Dugesia benazzii, Lepori, 1951

Dols-Serrate, Daniel, Leria, Laia, Aguilar, Juan Pablo, Stocchino, Giacinta Angela & Riutort, Marta, 2020, Dugesia hepta and Dugesia benazzii (Platyhelminthes: Tricladida): two sympatric species with occasional sex?, Organisms Diversity & Evolution (New York, N. Y.) 20 (3), pp. 369-386 : 379-380

Dugesia benazzii species status

The clues provided by the phylogenetic Cox1 tree obtained in this study (Fig. 3) point out that D. benazzii could constitute more than one species. We base this suggestion on the paraphyletic arrangement of the D. benazzii clades in the tree and especially on the high genetic differentiation among them. The genetic differentiation between the populations from Corsica and Sardinia for COI has a mean value of 5.8 ± 0.8%, far superior to the ones found between populations within each of the islands ( Table 3). Moreover, the values found between islands concur with some of the interspecific genetic distance values for Dugesia species from the Western and Eastern Mediterranean ( Lázaro et al. 2009; Solà et al. 2013) that vary between 2.8% for closely related species in the Aegean region and 11% for some species on the Western region. On the other hand, the phylogenetic tree obtained from the nuclear marker (Dunuc12, Fig. 4 ) showed a monophyletic D. benazzii clade, yet the two individuals from Corsica appeared again to be highly differentiated from the Sardinian populations. Nonetheless, the fact that only two sequences for the aforementioned nuclear marker of Corsican individuals were used in this study gives us little information regarding genetic diversity of the populations of the Corsican D. benazzii . On the other hand, populations of D. benazzii from both islands are identical regarding the ITS-1, which will support the monophyly of D. benazzii but not its division in more than one species, unless this is a very recent event and ribosomal clusters are still being kept similar by concerted evolution. As for the morphology, the original description of the species given by Lepori (1951) did not establish any remarkable differences between Corsican and Sardinian D. benazzii populations regarding the copulatory apparatus, but it did point out some minor dissimilarities that he deemed insufficient neither to consider the Corsican and Sardinian populations as distinct geographical subspecies. However, considering that modern descriptions take into account more characters than in the past, the possibility to find valid differences supporting a specific differentiation cannot be ruled out. To this purpose, a new detailed morphological study has been already undertaken on populations of D. benazzii from the two islands that will be the subject of a companion paper.

Hence, there is incongruence between the mitochondrial history and the nuclear and morphological accounts. In some cases, a potentially high degree of genetic variation may only be reflected by recondite morphological traits (according to Kucera and Darling 2002) that are not evident at first sight. Sibling species often have minor morphological differences that are only noticed once species are recognized for other reasons—such as karyological data or molecular evidences. The species that fit this profile are known as pseudo-cryptic ( Knowlton 1993), and this may be the case for Corsican and Sardinian D. benazzii populations. Nonetheless, speciation is a continuum. Theoretically, the further we stray from the starting point, the clearer and more evident should be the differences between descendant lineages, but in the first stages of speciation, there can be divisiveness among sources of evidence—i.e. genetic data versus morphological data, nuclear versus mitochondrial DNA—because changes do not accumulate uniformly and at a fixed rate. This interval of speciation is known as the “gray zone” ( De Queiroz 2007) and could explain why we find differences regarding the Cox1 sequences between Corsican and Sardinian D. benazzii populations but not in the ITS-1 or in their morphology.

These results point to the need for a revision of the taxonomic status of D. benazzii , based on more data ranging from an increase of the number of nuclear markers and the use of molecular methods for species delimitation to a morphological and karyological revision of the individuals. A similar situation has been resolved in a close relative, D. subtentaculata , by the concurrent use of all these lines of evidence in an integrative way, resulting in the description of three new species that are morphologically cryptic with D. subtentaculata ( Leria et al. 2020) .

Figures

Fig. 4 Bayesian inference tree of dataset II (Dunuc12). Node support values are displayed as in Fig. 3. Bar diagrams indicate the species assignation of samples based on the ITS-1 criterion and karyology. It is