Distinct lineages of Schistocephalus parasites in threespine and ninespine stickleback hosts revealed by DNA sequence analysis

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.

Figure 1. A reconstruction of the phylogenetic relationship of the 35 concatenated COX1 and NADH1 haplotype sequences from Schistocephalus collected from two different stickleback hosts and across three geographic regions.

The tree is rooted using Spirometra erinaceieuropaei sequence as an outgroup, which is the closest cestode relative for which there was sufficient mtDNA sequence. The tree was reconstructed using Parsimony, Maximum Likelihood, and Bayesian approaches, and all gave qualitatively similar results (Maximum Likelihood topology is shown). In particular, the deepest division is between sequences from P. pungitius hosts (top) and G. aculeatus host (bottom), with these sequences being different, on average, at nearly 20% of the sites in the sequence (substitution rate above branch). This division is highly supported regardless of being measured by Bayesian Posterior Probability, Bootstrap, or Likelihood Ratio (support below branch). The next supported division is between Schistocephalus from Wales as compared to Oregon and Alaska, but with a sequence divergence less than 5%. The final strongly supported division are Schistocephalus from Oregon as compared to Alaska, but with a sequence divergence of less than 1%. Black dots represented Schistocephalus samples collected from either threespine or ninespine hosts from within Mud Lake, Alaska.

Figure 2. An alignment of the translated protein sequences Schistocephalus from Alaskan, Oregon and Wales threespine hosts, as well as Schistocephalus from ninespine hosts.

Only positions that are variable in at least one comparison are represented in this alignment. Using the Alaskan threespine host sequence as a reference, only a single AA difference (red highlighting) occurs at position 152 of NADH1 from a Valine to Isoleucine in the Oregon threespine hosts. Nine differences occur between Schistocephalus collected from threespine hosts in the Pacific vs. Atlantic basins. In comparisons between Schistocephalus collected from threespine and ninespine hosts in Alaska twenty one residue changes exist (position numbers in red), many of which are likely to change the structure of the proteins.