Redescription of Placodira lobata Thorne, 1937 (Rhabditida: Cephalobidae) with a discussion of the systematic position of the genus

Abstract

A population of Placodira lobata from the USA is described using both light and scanning electron microscopy and compared with the descriptions of the type specimens of the same species. The phylogenetic relationships of the species are inferred from molecular data and places P. lobata in a clade that unites genera with mostly simple morphology of the labial region, like Cephalobus, Acrobeloides, Heterocephalobellus and Metacrobeles, but also includes species of Zeldia and Chiloplacus.A sample containing specimens of P. lobata was collected by the first author during a desert sampling trip in 2006. Since the number of recovered specimens was too low (only a few adults were found), the same locality was revisited in 2009 and more specimens were recovered from soil. Nematodes were extracted by a modified tray extraction method and fixed in 4% formaldehyde for morphological studies and DESS (Yoder et al., 2006) for molecular analysis. For LM, specimens were transferred to pure glycerin by a slow evaporation method and mounted on permanent slides in glycerin with paraffin as support for the cover slip. For SEM studies, some specimens were postfixed in 1% osmium tetroxide (OsO 4 ) and transferred to pure acetone through an acetone/distilled water series. Specimens were critical point dried in liquid CO 2 , mounted on stubs, gold-plated under vacuum to a thickness of 200 Å using a sputter coater, and examined in a Hitachi S-4300 SEM at an accelerating voltage of 10 kV.

Twenty Years after De Ley and Blaxter—How Far Did We Progress in Understanding the Phylogeny of the Phylum Nematoda?

“…The relationships between the three families of the infraorder Cephalobomorpha sampled so far in molecular phylogenetic analyses (Cephalobidae, Osstellidae and Bicirronematidae) are not resolved (T6 in Figure 5 a). Both Osstellidae and Bicirronematidae are represented by a single taxon in published analyses, while the largest family of this infraorder, Cephalobidae, is paraphyletic [ 13 , 60 ]. It also includes Daubayliidae (not included in [ 2 , 3 ]) [ 48 ] and Drilonematomorpha, represented by Drilonematidae and Ungellidae in published phylogenies [ 60 , 61 ].…”

“…Both Osstellidae and Bicirronematidae are represented by a single taxon in published analyses, while the largest family of this infraorder, Cephalobidae, is paraphyletic [ 13 , 60 ]. It also includes Daubayliidae (not included in [ 2 , 3 ]) [ 48 ] and Drilonematomorpha, represented by Drilonematidae and Ungellidae in published phylogenies [ 60 , 61 ]. The remaining families Alirhabditidae and Elaphonematidae from Cephalobomorpha and Homungellidae and Pharyngonematidae from Drilonematomorpha have not been included in molecular phylogenies.…”