Hybridogenesis in Water Frogs


Several water frogs from the genus Pelophylax have a hybrid origin and perpetuate by hybridogenesis, a peculiar mode of reproduction ruled by complex phenomena such as clonality and polyploidy, and which can constitute a transient stage towards the formation of novel species. Different kinds of hybridogenetic complexes have been documented throughout Europe, and a tremendous diversity of breeding systems allows their maintenance in space and time, each with its own subtilities to bypass the numerous challenges posed by a semi‚Äźsexual life cycle. The other side of the coin is that hybridogenesis can boost the invasive potential of introduced frogs by altering the fragile dynamics of these natural populations, hence illustrating the thin line that separates ephemeral hybrid forms versus perennial reticulate taxa.

Key Concepts

  • Hybrids between diverged species can avoid sterility by abandoning sexual reproduction.
  • Hybridogenetic hybrids typically transmit a single, clonal genome to offspring.
  • Clonality promotes genetic decay and the need to breed with a sexual species.
  • Reliance on a sexual host can be bypassed by polyploidy.
  • Clonal genomes have various opportunities to recombine in populations.
  • Genetic integrity, ecomorphological differentiation and sexual independence of hybridogenetic hybrids may represent steps towards hybrid speciation.
  • Hybridogenetic systems can collapse due to invasive parental species.

Keywords: edible frog; hemiclone; hybridisation; hybridogenesis; hybrid speciation; marsh frog; Pelophylax; polyploidy; pool frog

Figure 1. Steps of genome elimination in Pelophylax hybridogens, exemplified here by a L‐eliminating L individual obtained from a P. fortis (FF) × P. lessonae (LL) cross. When gonads maturate (during tadpole development), one selected genome (here the L) is progressively eliminated from the germline and degraded by autophagy. The remaining F genome is then endoreplicated, and subsequent meiosis only produces gametes with that genome, which is thus transmitted in a clonal manner (noted). Source: Adapted from Dedukh D, Riumin S, Chmielewska M et al. Micronuclei in germ cells of hybrid frogs from Pelophylax esculentus complex contain gradually eliminated chromosomes. Scientific Reports 10: 8720.
Figure 2. (a) Primary hybridisation scheme (L‐E‐F) and breeding systems enabling the maintenance of LF hybridogens (‘P. esculentus’) through backcrossing with P. l. lessonae (L‐E systems) and P. fortis (F‐E systems). The eliminated genomes (−) and the clonal genomes ([]) are shown. (b) Maintenance in all‐hybrid populations with triploids, obtained by the production of diploid gametes (schematised eggs and sperm). In the well‐studied Baltic populations, LF eggs develop in LLF or LFF individuals upon fertilisation by L and F sperm, respectively. These triploids recombine the genome in double dose (labelled L‐rec and F‐rec). Additional genotypes are produced but do not actively contribute to the populations, e.g. LL and FF individuals clones are not fit. Note that triploids are also found in some L‐E and F‐E systems from central and eastern Europe.
Figure 3. Native distribution ranges of Pelophylax taxa involved in hybridogenetic complexes, and breeding systems of the LF hybridogens (‘P. esculentus’), compiled from a literature search spanning 702 localities taken from 101 different publications. The presence of polyploids is reported, when documented. Note that the diversity of western European ranges has been modified by introductions and biological invasions (not shown): P. l. lessonae is admixed by P. l. bergeri (Dufresnes and Dubey, ), and P. fortis is now found all the way through the Atlantic and Mediterranean coasts (Pagano et al., ). Photo: LF hybridogen (credit: CD). Source: Compilation by the authors.
Figure 4. Maintenance of the P. perezi / fortis hybridogens in France (‘P. grafi’, PF) and P. l. bergeri / fortis hybridogens in Italy (BF). Original crosses are unclear, as they may involve either the parental P. fortis (FF), or the L hybridogen P. esculentus. In both cases, the F genome is hemiclonally transmitted, and hybrids rely on the associated sexual parent (P. perezi and P. l. bergeri).
Figure 5. The water frogs involved in hybridogenesis, with parental species on the left, and associated hybridogens on the right (arranged as in the top‐right labels). Photo credit as follows: P. fortis: M. Denoël; P. l. lessonae: CD; P. l. bergeri: S. Dubey; P. perezi: CD; LF hybrid: CD; BF hybrid: W. Beukema; PF hybrid: CD.


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Further Reading

Avise JC (2008) Clonality: the Genetics, Ecology, and Evolution of Sexual Abstinence in Vertebrate Animals. Oxford University Press: Oxford.

Beukeboom L and Perrin N (2014) The Evolution of Sex Determination. Oxford University Press: Oxford.

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Coyne JA and Orr HA (2004) Speciation. Sinauer Associates: Sunderland, MA.

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Mallet J (2008) Hybridization, ecological races and the nature of species: empirical evidence for the ease of speciation. Philosophical Transactions of the Royal Society of London B: Biological Sciences 363: 2971–2986.

Ogielska M (2009) Reproduction of Amphibians. Biological Systems in Vertebrates. Science Publishers: Enfield, NH.

Schmid M, Evans BJ and Bogart JP (2015) Polyploidy in amphibia. Cytogenetics and Genome Research 145: 315–330.

Schön I, Martens K and van Dijk P (2009) Lost Sex. The Evolutionary Biology of Parthenogenesis. Springer: Dordrecht.

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How to Cite close
Dufresnes, Christophe, and Mazepa, Glib(Dec 2020) Hybridogenesis in Water Frogs. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0029090]