Two new riverine cichlid monotypic genera, is the genus concept being extended?

A male of Lufubuchromis relictus from the Lufubu River, Lake Tanganyika drainage [Zambia], in the aquarium of adrian Indermaur [Switzerland]. Foto von Adrian Indermaur. (2013-03-10). identifiziert durch Adrian Indermaur

In 2015, Juliane Weis et al. (2015) carried out a DNA comparison study based on mtDNA (mitochondrial DNA) with the main aim to spot hybridization events in the evolutive history of the cichlids of Lake Tanganyika. To this end, they sampled tissues of 91 species from the lake and surrounding rivers, as part of them a group of Pseudocrenilabrus species, of which one of them turned out to be an undescribed form they called “New Kalungwishi cichlid” (coined from the Kalungwishi River drainage flowing into Lake Mweru where it was collected). In their resulting maximum parsimony phylogenetic tree (which represents the phylogenetic tree with fewer required steps to reconstruct the evolution of the sampled species), their data showed that this so-called “New Kalungwishi cichlid” was at the base of the tree for the group of species of Pseudocrenilabrus, which means that the evolutive lineage of this species is as old as that of the whole group (P. philander, P. multicolor and P. nicholsi) with Pseudocrenilabrus species at one side and the “New Kalungwishi cichlid” at the other. This particular species was later found to inhabit also the Luongo River (Schedel et al., 2020), a tributary of the Luapula River which flows into Lake Mweru. Both the Kalungwishi and the Luapula rivers are part of the Congo River drainage.

In another research Stephan Koblmüller et al. (2012) analyzed the potential ancient connection of the upper reaches of the Lufubu River flowing into Lake Tanganyika with the upper reaches of the Congo River, comparing mitochondrial DNA of the species in both rivers. Their results suggested the existence of an undescribed haplochromine in the Lufubu river that most likely represents the most basal branch of the genus Pseudocrenilabrus, which they denominated Pseudocrenilabrus sp. Lufubu A, with an evolutive lineage even older than the “New Kalungwishi cichlid”.

While studying the successful adaptation of hybrid species to ecological opportunities in Lake Bangweulu and Lake Mweru, Joana Meier et al. (2019), also using mitochondrial DNA, found that the two previously mentioned Pseudocrenilabrinae species belong to old lineages in Pseudocrenilabrus with a hybrid origin with Ortochromis species, to which they are closely related. In fact, they provisionally called one of the species Ortochromis sp. “New Lufubu” (the one known as Pseudocrenilabrus sp. Lufubu A), and the other Pseudocrenilabrus sp. “New Kalungwishi” (the “New Kalungwishi cichlid”). A small phylogenetic tree in this paper exposes the relationships.

In a newly published paper, Frederic Schedel et al. (2020) have determined to describe both species as new, which is not surprising. What is surprising is that they have also erected two monotypic genera, one for each of them: Lufubuchromis relictus from the species in the upper reaches of the Lufubu River (flowing into Lake Tanganyika) and Palaeoplex palimpsest from the form of the Luongo and Kalungwishi Rivers (Flowing into Lake Mweru and eventually the Congo).

When we think of a genus, it understandably comes to our mind as a group. After all, from Latin genus meaning is birth, race, or kind. In taxonomy, one definition of a genus states that “a genus is taxonomic category rank used in biological classification that is below a family and above a species level, and includes the group(s) of species that are structurally similar or phylogenetically related”.

A genus in phylogeny should comply with three principles: monophyly: which implies that all descendants of an ancestral taxon are grouped together; reasonable compactness: establishing that a genus should not be expanded needlessly; and distinctness: which establishes that its members have characteristics to readily separate them from other evolutionary lineages (genera).

Monotypic genera, those genera comprised of only one species, are then a contradiction of terms. We however know of many monotypic genera, which are formed of only one species, a species that is so distinct to otherwise closely related species that taxonomists decide not to group with them, and in this sense it monotypic genera have a place in taxonomy. Monotypic genera presumably should represent significantly distinctive forms and are expected to evolve in a different direction than their sister species, forming a new kind. In Africa, we can think for example in Boulengerochromis microlepis from Lake Tanganyika; Etia nguti from the Cross River drainage in Cameroon; Fossorochromis rostratus from Lake Malawi; Pterochromis congicus from the upper Congo River. In all these species, we can readily recognize their morphological distinctiveness and in most cases, it extends to behavioral and/or biological uniqueness. A diagnosis formulation (description of how they are different from other forms) for these genera poses no problem for the taxonomist.

The unique Boulengerochromis microlepis, the unchallenged single species in the genus Boulengerochromis. Foto von Juan Miguel Artigas Azas. (2010-09-11). identifiziert durch Patrick Tawil

With the two newly described genera things are however different. The species are hardly distinguished from Pseudocrenilabrus, and their diagnoses are exquisitely elaborated, relying mainly on the separation of the first tiny bone (lachrymal) from the second in a series of (five in this case) bones found under the eye (infraorbital bones), and even more subtle is the diagnosis between the two new genera that already share the previous aspect and relies on the size of the spines and other small morphological characters and the color pattern, not those traits that you expect to define genera separation, but species.

These differences alone are very weak to be used to support new genera, particularly when the species' appearance tells nothing of their uniqueness. The species would morphologically fit much better in the genus Pseudocrenilabrus, but Schedel et al. have decided to give a higher value to the long evolutive lineage and position at the base of the Pseudocrenilabrus phylogenetic branch of this species, and their potential ancient hybridization.

One aspect of caution when taking those decisions is the fact that for DNA comparisons gene selections constantly change, as well as the mathematical algorithms used by computers to count the gene differences and reconstruct the phylogeny. So although the phylogenies obtained with these models are becoming stronger every year, they are not the last word and there is room for improvements and corrections, particularly in such thin cases.

One problem with the description of the new genera is that their relationship with the rest of the species of Pseudocrenilabrus is blurred by this action, people may be hinted at their close relationship by their appearance, but to learn about it they need to dig into the research. The concept of the genus, that which groups species together as members of a monophyletic group, is therefore lost at least in this sense.

In middle America, we have seen recently a proliferation of the description of monotypic genera, that while not without criteria by their authors, firsthand blurs the relationship of related taxa and hides from us the sometimes dramatic and fast effects of evolution in shaping new forms. Unfortunately in my view, these seem to be the starting states of a new taxonomic trend, at least in cichlids.