Phylogeny and Taxonomic Revision of Tylenchidae with Emphasis on the Genus Cephalenchus (Tylenchina: Tylenchomorpha)
Historically, nematode systematics has been heavily driven by morphology, and for many groups, morphology-based classifications are the basis for establishing species relationships. Plant parasitic nematodes of the family Tylenchidae are a great example; they are highly diverse but their phylogenetic relationships remain poorly studied. Molecular phylogenetic analyses of Tylenchidae, including the redescribed Filenchus annulatus, differ in results depending on the gene: 28S ribosomal RNA (rRNA) gene strongly supports Filenchus as monophyletic, whereas 18S rRNA shows Filenchus as polyphyletic. Relationships between Filenchus and other Tylenchidae genera are also gene dependent. Molecular phylogenies also suggest that Cephalenchus and Eutylenchus belong to a separate clade other than Tylenchidae. rRNA phylogenies often assume concerted evolution, so that intraspecific polymorphism for these genes is expected to be eliminated. This phenomenon is further explored in the genus Cephalenchus, another intriguing genus within Tylenchomorpha. Sequence variation of 28S and ITS rRNA genes suggest that not all Cephalenchus species undergo concerted evolution. High levels of intraspecific polymorphism are found in Cephalenchus sp1 (BRA-01). Secondary structure analyses suggest the functionality of these rRNA copies (i.e. not a pseudogene); and potential cross-fertilization in some Cephalenchus species, might contribute to both intragenomic and intraspecific polymorphism. These results reinforce the implications of intragenomic and intraspecific genetic diversity on species delimitation, especially in studies based solely on molecular approaches. The phylogenetic position of Cephalenchus within Tylenchomorpha is further investigated based on 26 populations (11 species) sampled worldwide. Molecular analyses based on three rRNA genes and different alignment methods always supported Cephalenchus as a monophyletic group. A sister relationship between Cephalenchus and Eutylenchus is also recovered; branch support for this relationship varies depending on the method used. Placement of Cephalenchus + Eutylenchus within Tylenchidae is not supported by 18S and 28S genes; nevertheless, the position of both taxa within Tylenchomorpha remains ambiguous and highlights the importance of sampling additional genes and taxa. Within Cephalenchus, amphid opening morphology shows congruence with molecular-based phylogenetic relationships. All three rRNA genes support the non-monophyly of four morphologically defined Cephalenchus species. In light of this evidence, recommendations for the synonymization of Cephalenchus species are given.