Leduc, D.; Zhao, Z. Q.; Verdon, V.; Xu, Y. (2018). Phylogenetic position of the enigmatic deep-sea nematode order Rhaptothyreida: A molecular analysis. Molecular Phylogenetics and Evolution. 122: 29-36.
The placement of the rare deep-sea nematode order Rhaptothyreida remains unclear due to the unique morphology
of this group, an unknown life cycle with morphologically distinct juvenile stages which may or may not
be parasitic, and lack of molecular sequences. Here, we investigate the phylogenetic placement and status of the
Rhaptothyreida based on SSU and D2-D3 of LSU rDNA sequences of Rhaptothyerus typicus specimens obtained
from the continental slope of New Zealand. Molecular sequences of three adults and a late stage juvenile were
identical, confirming that they belong to the same species despite pronounced morphological differences. We
observed the presence of the rare nucleotide transition A?G and transversion G?Y in the loops of Hairpin 35
and 48 regions, which is consistent with the placement of R. typicus within the order Enoplida. Rhaptothyreus
typicus was consistently recovered as a long branch clade in SSU and D2-D3 of LSU analyses, which can have a
destabilising effect on tree topology. After Gblocks were used to remove sites of questionable alignment, R.
typicus was placed in a clade comprising Trissonchulus, Dolicholaimus and Ironus sequences (family Ironidae, order
Enoplida) in both Bayesian and Maximum Likelihood SSU topologies. Depending on which alignment algorithm
was used, analyses of LSU sequences focusing on enoplid taxa either suggested a relationship between R. typicus
and Halalaimus (family Oxystominidae) or did not identify any clear relationships. Overall, our results provide
strong evidence for placing R. typicus and the family Rhaptothyreidae within the order Enoplida, although
further work is required to clarify relationships between rhaptothyreids and other enoplid taxa. A parasitic
lifestyle could explain the unique morphology of this group, their highly divergent SSU and LSU rDNA molecular
sequences, and the marked morphological differences between late juveniles and adults. Further molecular investigations
targeting both free-living and parasitic early juvenile life stages in potential deep-sea hosts are
needed to better understand the evolution of this unusual nematode taxon.