Hybridogenesis in water frogs

The fertile hybrids of European water frogs (genus Pelophylax) reproduce by hybridogenesis (hemiclonally).

esculentus (mostly RL genome), which is a hybridogenetic hybrid of the marsh frog P. ridibundus (RR) and the pool frog P. lessonae (LL), usually excludes the lessonae genome (L) and generates gametes of the P. ridibundus (R).

[2][3][4] The hybrid populations are propagated, however, not by the above primary hybridisations, but predominantly by backcrosses with one of the parental species they coexist (live in sympatry[9][10]) with (see below in the middle).

[11][2][3][12][9][4] Since the hybridogenetic hybrids require another taxon as sexual host to reproduce, usually one of the parental species, they are called kleptons[13][14][5] (with "kl."

There are three known hybridogenetic hybrids of the European water frogs: Hybridogenesis implies that gametes of hybrids don't contain mixed parental genomes, as normally occurs by independent chromosome segregation and crossover in meiosis (see also second Mendel's law, recombination).

[2] Hybridogenesis is a hemiclonal mode of reproduction — half of a hybrid genome is transmitted intact clonally from generation to generation (R genome in the L-E system) — not recombined with a parental species genome (L here), while the other half (L) is transmitted sexually — obtained (replaced) each generation by sexual reproduction with a parental species (sexual host[5][1][4], P. lessonae in the L-E system).

[2] Most of above hybridogenic systems consist of a hybrid coexisting (living in sympatry[9][10]) with one of the parental species required for its reproduction.

The Pelophylax esculentus complex consists of the hybrid taxon – edible frog P. kl.

esculentus (genotype RL) exclude here the P. lessonae genome (L) and make exclusively clonal P. ridibundus gametes (R).

esculentus hybrids (RL) can mate also with each other, but only 3% of resulting tadpoles (RR) survive to sexual maturity (97% do not).

[2] Triploids LLR and RRL are providers of P. lessonae (L)[2] and P. ridibundus gametes (R) respectively in this system lacking both of parental species.

[3][6] Such modified hybridogenesis[19] (or gametogenetic system[20]) occurring in allotriploid hybrids, where during meiosis chromosomes (genomes) from the doubled set (LL from LLR or RR from RRL here) are used to produce haploid gametes (L or R respectively), whereas the remaining ones may be excluded (R from LLR or L from RRL) is known as meiotic hybridogenesis.

[6] P. lessonae (LL) and P. ridibundus (RR) offspring do not survive to sexual maturity in the E system.

L, R – P. lessonae and P. ridibundus haploid genomes;LL, RR – do not survive to sexual maturity;* females only (eggs); ** L gametes are produced by LR, but author doesn't write whether they take part in reproduction or not.

It is not clear, whether the primary hybridisation which originated Graf's hybrid frog Pelophylax kl.

grafi (PR) discard the P. perezi genome (P) and make exclusively clonal P. ridibundus gametes (R).

[2] The primary hybridisation which originated Italian edible frog Pelophylax kl.

hispanicus (RB) discard the P. bergeri genome (B) and make exclusively clonal P. ridibundus gametes (R).

[2] Matting patterns of hybridogenetic water frogs don't fit precisely known definitions of hybridogenesis:[21][1][7] The Pelophylax kl.

esculentus occur between P. ridibundus females (large) and P. lessonae males (small) and later are maintained through backcrosses P. kl.

esculentus females with P. lessonae males (L–E system[2]), the expected mtDNA phenotype of P. kl.

[2] During the ice ages,[clarification needed] the population of the common ancestor of both parental species of the edible frog was split into two.

in the Danube delta
Example crosses between pool frog ( Pelophylax lessonae ), marsh frog ( P. ridibundus ) and their hybrid - edible frog ( P. kl. esculentus ). The first example is the primary hybridization-generating cross. [ 2 ] The second one is an example of hybridogenesis and occurs in the most widespread hybridogenetic L–E system, [ 11 ] [ 2 ] [ 3 ] [ 4 ] [ 9 ] [ 12 ] [ 8 ] the third example occurs in the R–E system, is less frequent in nature [ 2 ] [ 4 ] , but is considered as possible e.g., if an L-E system is invaded by P. ridibundus [ 8 ] . P. kl. esculentus × P. kl. esculentus crossings result in inviable P. ridibundus tadpoles and are not shown here. [ 2 ] [ 3 ] Large circles - adult frogs, small circles - gametes , × - lack of gametes containing genome of one of the parental species.
Edible frog Pelophylax kl. esculentus
Pelophylax kl. esculentus are a hemiclone here, because they share half of their genome ( R haplotype , red arrows). L-E system.
Typical gametogenesis in Pelophylax kl. esculentus (in the L-E system). 1 - exclusion of the P. lessonae genome, 2 - duplication ( endoreduplication ) of the P. ridibundus genome - restoration of diploidy , 3 - meiosis , L and R - P. lessonae and ridibundus genomes. [ 16 ]
Gametes of a hybridogenetic hybrid contain the unrecombined genome of one parental species (C), instead of all possible combinations of both parental (red and green) chromosomes (B).
A – somatic cell.
Hybridogenesis – L–E and R–E systems.
Hybridogenesis – All-hybrid (E) system.
Hybridogenesis in Graf's hybrid frog Pelophylax kl. grafi (P–G system).
Hybridogenesis in Italian edible frog Pelophylax kl. hispanicus (B–H system).