Comparative morphological and molecular analyses of Acrobeloides bodenheimeri and A. tricornis Cobb, 1924 (Rhabditida, Cephalobidae) from Tunisia

Four populations of Acrobeloides nematodes were isolated from Tunisian soils. Based on morphological and morphometric data, two populations, TC9 and HWO, were identified as A. tricornis and the other two, TC7 and K18g, as A. bodenheimeri. Acrobeloides tricornis is distinguished from its closely related species by its high labial probolae with arcuate termini, inconspicuous post-uterine sac, and five lateral incisures, while A. bodenheimeri is distinguished from other acrobeloids by having a low and rounded labial probolae, distinct post-uterine sac, five lateral incisures, and vulva frequently with vaginal plug. The A. tricornis and A. bodenheimeri reported in this study are distinguished by the shape of labial probolae (high with acute distal part vs low and rounded), post-uterine sac (inconspicuous vs distinct), position of the reproductive system with respect to the intestine (sinistral vs dextral), vulva with vaginal plug (present vs absent) and some morphometric characters (such as body length). Further, these species were also molecularly characterised using the sequences of the D2-D3 region of the 28S rRNA gene, the 18S rRNA gene, the ITS rRNA gene, and of the mitochondrial cytochrome c oxidase subunit I (mtCO1) gene. Phylogenetic trees using ribosomal RNA gene sequences clearly separate species of the 'Maximus' group, but these sequences appear to be identical in several other Acrobeloides species, and thus are unlikely to be useful as molecular markers for species-level molecular identification in the genus Acrobeloides. The mtCO1 gene sequences of two A. tricornis populations differ in 58 and 64 nucleotide bases from A. varius and A. nanus, respectively, indicating that the mtCO1 gene sequences have greater phylogenetic resolving power than nuclear rRNA gene sequences, and that this gene marker can differentiate closely related species in the genus Acrobeloides.