The current ITS-based phylogeny covering the most comprehensive sampling of Xylodon till now recovered the four lineages of X. laurentianus, X. novozelandicus, X. patagonicus and X. raduloides, each not receiving strong support; the lineage of X. subtropicus being composed of two original Asian specimens described by Chen et al. [15] occupied a basal position of the four lineages (Figure 1). Like the phylogeny based on ITS region (Figure 1), that based on three genes also recovered the four lineages, and no one was strongly supported, neither was the clade being composed of these four lineages (Figure 2). Alternatively, these four lineages together with the basal lineage of X. subtropicus formed a strongly supported clade (Figure 2). In the phylogeny based on seven genes (Figure 5), the two original specimens of X. subtropicus with only ITS and nLSU regions available were not included; the lineage of X. raduloides was strongly supported, whereas those of X. laurentianus, X. novozelandicus and X. patagonicus were weakly to moderately supported; the clade consisting of these four lineages was strongly supported. In these three phylogenies (Figure 2 and Figure 5), the newly sequenced Australian specimens merged in the lineage of X. novozelandicus. Besides the topologies, the branch lengths among these lineages are also too short to clearly distinguish species and fall within the infraspecific distances observed in several other well accepted species in the genus (Figure 2 and Figure 5). Taking into consideration the morphological similarity, the current phylogenies and the low level of divergence, we consider X. laurentianus, X. novozelandicus, X. patagonicus, X. raduloides and X. subtropicus to be conspecific with the last one as the correct name.

The more or less geographically structured phylogenetic signal recovered by Fernández-López et al. [41] indicates a species perhaps in the process of incipient speciation. It is also of interest that Fernández-López et al. [41] found some niche differentiation among the four lineages they recognized as species, and this differentiation merits further exploration on a wider sampling. Furthermore, Paulus et al. [87] found that New Zealand samples of X. subtropicus (as S. radula) had a high mating compatibility (73.7% crosses completely positive and 1.2% lacking clamps) but there was also compatibility between these and isolates from the Northern Hemisphere, albeit at a reduced level (47.8% crosses completely positive and 12.2% lacking clamps). Xylodon subtropicus presents an interesting test case for applying genome-wide markers such as restriction site-associated DNA markers (RADSeq) [88] to further investigate the boundary between population genetic and species level variation, especially if integrated with mating studies of this and closely related species.