Hybridogenesis in water frogs

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Reference excerpt

The fertile hybrids of European water frogs (genus Pelophylax) reproduce by hybridogenesis (hemiclonally). This means that during gametogenesis, they discard the genome of one of the parental species and produce gametes of the other parental species (containing a genome not recombined with the genome of the first parental species). The first parental genome is restored by fertilization of these gametes with gametes from the first species (sexual host). In all-hybrid populations of the edible frog Pelophylax kl. esculentus, however, triploid hybrids provide this missing genome. Because half of the genome is transmitted to the next generation clonally (not excluded unrecombined intact genome), and only the other half sexually (recombined genome of the sexual host), the hybridogenesis is a hemiclonal mode of reproduction. For example, the edible frog Pelophylax kl. 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). In other words, edible frogs produce gametes of marsh frogs. 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) with (see below in the middle).

Since the hybridogenetic hybrids require another taxon as sexual host to reproduce, usually one of the parental species, they are called kleptons (with "kl." in scientific names).

There are three known hybridogenetic hybrids of the European water frogs:

edible frog Pelophylax kl. esculentus (usually genotype RL):pool frog P. lessonae (LL) × P. ridibundus (RR) Graf's hybrid frog Pelophylax kl. grafi (PR):Perez's frog P. perezi (PP) × P. ridibundus (RR) orPerez's frog P. perezi (PP) × edible frog P. kl. esculentus (RE)(it is unclear which one crossing was the primary hybridisation) Italian edible frog Pelophylax kl. hispanicus (RB):Italian pool frog P. bergeri (BB) × P. ridibundus (RR)

Parental genome exclusion 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). Instead, each gamete carries a complete (haploid) genome of only one parent species. Usually one entire genome of the parental species is excluded prior to meiosis during gametogenesis, such that only one (remaining) parental genome is represented among gametes and genes from the other parent are not passed on by the hybridogen. This discarding occurs gradually during subsequent mitotic divisions, not in one step.

Hemiclones 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, P. lessonae in the L-E system).

Hybridogenetic systems overview There are at least three hybridogenetic species (hybrids) of water frogs in Europe – edible frog Pelophylax kl. esculentus, Graf's hybrid frog Pelophylax kl. grafi and Italian edible frog Pelophylax kl. hispanicus. Their mating patterns are classified into several hybridogenetic systems:

(capital abbreviations below scientific names are genotypes) All these hybrids contain genome of marsh frog P. ridibundus (R) and genome of second parental species (L, P or B). Most of above hybridogenic systems consist of a hybrid coexisting (living in sympatry) with one of the parental species required for its reproduction. P. kl. esculentus for example in the most frequent L-E system must mate with P. lessonae to produce new hybrids, in the R-E system with P. ridibundus. Because these hybrids depend on other taxa as sexual hosts to reproduce ("parasitize" on them sexually), they are kleptons ("kl." in scientific names).

Edible frog Pelophylax kl. esculentus The Pelophylax esculentus complex consists of the hybrid taxon – edible frog P. kl. esculentus (genotype RL) and parental species – marsh frog P. ridibundus (RR) and pool frog P. lessonae (LL). Hybrids are females and males, which is unusual, because hybrids of other hybridogenic species are only females. The primary hybridisation originating P. kl. esculentus (genotype RL) is:

P. lessonae (LL) × P. ridibundus (RR) It occurs between P. lessonae (LL) males and P. ridibundus (RR) females, because smaller P. lessonae males prefer larger females. The lineages of hybrids are maintained later through other matings, described below. P. lessonae and P. ridibundus have distinct habitat requirements and usually don't live together.

P. lessonae – P. kl. esculentus (L–E) system The P. lessonae – P. kl. esculentus (L–E, LE, lessonae–esculentus) system is most widespread hybridogenetic system. It is found in Western Europe. Hybrids P. kl. esculentus (genotype RL) exclude here the P. lessonae genome (L) and make exclusively clonal P. ridibundus gametes (R). In other words, edible frogs produce gametes of marsh frogs! Their lineages are maintained usually through backcrosses of a female P. kl. esculentus (RL) with a male P. lessonae (LL). The offspring consist of only P. kl. esculentus. P. kl. esculentus hybrids (RL) can mate also with each other, but only 3% of resulting tadpoles (RR) survive to sexual maturity (97% do not). The genomes of interhybrid crosses are female, because of carrying X chromosomes of females from primary hybridisation.

P. ridibundus – P. kl. esculentus (R–E) system The P. ridibundus – P. kl. esculentus (R–E, RE, ridibundus–esculentus) system inhabits Eastern Europe. It is essentially a reverse form of the L–E system. Hybrids P. kl. esculentus (genotype RL) exclude here the P. ridibundus (R) or P. lessonae (L) genome in a 3:1 ratio and make mainly clonal P. lessonae (L), less P. ridibundus gametes (R). One frog produce either L or R gametes or a mixture of both. Their lineages are maintained through backcrosses of a male P. kl. esculentus (RL) with a female P. ridibundus (RR). The offspring consist of P. kl. esculentus males (75%) or P. ridibundus females (25%). This is called hybrid-amphispermy.

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Illustrations

Hybridogenesis in water frogs: 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, @media screen{html.skin-theme-clientpref-night .mw-parser-output div:not(.notheme)>.tmp-color,html.skin-theme-clientpref-night .mw-parser-output p>.tmp-color,html.skin-theme-clientpref-night .mw-parser-output table:not(.notheme) .tmp-color{color:inherit!important}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output div:not(.notheme)>.tmp-color,html.skin-theme-clientpref-os .mw-parser-output p>.tmp-color,html.skin-theme-clientpref-os .mw-parser-output table:not(.notheme) .tmp-color{color:inherit!important}}× - lack of gametes containing genome of one of the parental species. — 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, @media screen{html.skin-theme-clientpref-night .mw-parser-output div:not(.notheme)>.tmp-color,html.skin-theme-clientpref-night .mw-parser-output p>.tmp-color,html.skin-theme-clientpref-night .mw-parser-output table:not(.notheme) .tmp-color{color:inherit!important}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output div:not(.notheme)>.tmp-color,html.skin-theme-clientpref-os .mw-parser-output p>.tmp-color,html.skin-theme-clientpref-os .mw-parser-output table:not(.notheme) .tmp-color{color:inherit!important}}× - lack of gametes containing genome of one of the parental species.

Hybridogenesis in water frogs: Pelophylax kl. esculentus are a hemiclone here, because they share half of their genome (R haplotype, red arrows). L-E system. — Pelophylax kl. esculentus are a hemiclone here, because they share half of their genome (R haplotype, red arrows). L-E system.

Hybridogenesis in water frogs: 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] — 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]

Worked examples

Example 1 — a first encounter with Hybridogenesis in water frogs

Start with the simplest possible case. Write down what Hybridogenesis in water frogs claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Hybridogenesis in water frogs before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Hybridogenesis in water frogs ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Hybridogenesis in water frogs

In research: Hybridogenesis in water frogs appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Hybridogenesis in water frogs in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Hybridogenesis in water frogs is common in secondary-school and first-year university syllabi. It links to neighbouring topics Amphibia hybrids, Asexual reproduction in animals, so understanding it makes those chapters shorter.
In everyday life: Look for Hybridogenesis in water frogs outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Hybridogenesis in water frogs in 20 minutes

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Frequently asked questions

What is Hybridogenesis in water frogs in simple terms?

The fertile hybrids of European water frogs (genus Pelophylax) reproduce by hybridogenesis (hemiclonally). This means that during gametogenesis, they discard the genome of one of the parental species and produce gametes of the other parental species (containing a genome not recombined with the geno…

Why does Hybridogenesis in water frogs matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Hybridogenesis in water frogs?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

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It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Hybridogenesis in water frogs.