Abstract
Parasitic interactions are often part of complex networks of interspecific relationships that have evolved in biological communities. Despite many years of work on the evolution of parasitism, the likelihood that sister taxa of parasites can co-evolve with their hosts to specifically infect two related lineages, even when those hosts occur sympatrically, is still unclear. Furthermore, when these specific interactions occur, the molecular and physiological basis of this specificity is still largely unknown. The presence of these specific parasitic relationships can now be tested using molecular markers such as DNA sequence variation. Here we test for specific parasitic relationships in an emerging host-parasite model, the stickleback-Schistocephalus system. Threespine and ninespine stickleback fish are intermediate hosts for Schistocephalus cestode parasites that are phenotypically very similar and have nearly identical life cycles through plankton, stickleback, and avian hosts. We analyzed over 2000 base pairs of COX1 and NADH1 mitochondrial DNA sequences in 48 Schistocephalus individuals collected from threespine and ninespine stickleback hosts from disparate geographic regions distributed across the Northern Hemisphere. Our data strongly support the presence of two distinct clades of Schistocephalus, each of which exclusively infects either threespine or ninespine stickleback. These clades most likely represent different species that diverged soon after the speciation of their stickleback hosts. In addition, genetic structuring exists among Schistocephalus taken from threespine stickleback hosts from Alaska, Oregon and Wales, although it is much less than the divergence between hosts. Our findings emphasize that biological communities may be even more complex than they first appear, and beg the question of what are the ecological, physiological, and genetic factors that maintain the specificity of the Schistocephalus parasites and their stickleback hosts.
Highlights
The processes of competition and predation have historically received the lion’s share of focus in evolutionary ecology research, the importance of parasitism as an evolutionary force – and its potential in structuring community dynamics – is being increasingly recognized [1,2,3,4,5]
We have shown that the relationships between Schistocephalus parasites and stickleback intermediate hosts can be quite specific, even when those species occupy similar niches in the same geographic locale and even the same lake
Schistocephalus from ninespine stickleback are very differentiated from those extracted from threespine stickleback, even in lakes where these host species co-occur
Summary
The processes of competition and predation have historically received the lion’s share of focus in evolutionary ecology research, the importance of parasitism as an evolutionary force – and its potential in structuring community dynamics – is being increasingly recognized [1,2,3,4,5]. One major way in which parasites can influence food webs, for example, is by influencing the probability of survival of host organisms through alterations of their antipredator behavior [7]. Such behavioral changes are potentially adaptive where parasite transmission relies on one host being consumed by the [8,9,10,11,12]. Through their myriad effects on host organisms, parasites can alter the competitive ability, growth, sexual maturation, sexual attractiveness and parental ability of host organisms, and have considerable fitness implications for hosts. Understanding parasitic interactions in the wild is very important for accurately describing community complexity [6]
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