Shabanov et al. (2009) Which green frogs inhabit the Kharkiv region?

Shabanov D. A., Korshunov O. V., Kravchenko M. O. Which of the water frogs inhabit Kharkiv oblast? Terminological and nomenclatural aspects of the problem // Biology and Valeology. – Iss. 11. – Kharkiv: KhSPU, 2009. – pp. 116–125.

UDC 597.851(477.54)

Which of the water frogs inhabit Kharkiv oblast? Terminological and nomenclatural aspects of the problem

D. A. Shabanov, O. V. Korshunov, M. O. Kravchenko

V. N. Karazin Kharkiv National University

The water frogs that inhabit Kharkiv oblast, according to the latest revisions, belong to the genus Pelophylax Fitzinger, 1843. These are two parental species, the pool frog Pelophylax lessonae (Camerano, 1882) and the lake frog, P. ridibundus (Pallas, 1771), as well as their diploid and triploid hybrids, which are called edible frogs, P. esculentus (Linnaeus, 1758). The diploid hybrids P. esculentus are represented by various forms that differ in the character of gametogenesis; the triploids — by two forms that differ in the composition of the genomes in the genotype. The reproduction of the hybrids is connected with the phenomenon of hemiclonal inheritance. All the named forms of frogs can form hemiclonal population systems (HPS), in which, in the course of joint reproduction, both clonal and recombinant genomes are transmitted. In the Siversky Donets basin in the Kharkiv and Donetsk oblasts is located the Siversky Donets centre of water frog diversity, which is characterized by the presence of polyploid hybrids and the (complete or almost complete) absence of P. lessonae.

Key words: water frogs, hemiclonal inheritance, hemiclonal population systems, Siversky Donets, Kharkiv oblast.

Which of the water frogs inhabit Kharkiv oblast? Perspectives on terminology and nomenclature.D. A. Shabanov, O. V. Korshunov, M. O. Kravchenko.V. N. KarazinKharkiv National University

Latest taxonomic revisions place water frogs that inhabit Kharkiv oblast within the genus Pelophylax Fitzinger, 1843. These are two parent species – pool frog Pelophylax lessonae (Camerano, 1882)and lake frog P. ridibundus (Pallas, 1771)– as well as their hybrids: which are referred to as an edible frog P. esculentus (Linnaeus, 1758), diploidand triploid ones. Diploid hybrids are represented by several forms, which differ in the mode of gametogenesis. Triploid hybrids are represented by two forms, which differ in the ratio of the parent genomes in the genotype. Reproduction of hybrids is associated with a phenomenon of hemiclonal inheritance. All green frogs mentioned can form hemiclonal population systems and reproduce together passing both clonal and recombinant genomes to descendants. In the Siversky Donets basin in Kharkiv and Donetsk oblasts there exist a peculiar centre of green frogs diversity. In this centre polyploid hybrids occur while P. lessonaeis absent (or at least is very rare).

Key words: water frogs, hemiclonal inheritance, hemiclonal population systems, Siversky Donets, Kharkiv oblast.

Statement of the problem in general form

Water frogs are one of the widely known symbols of zoology. They are among the most widespread animals in the countries of Europe with a temperate climate, a classical object of laboratory research. This group of animals has now become an object of intensive research by European zoologists thanks to the phenomenon of hemiclonal hybridization discovered in them (18; 28, etc.). It is possibly precisely with this that the serious nomenclatural and terminological confusion regarding the names of water frogs and the modes of their reproduction is connected, the manifestations of which are characterized below.

The aim of this work is to consider the alternative systems of nomenclature and terminology used to describe water frogs, and to choose a set of names and terms for further use. It should be noted that, in choosing certain names and concepts in this work, the authors do not reject other variants as incorrect or unfortunate.

Presentation of the main material of the research

The generic name. Throughout the predominant part of the history of the study of water frogs, their division into separate species or other taxa was carried out by external features: at first by general features of external appearance, later — by morphometric characters. In the opinion of P. V. Terentyev (11), the «batrachos» mentioned by Aristotle were lake frogs. Water frogs were mentioned by C. Gesner, A. Leeuwenhoek, J. Ray, and other classics of biology.

C. Linnaeus, who laid the foundations of the modern system of animals, assigned all tailless amphibians to the genus Rana — Frog, and described representatives of the water frogs as the species Edible frog, Rana esculenta. Ten years after the appearance of the 10th edition of the «System of Nature» (27), which became the starting point of the systematics of the Modern era, in the work of J. Laurenti (26) frogs were separated from toads and other groups of tailless amphibians. Since European frogs are clearly divided into terrestrial (brown) and aquatic (green) ones, L. Fitzinger (23) singled out the latter into the genus Pelophylax. The majority of systematists did not accept this decision, but in the 20th century, when the genus Rana was divided into subgenera, the water frogs were assigned to the subgenus Pelophylax. In the 19th century many other names of water frogs appeared, almost all of which were in time reduced to synonyms of a few species.

During the larger part of the 20th century, two or three species of European water frogs were distinguished. Thus, zoologists of the Soviet period until the end of the 1960s most often distinguished two species of frogs (11): the lake frog, R. ridibunda Pallas, 1771, and the «pool» one (R. esculenta Linnaeus, 1758); quite often two subspecies were included in the composition of the latter species: R. esculenta esculenta Linnaeus, 1758, and R. esculenta lessonae Camerano, 1882. When using the works of that time, it should be taken into account that the notions about the taxa of water frogs did not coincide with the modern ones.

The works of L. Berger (17, etc.) radically changed the situation in the study of water frogs; he obtained the form «esculenta» when crossing the forms «ridibunda» and «lessonae», as well as representatives of the parental forms in the offspring of hybrids. H. Tunner (31) was able to explain this phenomenon, and later to prove his assumptions by means of protein electrophoresis. After the works of these authors and their followers, among European water frogs they began to distinguish two parental species, the pool frog R. lessonae Camerano, 1882, and the lake frog R. ridibunda Pallas, 1771, as well as their hybrids, the edible frog, R. esculenta Linnaeus, 1758.

At the end of the 20th – beginning of the 21st century, molecular-biological data accumulated regarding the rather ancient separation of the water (green) and brown frogs (for example, 33), which led to the revival of the generic name proposed by L. Fitzinger (23). This was done in an authoritative taxonomic revision with the participation of D. Frost (24), which introduced into circulation a considerable number of new and renewed names. At the time of writing this work, the scientific community had not yet adopted a single point of view regarding the new systematics. One of the causes of such conservatism is that, by dividing genera into small parts, the new systematics partly levels the significance of the genus as a taxonomic category. For example, of the three species of toads that inhabit Ukraine (the common toad, Bufo bufo (Linnaeus, 1758); the green toad B. viridis Laurenti, 1768, and the natterjack toad B. calamita Laurenti, 1768), only the first species remained within the genus Bufo. The two others, which had always been considered rather close, turned into representatives of different genera: Pseudepidalea viridis (Laurenti, 1768) and Epidalea calamita (Laurenti, 1768). The generic name in this (as in many other) case ceased to unite close species.

The position of the proponents of including water frogs in the genus Rana remains rather strong (for example, 29). The authors who share these views consider that the taxon Pelophylax is a subgenus. In that case the full name of the lake frog with indication of the subgenus is the following: Rana (Pelophylax) ridibunda Pallas, 1771. However, more and more works are appearing (especially American ones) in which water frogs are assigned to a separate genus. Some authors who use the name Pelophylax accept the systematics of D. Frost and his co-authors (24) with respect to water frogs, but do not use it with respect to green toads (for example, 12). Among the authoritative compendia available on the Internet, representatives of both points of view can also be found. The AmphibaWeb Species Lists (15) treats water frogs within the genus Rana, while the Amphibian Species of the World Database (14) places them in the genus Pelophylax. A drawback of the transition to the new nomenclature is a certain gap between the old and new works, while a positive consequence is a clearer delineation of precisely the group of water frogs.

Adopting the new point of view (24), one may conclude that Kharkiv oblast is inhabited by the pool frog Pelophylax lessonae (Camerano, 1882), the lake frog, P. ridibundus (Pallas, 1771), and the edible frog, P. esculentus (Linnaeus, 1758).

The taxonomic status and composition of the parental species. Molecular data not only separated the water frogs from the brown ones, but also demonstrated the complex structure of the wide-range parental species that were previously considered single. This especially concerns the lake frog. One of the recent reviews (28) names the following species singled out from the composition of R. ridibunda (according to the nomenclature adopted by the author): R. bedriagae Camerano, 1882; R. terentievi Mezhzherin, 1992; R. cretensis Beerli, Hotz, Tunner, Heppich& Uzzell, 1994; R. epeirotica Schneider, Sofianidou& Kyriakopoulou-Sklavounou, 1984. First of all, an independent status is received by the species located at the borders of distribution of the whole group. But in the near future changes may affect even the central- and east-European representatives of this group. J. Plötner considers the lake frog a «superspecies» and points to the division of its range among several forms that differ substantially from one another genetically. Since P. S. Pallas described the lake frog from the coast of the Caspian, the name «ridibunda» in its narrow sense will remain with the south-eastern representatives of this «superspecies». In Ukraine the form ridibunda, according to Plötner, is distributed only in the Crimea, while the continental part of the country's territory is inhabited by the form fortis Boulenger, 1884. The term «form» is used here without any connection to its taxonomic rank, in the sense proposed by E. Mayr (7). Whether this form has the status of a subspecies or a species will be shown by further research.

The species pool frog has lost parts of its range which, according to modern notions, are inhabited by R. bergeri Günther, 1985, and R. shqiperica Hotz, Uzzell, Günther, Tunner& Heppich, 1987 (28). Possibly, in time taxonomic changes will affect the representatives of this species inhabiting the territory of Ukraine as well.

The taxonomic status of diploid hybrids. The oldest of the scientific names of water frogs, given as early as by C. Linnaeus, belongs to the hybrid frog (28). Usually hybrids do not have independent names, but neither is hybridization in water frogs usual. During gametogenesis in hybrid frogs some of the parental genomes (the clonal ones) pass into the sex cells, while others are eliminated. The transmission of clonal genomes between generations takes place, as a rule, without recombination, although examples of the formation of interspecific recombinants are also known (28; 29).

In favour of granting the edible frogs an independent status is the fact that the hybrids do not arise time and again upon the crossing of the parental species, but are stably reproduced over a number of generations, transmitting the clonal genome and acquiring new features in the course of evolution. The hybrids have their own range, which extends beyond the limits of the intersection of the ranges of the parental species. Moreover, it is known that hybridization of the parental species occurs rather often in nature, but the products of such hybridization usually have reduced viability and are often sterile, and therefore lose out to the hybrids that reproduce by way of hybridogenesis (28). An essential circumstance is that the hybrids can form populations in which they exist without crossing with the parental species. In this case, as in many others, the richness of the manifestations of life does not fit into the Procrustean bed of unambiguous rules and unified categories. It remains a fact that after the discovery of the hybrid nature of the edible frogs, the scientific community continued to use the name R. esculenta (and in time — P. esculentus).

In order to give some taxonomic status to such hybrid forms, A. Dubois and R. Günther proposed introducing the taxonomic category klepton (21; 22). According to this approach, hybrid frogs should be called Rana kl.esculenta or Pelophylax kl.esculentus, but in the majority of modern works this requirement is not fulfilled, and the International Code of Zoological Nomenclature does not provide for such changes of species names (9). In a considerable number of modern publications P. esculentus is called simply a «species», but one should not forget that this is a hybridogenetic taxon with a name similar to a species one.

It should be noted that even diploid P. esculentus, which have a genotype consisting of the genomes of P. lessonae and P. ridibundus, not only arose repeatedly as a result of hybridization of the parental species, but also differ in the character of their gametogenesis. Among them, including on the territory of Kharkiv oblast (2; 13), there occur individuals that produce gametes with clonal genomes of P. lessonae, with genomes of P. ridibundus, and even such as simultaneously produce (in a certain ratio that is characteristic of each individual) gametes with clonal genomes of either P. lessonae or P. ridibundus. These forms of diploid hybrids do not have (and, probably, should not have) an independent taxonomic reflection. Denoting the genomes of the parental species as L and R according to the first letters of their species names, and also showing the clonal character of the genome by placing its symbol in parentheses, we can distinguish these forms of frogs by the symbolic designations of their genotypes: L(R), (L)R, and (L)(R). However, it should be noted that different forms of diploid hybrids may arise independently of one another and compete with one another.

The taxonomic status of triploid hybrids. The nomenclatural problems connected with water frogs are complicated by the presence in this complex of polyploid hybrids. During the last decade tri- and even tetraploid hybrids have been found in Kharkiv oblast (2; 13; 19). In accordance with the views of many specialists (for example, 28), the name P. esculentus cannot apply to polyploid forms. However, in many works the names R. esculenta or P. esculentus apply both to diploid and to triploid hybrids (for example, 18).

Certain drawbacks exist in all possible ways of solving this nomenclatural problem. If the name given by C. Linnaeus belonged to a diploid hybrid, it should be remembered that triploid hybrids are, undoubtedly, different forms. However, it is known that in some population systems (including those widespread in the Siversky Donets basin in Kharkiv oblast; 13) diploid and triploid hybrids exist jointly, cross, and, probably, give rise to one another. Moreover, it is known that diploid hybrids may represent different forms. If the name P. esculentus (Linnaeus, 1758) embraces all forms of diploid hybrids, it is logical to include in this group the triploids (and also the tetraploids) as well. Such an interpretation of the name P. esculentus does not correspond to the customary use of species names, but one should not forget that the hybrid frogs themselves have the status not of a species, but of a certain hybridogenic taxon.

The mode of reproduction of hybrid frogs. The mode of existence and reproduction of P. esculentus does not correspond to what can be considered a «typical case» according to modern biological notions. In the reproduction of hybrids Mendelian recombination of the parental genes does not take place; certain genomes are transmitted clonally (4; 16; 28). For the first time such a type of reproduction was described in the fishes Poeciliopsis (30) under the name «hybridogenesis». This name is not a fortunate one, because the main feature of such a phenomenon is not the appearance of hybrids but the mode of their reproduction. Other names for this phenomenon are «gametic (genomic) parasitism» (25), «creditogenesis» (1), «kleptogenesis» (21), «meroclonal inheritance in hybrids» (34), but the most widespread term is «hemiclonal inheritance».

A certain drawback of the term «hemiclonal» (that is, «half-clonal») is that in triploid hybrids it is by no means half of their genome that is transmitted clonally. What takes place in triploids is more precisely characterized by the term «meroclonal» («partly clonal») inheritance, and it is precisely by this that the logic of the authors (34) who proposed it for the study of water frogs is explained. But this term has hardly found wide use, and, moreover, it is used in protistology to describe inheritance in which it is not the whole genome but a certain part of it that is protected from recombination (for example, 20).

A specific feature of water frogs (as of some other species with hemiclonal inheritance) is their ability to form joint breeding groups consisting of representatives of different (in terms of the number of genomes, their origin, and the character of gametogenesis) forms. The composition of these breeding groups may include both representatives of the parental species and various hybrids. To call these groups «populations» is incorrect, because the composition of populations should include representatives of one species.

To designate such groups, terms have been used such as «population of the L-E system» (33) or «mixed population systems of the l-e type» (6). In such designations the letter L corresponds to P. lessonae, R — to P. ridibundus, E — to P. esculentus, and Et — to triploid representatives of P. esculentus. J. Plötner (28) emphasizes that the name «population systems» should be used only for such systems in which hybridogenetic (hemiclonal) reproduction of hybrids takes place. To emphasize this circumstance, we prefer to use the concept of a hemiclonal population system (abbreviated as HPS).

For example, we can say that for the Siversky Donets basin within Kharkiv oblast, populations and HPS of water frogs of the following types are characteristic: R, RE, Ret, and Et. In the Dnipro basin in this oblast, populations and HPS of the R, L, RE, and REL types are widespread (4). With such a use of the concepts, the expression «population of the R type» means an ordinary population of P. ridibundus.

The Siversky Donets centre of water frog diversity. The concept «centre of diversity» should be distinguished from the concepts «centre of origin», «centre of speciation», and others. For example, according to the definition adopted by the UN in the International Treaty on Plant Genetic Resources for Food and Agriculture, which came into force in 2004, the concept of a centre of diversity (in contrast to a centre of origin) means a geographical territory containing a high level of diversity (in the context of the treaty — of cultivated plant species) under in-situ conditions (8).

From this point of view it can be established that the territory of the Siversky Donets basin in Kharkiv oblast (and also downstream of this river in the Donetsk and Luhansk oblasts of Ukraine and as far as the Rostov oblast of Russia) is a centre of diversity of water frogs (2; 4; 13; 19). In the Siversky Donets river and the water bodies connected with it are located HPS of the RE, Ret, and Et types; this region of distribution of triploids is separated from other places of their permanent existence by approximately 1000 (Eastern Poland) and 1500 (Western Hungary) kilometres (19). In the Zmiiv district of Kharkiv oblast even single tetraploid individuals have been registered (2, 19). The most diverse populations of water frogs are found in Kharkiv oblast and the northern part of Donetsk oblast, where in sufficient quantity there are the biotopes necessary for the existence of P. esculentus. An unusual feature of this centre is the complete or almost complete (there are no reliable finds of sexually mature individuals) absence in it of P. lessonae (5). By the most characteristic feature — the connection with the Siversky Donets river — this centre can be called the Siversky Donets centre of water frog diversity.

This work was supported by a joint grant of the SFFR of Ukraine and the RFBR of Russia (agreement with the SFFR No. F 28/268-2009).

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