Two New Pristionchus Species (Nematoda: Diplogastridae) from Taiwan are Part of a Species-cluster Representing the Closest Known Relatives of the Model Organism P. pacificus.

Matthias Herrmann, Christian Weiler, Christian Rödelsperger, Natsumi Kanzaki, and Ralf J. Sommer (2016) Pristionchus pacificus is an important model organism in evolutionary biology and has been used to establish integrative studies that link developmental biology with ecology and population genetics. This species is part of the P. pacificus species-complex of the genus, many members of which occur in East Asia. While P. pacificus is hermaphroditic, the two most closely related known species are P. exspectatus and P. arcanus from Japan, both of which are gonochorists. P. exspectatus is so far the closest known relative of P. pacificus and thus, considered to represent the sister species. Here, we describe two new species of Pristionchus, P. taiwanensis and P. occultus from Taiwan using morphology, morphometrics, mating experiments and genome- wide sequence analysis. Both species are gonochorists and they are morphologically indistinguishable from P. exspectatus, P. arcanus and P. pacificus. However, reproductive isolation, namely the inability to produce interfertile hybrids, separates all species pairs in the species-complex. Phylogeny inferred from more than 700,000 genome-wide variable sites that were genotyped in all species suggest that P. taiwanensis and P. occultus are the sister species of P. arcanus and P. exspectatus, respectively. P. taiwanensis and P. occultus together with P. exspectatus and P. arcanus form a species-cluster with P. pacificus. The identification of these two novel gonochoristic species is invaluable for studies of population genetics, speciation, and macroevolution in the genus. We discuss the biogeography of Pristionchus in East Asia and the origin of hermaphroditism in the P. pacificus species-complex.