Edible frog – Pelophylax esculentus

The edible frog (Pelophylax kl. esculentus) was formerly considered a species (Rana esculenta), but today it is known to be a hybridogenetic hybrid form of the lake frog and the pool frog. Hybridogenesis refers to a reproductive system in which interbreeding of two species are able to reproduce with either parent species, but not with each other. There is no recombination at all in meiosis, but one of the parental chromosome sets remains completely excluded as gametes are formed. The gametes of the interbreeding of two species then contain only an entire chromosome set of either parent species, which is clonally transmitted from generation to generation. Hybrids breed only with individuals of the parent species, resulting in exclusively additional hybrids. Descendants produced by diploid hybrids among themselves are not viable. Therefore, the edible frog is not actually a species, that is way there is the abbreviation “kl.” in its scientific name. The acronym comes from the Greek word “klepton”, suggesting that hybridgenetic frogs in each generation “steal” one chromosome from either parent species to produce the next generation. It is, they need one of the parent species to be able to reproduce.

In most of Europe, including the Baltic countries, the edible frog lives in the same areas as the pool frog and reproduces with it. Populations in which the edible frog reproduces with the lake frog are less common, but they are known in the Baltic Sea region at least in the northern coast of Poland and in Bornholm.

In some populations of edible frogs, also triploid individuals with the additional chromosome of either the pool frog (LLR type) or the lake frog (LRR type) are present. When mating with a diploid, a triploid individual acts as a parent species, and in these populations the edible frog is able to reproduce without either parent species. Specifically this happens in Denmark and southern Sweden, where the edible frog is common but both parent species are absent.