Pristionchus hoplostomus, Ragsdale & Kanzaki & Röseler & Herrmann & Sommer, 2013

Ragsdale, Erik J., Kanzaki, Natsumi, Röseler, Waltraud, Herrmann, Matthias & Sommer, Ralf J., 2013, Three new species of Pristionchus (Nematoda: Diplogastridae) show morphological divergence through evolutionary intermediates of a novel feeding-structure polymorphism, Zoological Journal of the Linnean Society (Zool. J. Linn. Soc.) 168 (4), pp. 671-698 : 694-696

PRISTIONCHUS SPECIES

The existence of the cheilostom polymorphism in the three new species shows how two types of plasticity interact in a single organ to generate novel morphology. To our knowledge, the presence of a third form has never been reported for a nematode exhibiting mouth plasticity. As combinations of individually plastic traits can confer adaptive differences ( Griffith & Sultan, 2006), the composite spectrum of feeding forms in the new species could in principle respond to selection pressures differently than the canonical dimorphism of other diplogastrid species. Although the 12-plated and intermediate forms were normally only observed in eurystomatous mouth types, a single example of a 12-plated form was discovered in a stenostomatous individual in P. hoplostomus sp. nov. The occurrence of this form suggests that some developmental module of the 12-plated form operates independently of what regulates the canonical mouth dimorphism. In this case, the ability to produce six or 12 plates (or intermediate numbers of plates) could arise or become uninhibited in either the stenostomatous or eurystomatous developmental programme and thus also be selected upon within a single form. The presence of exaggerated armature, notably the larger left subventral denticles, in the observed 12-plated stenostomatous individual further suggests an independent developmental module for the 12-plated condition, although the observation of only one individual prevents meaningful comparison with this form. Finally, independent selection on mouthpolymorphism traits may be possible between sexes. Consistent with findings in P. pacificus ( Serobyan et al., 2013) , males were highly stenostomatous in the three new species, in contrast to the relative abundance of the eurystomatous form in females and hermaphrodites.

With a growing understanding of genetic mechanisms of the canonical mouth dimorphism in Pristionchus , developmental regulators of the megastomatous form of the new species can be studied empirically. For example, one could examine the extent to which the megastomatous form differentially responds to cues that have been shown in P. pacificus to induce the eurystomatous form, such as starvation and pheromones ( Bento et al., 2010; Bose et al., 2012; Serobyan et al., 2013). Because the polymorphism was observed in a strain of P. triformis sp. nov. (RS5233) that has undergone extensive inbreeding and is putatively isogenic, it is likely that the plasticity in this species is not a result of genetic heterozygosity but is instead a polyphenism. In P. fukushimae sp. nov. and P. hoplostomus sp. nov., however, it is possible that genetic heterozygosity has persisted in culture populations and might account for the higher frequency of intermediate forms in those species. Artificial selection experiments in both hermaphroditic and gonochoristic species with the plate polymorphism will thus be necessary to test whether either the six-plated or 12-plated forms can be fixed.

For now, the potential fitness advantages of the megastomatous form can only be hypothesized, but the typical six-plated eurystomatous form, although slower in its postembryonic development than the stenostomatous form, could confer the benefit of access to a broader diet ( Serobyan et al., 2013). Namely, the eurystomatous form is associated with omnivory and predatory feeding ( Kiontke & Fitch, 2010). It is possible that selection for the these feeding modes would favour the evolution of the 12-plated form because of its greater flexibility, as would be offered by the doubled number of vertices in the cheilostom. Empirical investigation of the functional and fitness advantages of the dimorphism in P. pacificus as a model will provide the foundation for comparative studies, including the new polymorphic species.

INTERMEDIATE TAXA REVEAL ANCESTRAL STATES