Meleshko, Kravchenko (2012) Study of green frogs of the Iskov pond

The complex of Central European green frogs, Pelophylax esculentus complex, consists of two parental species — the pool frog, Pelophylax lessonae (Camerano, 1882), the lake frog, Pelophylax ridibundus (Pallas, 1771), and their hybrid (which has a name similar to a species) — the edible frog, Pelophylax esculentus (Linnaeus, 1758) [13]. One of the unusual features of this complex is that for diploid P. esculentus, hemiclonal (semiclonal) inheritance is characteristic: in hybrid frogs, one of the parental genomes passes into the gametes. This is achieved through premeiotic elimination of one genome, endoreduplication of the other (clonal) genome, and gametogenesis with the formation of identical gametes [5;15]. If, when crossing hybrid individuals, the offspring receives clonal genomes from the same parental species, its viability significantly decreases: such individuals do not reach sexual maturity [15].

The three obtained population estimates (92±25, 286±82, and 697±224) differ significantly from each other. This discrepancy can be explained by the short time interval between the release of tagged individuals and the first and second recapture events. Probably, the calculations based on the first and second captures are not reliable: the tagged individuals did not have enough time to mix with other individuals in the population. At the same time, the third capture was carried out three weeks after the last release of tagged individuals, so, from the perspective of meeting the method's conditions (equal probability of capturing tagged and untagged animals), the population estimate obtained from its results can be considered quite correct. The obtained data allow for an approximate determination of the sex ratio in the studied HPS. We separately estimated the number of males and females using the Peterson method. Since females were not included in the sample obtained during the first recapture (Table 1), the sex structure was assessed based on data from the second and third recaptures. The obtained data are shown in Table 2. Table 2. Sex ratio based on recapture results (see above for notation)

* m – males; f – females. Males were found to be significantly predominant, which may be due to the deficit of females recorded in 2005. Despite this, the sex ratio is stable based on the results of two captures separated by a significant time interval. As already indicated, we consider the population estimate based on the results of the third and final capture to be more reliable. At the time of the second capture, the tagged individuals had not yet redistributed evenly throughout the entire HPS (which consists of both individuals in the water body and those that have moved away from the pond and are on land). However, the fact that the sex ratio estimates coincide suggests that mixing occurred equally for females and males. It is likely that the calculated sex ratio is close to the actual one. P. esculentus individuals predominated in species composition, with only 1% represented by individuals of the parental species P. ridibundus. Ploidy was determined in 107 green frogs. Determination of ploidy in the studied hybrids showed that almost all individuals in the sample are diploid (only one triploid individual was recorded). Compared to 2011, the proportion of females in catches has significantly increased. Thus, the obtained results reflect the tendency of the HPS to return to the state recorded by G. A. Lada in 1995 [3; 8]. Modeling of the transformations of the green frog HPS shows that for populations consisting only of hybrid forms, a necessary condition for reproduction is the presence and transmission from generation to generation of various clonal genomes (both P. ridibundus and P. lessonae, including both male and female) [4; 9]. The crisis of this HPS was likely due to the loss of all clonal genomes except the male genome of P. lessonae [3]. Fortunately, the Is'kiv pond is not isolated from other nearby water bodies (Fig. 1). It is likely that the improvement in the HPS condition we observe is a result of the introduction of a set of clonal genomes, allowing it to transition to a stable state [4]. Conclusions In the course of the work, an assessment of the demographic status of a scientifically interesting green frog HPS was obtained. The HPS includes 161 individuals with tags that allow for further individual identification. It was established that at this stage of transformation of the studied HPS, we observe an E-type system (i.e., consisting mainly of diploid P. esculentus) with a small proportion of P. ridibundus and triploid P. esculentus. The calculated number of sexually mature individuals in this HPS is about 700, with males outnumbering females approximately fivefold. The studied HPS is recovering after a crisis caused by the draining of the pond in 2000. The shortcomings of the Peterson population estimation method became apparent during the work, as its accuracy is significantly affected by the movement of groups of individuals within and around the pond. For a comprehensive study of the HPS transformations of the Is'kiv pond green frogs, further research using individual tagging methods is necessary. Acknowledgments The authors express their gratitude to many colleagues who contributed to the study of the Is'kiv pond green frog HPS. G. A. Lada initiated these studies. S. M. Lytvynchuk, L. Ya. Bokin, and Yu. M. Rozanov provided invaluable assistance in identifying the studied frogs. Field studies involved E. Malchenko, R. Myronov, Yu. Kravchenko, S. Ohiyenko, as well as many other students who underwent field practice in vertebrate zoology at the V. N. Karazin Kharkiv National University biological station. O. V. Korshunov and D. A. Shabanov played a key role in all studies of the Is'kiv pond. 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Bielefeld: Laurenti-Verlag, 2005. — 161 p. Article submitted to the editorial office on 10/15/2012; accepted for publication on 10/24/2012.