Hybridogenesis in Green Frogs

In this work, I will try to bring together all my understanding of the formation of the phenomenon of hybridogenesis in green frogs (Pelophylax). First, let's focus on the term "hybridogenesis". As the dictionary states, it is:

A mode of existence of individuals (populations) in which the hybrid composition of their genome is constantly maintained (without impoverishment of parental genomes); a classic example of hybridogenesis is the pond frog P. esculentus, which is a constantly reproducing hybrid of the frogs P. ridibundus and P. lessonae; the pond frog has both parental haploid genomes in its somatic cells, and only the P. lessonae genome in its gametes; also, in sympatric populations, constant hybridization of P. ridibundus and P. lessonae occurs.

The Department of Zoology and Animal Ecology has a long history of studying hybridogenesis, which makes understanding the development of this phenomenon worthwhile. Most animal species do not interbreed because effective isolation mechanisms are in place. According to Mikhalevsky, isolation can be classified into prezygotic and postzygotic types. Prezygotic isolation, such as geographic, ecological, or behavioral barriers, prevents mating even between closely related species. Postzygotic isolation comes into play when prezygotic mechanisms fail and mating occurs, but the resulting offspring are infertile, thus ending the process. It is known that hybridization is possible when these isolation barriers are artificially overcome, a fact proven in laboratory settings. Below is a list of amphibians that have been successfully crossbred in laboratory conditions, based on literature data: * *Rana clamitans* × *R. sylvatica* * *R. clamitans* × *R. pipiens* (Moore 1941; Denis, 1970) * *Rana pipiens* × *R. sylvatica* (Moore, 1946, 1947, 1948) * *R. dunni*, *megapoda*, and *pipiens* (Moore, 1966) * *Discoglossus pictus* × *Xenopus laevis* (Woodland and Gurdon, 1969) * *R. pipiens* × *R. palustris* (Hennen, 1972, 1973; Liepins and Hennen, 1977) * *R. pipiens* × *R. catesbeiana* (Reynhout and Kimmel, 1969) * *P. esculentus* × *R. temporaria* * *R. pipiens* × *R. sylvatica* * *Bufo arenarum* × *R. temporaria* (Denis, 1970) * *R. catesbeiana* × *R. clamitans* (Elinson, 1975a,b; Elinson and Briedis, 1981) * *Hyla arborea* × *Pelobates cultripes* * *H. arborea* × *Bufo calamita* * *Pleurodeles waltlii* × *Euproctus asper* * *P. waltlii* × *Triturus palmatus* * *P. waltlii* × *Ambystoma mexicanum* (reviewed by Subtelny, 1974) * *P. waltlii* × *P. poireti* (Gallien and Aimar, 1971; Guillet and Aimar, 1971) * *A. mexicanum* × *A. dumerilli* (Boucaut and Gallien, 1975) * *P. waltlii* × *A. mexicanum* (Aimar et al., 1976) * *Triturus cristatus carnifex* × *T. vulgaris meridionalis* (Mancino et al., 1978) Closely related amphibian species with sympatric origins sometimes form interspecies hybrids. Most of these hybrids perish in the early stages of development, but some survive. It is important to note that hybrids are often infertile. However, some species form stable hybridogenetic populations. Examples include: * *Bombina bombina* × *B. variegata* (Madej, 1964; Szymura, 1993; Szymura, Barton, 1986, 1991) * *Bufo americanus* × *B. hemiophrys* (Green, 1983) * *Triturus cristatus* × *T. marmoratus* (Valle, 1959; Arntzen and Wallis, 1991) * *Triturus vulgaris* × *T. montandoni* (Kotlik and Zavadil, 1999; Litvinchuk et al., 2003; Babik et al., 2003; Babik, Rafinski, 2004) The description of such populations is a separate topic actively researched by scientists, notably Hewitt (1988) and Harrison (1993). Let's focus more on the green frog complex (*Pelophylax*). Two major groups of green frogs form hybrids: one complex is associated with *Rana pipiens* in the Nearctic (discussed by Hillis, 1988), and the other is linked to *P. ridibundus* in the Palearctic (Graf, Polls Pelaz, 1989). The *Rana pipiens* complex comprises at least 27 extant or recently extinct species, divided into alpha and beta complexes, each further subdivided into two groups with stable hybridization between them (Hillis, 1988). The Palearctic green frogs (*Pelophylax*) include 26 recognized species, among them three hybridogenetic taxa: *Pelophylax esculentus*, *Pelophylax grafi*, and *Pelophylax hispanicus* (overview by Dubois and Ohler, 1994a,b). The Nearctic and Palearctic green frogs were most likely separated when land connections between North America and Eurasia disappeared during the Eocene. Since then, these two groups of frogs have evolved independently. Hybridization occurs in both groups, and diploid eggs are spontaneously produced in *R. pipiens* females (Richards and Naisu, 1977) and *Pelophylax esculentus* (Berger et al., 1986). However, triploid hybrids of *R. pipiens* are sterile, whereas triploid males and females of *Pelophylax esculentus* can produce normal gametes due to a special modification of meiosis. There are two concepts that explain the origin of hybridogenesis in green frogs.

The first concept was proposed by Uzzell (1979, 1982). He suggested that P. ridibundus and P. lessonae diverged at least 9-15 million years ago. During glaciation, interspecific hybridization occurred, resulting in an F1 hybrid with a mutation that enabled reproduction in hybrids. As Ogielska points out, this concept was supported by mtDNA analysis of parental forms and hybrid species (Spolsky and Uzzell, 1986). The second concept was proposed by Pelaz (1994) and named the mutation hypothesis of hybridogenesis (MHH). The mutation hypothesis of hybridogenesis states that the cytological phenomenon of hybridogenesis is genetically encoded, and in theory, all animals are capable of hybridization, but their living conditions do not require resorting to this phenomenon. Pelaz believes that the mutation underlying this phenomenon occurred after the Eocene, when the Nearctic was already separated from the Palearctic. This opinion is supported by the fact that the phenomenon of hybridogenesis is found only in the Palearctic, unlike Nearctic amphibians.

In her textbook "Reproduction of Amphibians," Maria Ogielska leans towards the MHH. She argues that research on hybridogenesis in green frogs has reliably demonstrated the cytological mechanisms of its activation. Furthermore, it has been proven in laboratory conditions that frogs belonging to the Pelophylax complex, which do not encounter such phenomena in natural conditions, exhibit the ability for hybridogenesis. In other words, it is likely that in all frogs of the Pelophylax complex, the predisposition to hybridogenesis is encoded in their genetic information. German scientist Pletner (2005) also paid considerable attention to this issue. His works indicate that it is difficult to establish the exact time of origin of the hybridogenetic mode of reproduction. He believes that hybridogenesis is not at all related to hybridization between related species or genetically similar taxa. Most likely, in such forms, genomes interact in already clearly differentiated species, the isolation of which, according to current data, occurred at the beginning of the Pliocene, i.e., more than 5 million years ago. Unfortunately, very little is known about the molecular and cytogenetic processes and mechanisms underlying hybridogenetic reproduction in green frogs. Literature:

Arefiev V.A., Lisovenko L.A. English-Russian Explanatory Dictionary of Genetic Terms. Moscow: VNIRO Publishing House, 1995.

Plotner J. Die wastpalaarkischen Wasserfrosche von Martyrerm der Missenschaft zuz biologis chen Sensation / J. Plotner. – Bielefeld, 2005

Maria Ogielska "Reproduction of Amphibians" pp. 363-395