Traditionally, taxonomy has relied on the use of morphological features to identify distinct species. However, as molecular techniques have improved, a new era in species identification is underway. Using molecular markers, it is now possible to separate species that appear to be morphologically identical, but are in fact genetically distinct. In a recently published study in Frontiers in Zoology, Katharina Jörger and Michael Schrödl from the SNSB-Bavarian State Collection of Zoology, Germany, demonstrate the use of molecular techniques to identify cryptic species. Biome interviews Alessandro Minelli, who is on the editorial board of Frontiers in Zoology, in order to examine how the increasing use of molecular techniques is changing taxonomy.
Minelli is Emeritus Professor of Zoology at the University of Padova, Italy, where his main research interests presently lie in evolutionary developmental biology – specifically the origin and evolution of segmentation and appendages. His wide ranging expertise also includes biological systematics as well as phylogeny and taxonomy of myriapods. Minelli is a commissioner, and former president (1995-2001) of the International Commission on Zoological Nomenclature, and an honorary member of the UK’s Royal Entomological Society of London.
What is a cryptic species?
Faithful to the adjective’s literal meaning, a cryptic species is a ‘hidden’ species. Hidden because we are not able to find out morphological traits differentiating this species from one (or more) of its closest relatives. Thus, to be precise, there cannot be a cryptic species in isolation (quasi a ghost duplicate of a ‘true,’ legitimate species) but only a pair (or a larger set) of morphologically indistinguishable species. The reasons why we believe they nevertheless are separate species are different, and to some extent related to our species concepts.
How important are new molecular techniques in the identification of cryptic species?
Techniques are important, but the results of observations and experiments must be eventually interpreted and it is here that theory becomes relevant – in our case, in the form of alternative species concepts. Fifty years ago, mainly under the influence of the evolutionary biologist and ornithologist Ernst Mayr and the population geneticist Theodosius Dobzhansky, most biologists accepted the so-called biological species, according to which the criterion for recognizing species is reproductive compatibility. Thus, if a cross between individuals from two separate populations failed completely, the two populations were considered to belong to separate species. If no reliable morphological trait could be found to differentiate one of them in respect to the other, these biological species were described as cryptic species.
In the article by Jörger and Schrödl, molecular techniques are used rather than hybridization tests, to recognize the new Pontohedyle species. Which species concept are they ultimately advocating – molecular or morphological?
Despite its theoretical appeal (in a sense, it is left to the animals themselves to show whether they recognize a potential partner as belonging to the same species, or not), the biological species concept has both conceptual and operational limitations. In practice, rather than testing representatives of different populations for their reproductive compatibility, taxonomists search for consistent diagnostic traits. Trait consistency is usually taken as an indirect proof of reproductive isolation. How large the differences should be to justify the recognition of separate species, this may depend on the taxonomic group. Some degree of arbitrariness is unavoidable, although mathematical methods (and corresponding software) have been recently proposed to reduce subjectivity. One of these methods has been used by Jörger and Schrödl in their study. Diagnostic traits are not necessarily morphological, but can instead be molecular.
How do scientists distinguish different species when morphological characteristics aren’t reliable or available?
In this case, species are differentiated mainly, or even exclusively, on the basis of molecular (sequence) data. For example, Jörger and Schrödl recognize at least 12 more or less cryptic species in a genus of tiny (0.7-6 mm) marine slugs based on the unique or preferred presence of a specific nucleotide in one or more sequence positions in two mitochondrial and two nuclear genes.
Are there different challenges faced when using molecular data than when following more ‘traditional’ taxonomic methods?
In principle, none. In either case, scientists must explicitly enumerate diagnostic traits (in this case, molecular rather than morphological) that differentiate the new species from its closest relatives. In practice, however, there are differences. The methods used to gather information about the specimens are destructive, in so far as these require crushing of tissues from which DNA will be extracted. This is a serious problem in the case of very small organisms, of which nothing or little, except for molecules, can be saved for records. Taking pictures before sacrificing the specimen for the molecular analysis may help with saving a record, albeit an indirect one, of its morphology. Hard parts, if any, have a better chance to be preserved, but this is also tricky when these are minute, as are the teeth armature (the radula) in the mouth cavity of the tiny slugs described by Jörger and Schrödl, which is easily lost together with the remnants of the soft tissues.
What implications does an entirely molecular description have when using the International Code of Zoological Nomenclature?
For valid species descriptions, the Code requires, besides publication, the fixation of a type. Traditionally, this is a preserved specimen, such as a stuffed bird or a pinned butterfly. Today, however, there may be reasons for suggesting the use of a DNA sample, rather than a whole animal, as the type of a new species. One reason in support of this is a strong concern for the rarity, and thus vulnerability, of the newly discovered species. In 1991, a bird species, the African shrike Laniarius liberatus, was described as new, but the only specimen collected at the time was released after taking a sample of DNA from it. A few other examples followed, always motivated by conservation reasons. More recently, DNA-only types are suggested because of the technical problems I have just mentioned – the destructive methods required to obtain the diagnostic DNA sample. One might add, that in the case of cryptic species a conventional type would be useless, as its morphology would not be diagnostic. In any case, fixing a type, even if represented by a DNA sample only, means also fixing a type locality, and this may prove of crucial importance, if the two or more cryptic species are allopatric, i.e. have non-overlapping geographical distributions.
Do you think molecular descriptions will be more widely used by the taxonomy community, or does more research need to be done?
Yes, this approach will certainly be more and more frequent in the near future. A strong stimulus for these studies has grown out of the systematic efforts to associate a diagnostic molecular signature (the barcode) to an increasingly large number of living species. This effort, by now extended to a million sequences, has revealed a number of cryptic or very subtly differentiated pairs of species; for more on this visit BOLD systems.