Who do you love more… mom (species 1) or dad (species 2)?

The latest blockbuster ‘Jurassic World’ brings to our theaters a hybrid dinosaur. In their quest for the most terrifying creature ever, the movie’s ‘scientists’ combine traits of different dinosaur species to create the ultimate predator, which turns out to be big, insatiable… and intelligent. Quite obviously, the movie goes far beyond the state of the art of genetic engineering. That said, hybrids actually can be found everywhere in the real world. And some of them are ‘designed’ by us.

A hybrid is an individual that results from the combination of genomes of different species. Mankind has been raising hybrids from old, by controlled pairings of animals and plants to obtain desired traits in crops and cattle. For example, farmers have long been using mules (the hybrid offspring of a male donkey and a mare) to help laboring the fields, and lots of fruits, cereal crops and garden trees are hybrids selected by us to better suit our needs. Have you ever eaten frog legs? Yes, you’re right! Edible frogs are hybrids too.

Here in the Iberian Peninsula, we also have hybrid frogs. When an Iberian green waterfrog, or  Pérez’s frog (Pelophylax perezi) mates with an individual of other European species (P. ridibundus), a hybrid is formed: Pelophylax klepton grafi.  These three species together compose a hybridogenetic complex. This is because ADN of hybrid frogs contains 50% of each parental species but, amazingly, the eggs or sperm they produce contain exclusively P. ridibundus DNA. They can thus perpetuate a hybrid lineage just by mating with another P. perezi (see figure). If you think about it, they are certainly a mixture of two species but, when they mate, they genetically mimic just one of the parentals, so they perform as a ‘sexual parasite’ for the other parental species. That’s why they are called ‘klepton’ (from Greek, ‘robber’).

Example of the origin and perpetuation of a hybridogenetic lineage of Pelophylax klepton grafi. Matings may involve different sexes of each species than those in the figure. Note that, although adult hybrids are RP (2n chromosomes, half of them are from P. perezi and the other half are from P. ridibundus), they only produce R gametes, thereby discarding the whole P. perezi chromosome dotation in their germinal line. 

When hybrid frogs enter the ecosystem, they may outperform parental species, potentially leading them to extinction. Therefore, understanding the processes of hybridization and delineation of ranges of parental species and contact zones is critical for the conservation of involved species. This is challenging because all these species look extremely alike, and thus morphological identification is very difficult. For this reason, molecular tools are necessary to solve biological questions involving hybridization in water frogs.

A group of researchers from the University of Navarra, the Natural History Museum of Madrid and the Doñana Biological Station (CSIC) are developing sets of molecular tools to answer questions such as: ‘What is the distribution range of P. kl. grafi? Did this hybrid klepton originate naturally or as a result of human introductions? Can hybrids mate themselves and produce P. ridibundus offspring? Is the klepton displacing native P. perezi? These genetic markers have proven useful to distinguish among the three species within the complex and, by using them, we can assess the genetic variability of individuals to trace the history of hybrid lineages and solve these and other key issues.

So don’t panic in the theater. If a mad hybrid threatens you, we’ll be ready for it… as long as it is a frog!

Gregorio Sánchez-Montes

PhD Student

Department of Environmental Biology, University of Navarra