When all frogs are princes: Czech hybrids (with no females) resort to ‘sexual parasitism’, cloning


Czech scientists monitoring a green frog population in the Odra River Basin for twenty years have yet to find a female of the species. Known in Czech as the Skokan zelený (which translates as “Green Jumper”), the species are found in wetlands throughout Europe, and also known as the Common Water Frog, Green Frog – or Edible Frog. While they are on the menu in France, in the Czech Republic they are protected by law as a highly endangered species. Only in the Poodří wetlands is there a purely male population. How can this be?


The Institute of Animal Physiology and Genetics at the Czech Academy of Sciences has long studied the Skokan zelený all-male enclave. Evolutionary biologist Marie Kaštánková is a member of the research team that recently published a paper about the phenomenon in the international magazine Nature.


“After years of research and genetic monitoring, we discovered that they are a hybrid line of frog where females simply do not exist. We found the males all share the same genes, meaning the same clone ancestor. And that led us to conclude we have discovered a unique population that has the same ancestors as those frogs which have spread from its source on the Odra River north to the Baltic Sea.”


Like other amphibians, the frogs of Poodří develop from a fertilized egg. But because they had no females, nature found another way to breed: Hybridogenesis – a kind of sexual parasitism.


The way it works is the hybridogenetic hybrids use gametes from their sexual host for their own reproduction, but the female’s genome is later eliminated. This occurs through a process by which sexual gametes are captured, converted to clones, and returned to sexual populations in one generation.


The Green Jumper is itself a hybrid, of a Marsh Frog and a Pool Frog. It is the Marsh Frog females that the males of the Poodří wetlands use to reproduce itself. As such, hybridogenesis is an extraordinary mode of reproduction, comprising components from both asexual and sexual reproduction.


“The Green Jumper lives in the same pond. When the mating season comes, it infiltrates the male Marsh Frog population. They start calling and luring their females. And when the females are ready to mate, the Green Jumper jumps on the female and simply fertilizes her.”


As they say, you have to kiss a lot of frogs before you find your prince.


Generally, this mode of reproduction is found in hybrids arising from historical or ongoing hybridization events. But how is it possible that in the case of the Green Jumper, only males are born?


Remarkably, this hybrid frog produces two types of sperm, and in each there is DNA of only one parent species. Only if the egg of the Marsh Frog is fertilized by a Pool Frog does a male Green Jumper results. If the egg and the sperm have the same DNA, a female Marsh Frog is born.


This was verified in laboratory experiments because in nature it is impossible not to recognize how the female originated. Evolutionary biologist Marie Kaštánková again:


“I would say that we are witnessing something a transformation, where the male is trying to become an independent species. Because the Green Jumper can produce sexual partners for his own clonal sons, he is somehow trying to evolve not to be dependent on females.”

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