The phylogenetics of Anguillicolidae (Nematoda: Anguillicoloidea), swimbladder parasites of eels

Abstract

Background: Anguillicolidae Yamaguti, 1935 is a family of parasitic nematode infecting fresh-water eels of the genus Anguilla, comprising five species in the genera Anguillicola and Anguillicoloides. Anguillicoloides crassus is of particular importance, as it has recently spread from its endemic range in the Eastern Pacific to Europe and North America, where it poses a significant threat to new, naïve hosts such as the economic important eel species Anguilla anguilla and Anguilla rostrata. The Anguillicolidae are therefore all potentially invasive taxa, but the relationships of the described species remain unclear. Anguillicolidae is part of Spirurina, a diverse clade made up of only animal parasites, but placement of the family within Spirurina is based on limited data.

Results: We generated an extensive DNA sequence dataset from three loci (the 5' one-third of the nuclear small subunit ribosomal RNA, the D2-D3 region of the nuclear large subunit ribosomal RNA and the 5' half of the mitochondrial cytochrome c oxidase I gene) for the five species of Anguillicolidae and used this to investigate specific and generic boundaries within the family, and the relationship of Anguillicolidae to other spirurine nematodes. Neither nuclear nor mitochondrial sequences supported monophyly of Anguillicoloides. Genetic diversity within the African species Anguillicoloides papernai was suggestive of cryptic taxa, as was the finding of distinct lineages of Anguillicoloides novaezelandiae in New Zealand and Tasmania. Phylogenetic analysis of the Spirurina grouped the Anguillicolidae together with members of the Gnathostomatidae and Seuratidae.

Conclusions: The Anguillicolidae is part of a complex radiation of parasitic nematodes of vertebrates with wide host diversity (chondrichthyes, teleosts, squamates and mammals), most closely related to other marine vertebrate parasites that also have complex life cycles. Molecular analyses do not support the recent division of Anguillicolidae into two genera. The described species may hide cryptic taxa, identified here by DNA taxonomy, and this DNA barcoding approach may assist in tracking species invasions. The propensity for host switching, and thus the potential for invasive behaviour, is found in A. crassus, A. novaezelandiae and A. papernai, and thus may be common to the group.

Figure 1

MOTU analysis of three marker loci from Anguillicolidaea. Variation in the number of MOTUs inferred at cut-offs ranging from 0 - 14% sequence divergence for specimens for which all three genes were sequenced (nSSU*, nLSU*, COX1*). Results of the MOTU analysis of the expanded COX1 dataset are included for comparison. Critical cutoff intervals for the different datasets are indicated in letters (A – E). b. Comparison of morphological species identified sensu Moravec and Taraschewski [2] with MOTU composition at the critical cutoff intervals (A – E).

Figure 2

Phylogenetic analysis of nSSU sequences of Spirurina. Consensus phylogram of the analysis of the nSSU sequences from Spirurina using Bayesian Inference, rooted with Teratocephalus lirellus, a non-spirurine rhabditid. Branches are collapsed where possible based on taxonomic affiliations and major groups are highlighted. Bayesian posterior probabilities for internal branches are indicated. The scale bar indicates the average expected number of substitutions per site.

Figure 3

Phylogenetic analysis of nSSU sequences of Spirurina B. Consensus phylogram of the analysis of the nSSU sequences from Spirurina B using Bayesian Inference, rooted with Cucullanus robustus (Spirurina A). Bayesian posterior probabilities for internal branches are indicated. The scale bar indicates the average expected number of substitutions per site.

Figure 4

Phylogenetic analysis of nLSU sequences of Anguillicolidae. Consensus phylogram of the analysis of the nLSU sequences from Anguillicolidae and outgroups using Bayesian Inference. Bayesian posterior probabilities for internal branches are indicated. The scale bar indicates the inferred number of base substitutions per site.

Figure 5

Phylogenetic analysis of COX1 sequences of Anguillicolidae. Consensus phylogram of the analysis of the COX1 sequences from Anguillicolidae and outgroups using Bayesian Inference. Bayesian posterior probabilities for internal branches are indicated. The scale bar indicates the average expected number of substitutions per site.

Figure 6

Network analysis of COX1 sequences from Anguillicola crassus. Statistical parsimony network of 70 distinct A. crassus COX1 sequences. The three COX1 haplotypes containing four specimens that have the minority A. crassus 28 S rDNA D2-D3 haplotype are highlighted. A list of all specimens associated with each COX1 haplotype can be found in the Additional file 1.

Figure 7

Host switching in the Anguillicolidae. Cladograms of the eel genus Anguilla (left tree, blue) [60] and its swim bladder parasites, the family Anguillicolidae (Nematoda: Anguillicoloidea) (right tree, green). The known host parasite relationships are indicated by: black lines = traditional host-parasite relationship, displaying low abundances and low pathogenicity (parasite endemic in host); red lines = novel host-parasite relationship, displaying high abundances and pathogenicity; orange lines = novel host-parasite relationship, displaying low abundances and pathogenicity; dashed orange lines = novel host-parasite relationship where completion of the life cycle has not been demonstrated.