able to reproduce in Magellanic penguins Spheniscus magellanicus from Brazil. Detailed analysis of morphological and morphometrical data from acanthocephalans from penguins indicates that they belong to Corynosoma australe Johnston, 1937. Partial fragments of the 28S rRNA and mitochondrial cox1 genes were amplified from isolates from penguins and two pinniped species (i.e. South American sea lion Otaria flavescens and South American fur seal Arctocephalus australis) to confirm this identification. Infection parameters clearly differ between penguins and the two pinniped species, which were significantly lower in S. magellanicus. The sex ratio of C. australe also differed between penguins and pinnipeds; in S. magellanicus was strongly biased against males, while in pinnipeds it was close to 1:1. Females of C. australe from O. flavescens were smaller than those from S. magellanicus and A. australis. However, fecundity (i.e. the proportion of fully developed eggs) was lower and more variable in females collected from S. magellanicus. At first glance, the occurrence of reproductive individuals of C. australe in Magellanic penguins could be interpreted as an adaptive colonization of a novel avian host through favourable mutations. However, it could also be considered, perhaps more likely, as an example of ecological fitting through the use of a plesimorphic (host) resource, since the ancestors of Corynosoma infected aquatic birds.
Highlights
Most parasites are considered resource specialists, exploiting only a small number of the potential available hosts in the habitat [1]
In addition to the morphological characterization of the specimens studied, molecular data confirm the conspecificity of the specimens of C. australe collected from the Magellanic penguin S. magellanicus, the South American fur seal A. australis and the South American sea lion O. flavescens
Our results call for further studies on the real host range and geographical distribution of C. australe
Summary
Most parasites are considered resource specialists, exploiting only a small number of the potential available hosts in the habitat [1]. In purely descriptive terms, such host-switching events can accounted for by two factors, namely, the ability of the parasite to readily contact the new host (either passively or actively), and to survive and reproduce in (on) it [7,8,9]. Successful host shifts are usually explained as a positive adaptive balance between the risks of dispersal and transmission, and the realized fitness in novel hosts [10,11]. Parasites would survive and persist in novel hosts by means of characters that they already possess due to past interactions, i.e., no new mutations are required [1,15]. Ecological fitting would convincingly explain how host-parasite associations become diversified without the concurrence of cospeciation or rare ‘lucky’ mutations [1]
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