We are very grateful to Till Hill Ltd for allowing access to the study site. . Differences in body condition and macroparasite burden between cluster 2 and non-cluster animals captured at the same time. A. Cluster 2 animals (dark bar) had significantly better body condition (BW resid ) than non-cluster animals, adjusting for size and stage. B. Cluster 2 animals (dark bar) had greater grouped macroparasite burdens (PC1 P ) than non-cluster animals, adjusting for size and stage. PC1 P represents first principal component scores from a principal components analysis of common macroparasite species at Cotgrave Forest. This principal component accounted for 23% of total variation and showed high loadings of the same sign for most species. It was thus taken to represent the major pattern of positive covariation within the assemblage. Variable coefficients on PC1 P were: Skrjabinotaenia lobata 0.25, Brachylaemus recurvum 0.28, Calodium hepaticum 0.24, Syphacia nigeriana 0.52, Heligmosomoides polygyrus 0.50, Ixodes ricinus 20.040, laelapid mites 0.102, Polyplax serrata 0.52. There were individually significant associations with cluster 2 membership, in the same direction as PC1 P , for P. serrata and S. lobata, adjusting for host size and stage. doi:10.1371/journal.pone.0020070.g006 Spliced Leader RNAs, Mitochondrial Gene Frameshifts and Multi-Protein Phylogeny Expand Support for the Genus Perkinsus as a Unique Group of Alveolates The genus Perkinsus occupies a precarious phylogenetic position. To gain a better understanding of the relationship between perkinsids, dinoflagellates and other alveolates, we analyzed the nuclear-encoded spliced-leader (SL) RNA and mitochondrial genes, intron prevalence, and multi-protein phylogenies. In contrast to the canonical 22-nt SL found in dinoflagellates (DinoSL), P. marinus has a shorter (21-nt) and a longer (22-nt) SL with slightly different sequences than DinoSL. The major SL RNA transcripts range in size between 80–83 nt in P. marinus, and ∼83 nt in P. chesapeaki, significantly larger than the typical ≤56-nt dinoflagellate SL RNA. In most of the phylogenetic trees based on 41 predicted protein sequences, P. marinus branched at the base of the dinoflagellate clade that included the ancient taxa Oxyrrhis and Amoebophrya, sister to the clade of apicomplexans, and in some cases clustered with apicomplexans as a sister to the dinoflagellate clade. Of 104 Perkinsus spp. genes examined 69.2% had introns, a higher intron prevalence than in dinoflagellates. Examination of Perkinsus spp. mitochondrial cytochrome B and cytochrome C oxidase subunit I genes and their cDNAs revealed no mRNA editing, but these transcripts can only be translated when frameshifts are introduced at every AGG and CCC codon as if AGGY codes for glycine and CCCCU for proline. These results, along with the presence of the numerous uncharacterized ‘marine alveolate group I' and Perkinsus-like lineages separating perkinsids from core dinoflagellates, expand support for the affiliation of the genus Perkinsus with an independent lineage (Perkinsozoa) positioned between the phyla of Apicomplexa and Dinoflagellata. Perkinsus marinus is a pathogenic alveolate causing ''dermo'' disease in oysters in estuaries of the north and central American Atlantic and Gulf of Mexico coasts. Other species of Perkinsus cause similar afflictions in a wide variety of other marine molluscs worldwide, all of which yield serious losses for shellfish industries [1] . This genus belongs to the crown group of eukaryotes known as Alveolata, but its exact phylogenetic position is debated. Based on the presence of cell surface micropores and an apical complex, P. marinus is historically considered to be a member of the Apicomplexa (for review see [2] ), an exclusively parasitic lineage responsible for malaria and other infectious diseases in humans and animals. However, P. marinus shares cytological features with dinoflagellates, such as flagellar spurs and closed mitosis [2] . Phylogenetic studies based on small subunit ribosomal RNA (18S rDNA) and some conserved proteins such as actin and tubulin also conclude that P. marinus is closer to dinoflagellates than to apicomplexans (e.g. review by [2] - [4] ), and thus are an early branch of dinoflagellate [4] , [5] . These results challenge a proposition that both Perkinsus spp. and related Parvilucifera spp. parasites should constitute an independent phylum named Perkinsozoa [6] , [7] .