Planarian (Platyhelminthes, Tricladida) Diversity and Molecular Markers: A New View of an Old Group

Abstract

Planarians are a group of free-living platyhelminths (triclads) best-known largely due to long-standing regeneration and pattern formation research. However, the group's diversity and evolutionary history has been mostly overlooked. A few taxonomists have focused on certain groups, resulting in the description of many species and the establishment of higher-level groups within the Tricladida. However, the scarcity of morphological features precludes inference of phylogenetic relationships among these taxa. The incorporation of molecular markers to study their diversity and phylogenetic relationships has facilitated disentangling many conundrums related to planarians and even allowed their use as phylogeographic model organisms. Here, we present some case examples ranging from delimiting species in an integrative style, and barcoding them, to analysing their evolutionary history on a lower scale to infer processes affecting biodiversity origin, or on a higher scale to understand the genus level or even higher relationships. In many cases, these studies have allowed proposing better classifications and resulted in taxonomical changes. We also explain shortcomings resulting in a lack of resolution or power to apply the most up-to-date data analyses. Next-generation sequencing methodologies may help improve this situation and accelerate their use as model organisms.

The first multi-gene phylogeny of the Macrostomorpha sheds light on the evolution of sexual and asexual reproduction in basal Platyhelminthes

“…The separate origins of asexual fissioning contradict the traditional assumption that asexual fissioning is a basal macrostomorphan reproductive strategy shared with the Catenulida (Rieger, 1986(Rieger, , 2001, and casts additional doubts on the proposed plesiomorphy of asexual reproduction in Platyhelminthes, and even in early divergent Bilateria (Ehlers, 1985;Rieger, 1986). Indeed, Jondelius et al (2011) found asexual fissioning to be a derived feature in the Acoela, and also within the Tricladida there seem to be multiple origins of asexual fissioning (Riutort et al, 2012;Álvarez-Presas and Riutort, 2014). Interestingly, there seems to be a clear link between the presence of asexual fissioning and the capability of regeneration (Egger et al, 2007), suggesting that the presence of a totipotent neoblast stem cell system may play a crucial role in the evolution of asexual fissioning (Martín-Durán and Egger, 2012).…”

A molecular analysis of the phylogenetic position of the suborder Cavernicola within the Tricladida (Platyhelminthes), with the description of a new species of stygobiont flatworm from Benin

“…Although stygobiont freshwater triclads have received a good deal of attention (cf. Gourbault, , and references therein), many more studies have been published on planarians from epigean freshwaters, which have been extensively studied from all over the world (Sluys, Kawakatsu & Winsor, ; Sluys et al ., ; Lazaro et al ., ; Harrath et al ., ; Harrath et al ., ; Stocchino, Sluys & Manconi, ; Sluys, Kawakatsu & Timoshkin, ; Álvarez‐Presas & Riutort, ).…”

Convergent evolution: A new subfamily for bipaliin‐like Chilean land planarians (platyhelminthes)

“…Apparently, EF behaves the same way as the mitochondrial genes at this level, and it lacks sufficient resolution. However, concatenated genes produced better resolved phylogenies than the ones obtained from the individual genes alone, as stated in Álvarez‐Presas and Riutort (2014). This situation, where genes with different resolutions provide a more robust tree when concatenated, may be explained by the influence that each gene has at different levels of a tree (Baldauf et al., 2000) or simply by the fact that longer sequences would result in smaller variances (Gadagkar et al., 2005).…”

“…These sequences belong to representatives of most of the genera of the Neotropical Geoplaninae, plus representatives of the three other land planarian subfamilies. Representatives of the freshwater family Dugesiidae were used as outgroup (Álvarez‐Presas et al., 2008; Álvarez‐Presas & Riutort, 2014) (Table ). These data include the only two Chilean, undescribed species of land planarians that have DNA sequences available in the GenBank database, (i.e.…”