Allegany County, NY (USA)

Hybrids and sperm thieves: amphibian kleptons

In biology a hybrid is the result of the reproduction of two genetically different parents, although in most cases hybrids are either unviable or sterile. Yet in some species of amphibians, sometimes hybrids are not only viable, but also become new species with special characteristics. In this entry we’ll show you two cases of amphibian hybrids that form what is known as a klepton and that make the definition of species a little less clear.

WHAT IS A KLEPTON?

A klepton (abbreviated kl.) is a species which requires another species to complete its reproductive cycle. The origin of the word klepton comes from the Greek word “kleptein” which means “to steal”, as the klepton “steals” from the other species to reproduce. In the case of amphibians, kleptons have originated from hybridation phenomena. The amphibian’s potent sexual pheromones and the multispecies choirs in the case of anurans, causes some males and females of different species to try to mate together. Yet hybrids are only viable between closely related species.

Among the different klepton species we can encounter two different methods depending on the type of conception: zygokleptons, in which there’s fusion between the egg and the sperm’s genetic material, and gynokleptons, in which the egg needs the stimulation from the sperm but doesn’t include its genetic material.

The different amphibian kleptons are usually constituted entirely by females (there are usually few or no males) that use the sperm of another species to perpetuate the klepton. As some kleptons depend on various related species, this can promote the creation of “species complexes” in which various similar species present hybridization areas and very complicated relationships among them. Below you’ll find two klepton examples, one in European anurans and the other in American urodeles.

HYBRIDOGENESIS IN WATER FROGS

The European water frogs (Pelophylax genus) form what is known as a “hybridogenetic complex” in which the hybrids from different species form kleptons which can’t reproduce among each other but, which must reproduce with a member of one of the parental species, “stealing” or “parasitizing” its sperm in order to survive.

Pelophylax_esculentus_-_Amplexus_01
Photo by Bartosz Cuber of two edible frogs (Pelophylax kl. esculentus) in amplexus. This is the best known hybrid both because of its wide distribution, and for being considered a delicacy in France.

In the hybridogenesis of water frogs the genetic material of both parents combines to form the resulting hybrid (zygoklepton). This hybrids (almost always females) will have half their genome from one species and half from the other. Yet, not being able to reproduce with a similar hybrid, during gametogenesis the hybrids eliminate the genetic material from one of the parent species. This way, when reproducing with an individual from the species whose genetic material has been deleted, they will form another hybrid.

Hybirds
Scheme of the genetic composition of the different Pelophylax kleptons. In this hybridogenetic complex four “natural” species intervene: the marsh frog (Pelophylax ridibundus, RR genome), the pool frog (Pelophylax lessonae, LL genome), the Iberian waterfrog (Pelophylax perezi, PP genome)  and the Italian pool frog (Pelophylax bergeri, BB genome)

The edible frog (Pelophylax kl. esculentus, RL genome) comes from the hybridization between the marsh frog and the pool frog. The Italian edible frog (Pelophylax kl. hispanicus, RB genome) stems from a hybrid between the marsh frog and the Italian pool frog. Finally, the Graf’s hybrid frog (Pelophylax kl. grafi, RP genome) originated from the hybridization between the edible frog (in which the DNA of the pool frog is eliminated from their gametes) and the Iberian waterfrog.

Hybridogenesiisisisi
Schemes by Darekk2 of the hybridogenetic processes in the different European water frog’s kleptons. The bigger circles represent the individual’s genome and the smaller circles the gametes’ genetic material.

As we can see, the genetic information of the marsh frog is the only one present in all three kleptons. These kleptons delete the genetic material of the species with which they share their habitat from their gametes but keep the genetic material of the marsh frog (R). So for example, the edible frog (P. kl esculentus) deletes form its eggs the DNA of the pool frog (L) with which it encounters and breeds in its natural range, resulting in more edible frogs (RL). The marsh frog seldom reproduces with some of its hybrids and if it does, they produce normal marsh frogs.

SALAMANDERS WITH SEVERAL GENOMES

The salamanders of the Ambystoma genus, usually known as mole salamanders, are a genus endemic of North America and are the only living representatives of the Ambystomatidae family. Five of these species form what is known as the “Ambystoma complex”, in which these species contribute to the genetic composition of a unisexual lineage of salamanders which reproduce by gynogenesis (gynoklepton). Based on the mitochondrial DNA of the unisexual populations, it is thought that this complex originated from a hybridization event of about 2.4-3.9 million years ago.

ambystomert complexx
This complex consists of the five following species: the blue-spotted salamander (Ambystoma laterale LL genome, photo by Fyn Kynd Photography), the Jefferson salamander (Ambystoma jeffersonianum JJ genome, photo by Vermont Biology), the small-mouthed salamander (Ambystoma texanum TT genome, photo by Greg Schechter), the streamside salamander (Ambystoma barbouri BB genome, photo by Michael Anderson) and the tiger salamander (Ambystoma tigrinum TiTi genome, photo by Carla Isabel Ribeiro).

In the gynogenesis of this all-female lineage, the egg needs activation by a sperm to start division and development but, it first has to duplicate its genetic material by endomitosis to avoid the formation of an unviable haploid (with half the genetic information) zygote. Yet, as in parthenogenetic reptiles, in the long term the lack of genetic recombination can take its toll on the individuals. That’s why this lineage of unisexual salamanders has the capacity of occasionally incorporating the whole genome from the males of four of the species which constitute the complex (currently the reproduction of streamside salamanders with members of the unisexual lineage hasn’t been documented).

ginogino
Scheme from Bi, Bogart & Fu (2009) in which we can see the different paths that the gynogenetic mole salamanders can take while reproducing.

These individuals don’t mix the newly acquired genome, they add it. Therefore, among the members of this lineage we can find diploid, triploid, tetraploid and even pentaploid individuals (even if as the ploidy increases the individuals are less apt to survive) depending on how many different genomes the previous generations had incorporated.

mes ibrids
Among the klepton, the most common genome combination is that of triploids based on the blue-spotted salamander and the Jefferson salamander, with the genomes LLJ (left, image by David Misfud) and JJL (right, image by Nick Scobel), even though the number of combinations is incredibly large. For this reason why scientists haven’t been able to decide a valid scientific name for this group of hybrid origins.

Unlike the water frogs, it is very difficult to define a scientific name for the klepton inside Ambystoma, as the genomes of the different species can be found in different combinations and proportions in different unisexual individuals.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

Difusió-anglès

8 pensaments a “Hybrids and sperm thieves: amphibian kleptons”

  1. In insects, this particular ways of reproduction are named arrhenotoky, thelytoky and deutherotoky and it is part of a conplex system of reproduction strategies. For example, Hymenoptera show reproductive strategies like this.
    Furthermore insects, I knew it in some marine fishes. I can remember Amazon molly (Poecilia formosa). Is new for me in amphibians.

    1. Right, but thelytoky and arrhenotoky all occur into the same insect species, while hibridogenesis and gynogenesis occur in hybrids between two different species which must reproduce with some of the parental species. But I didn’t know the case of Poecilia formosa, which from what I just read is a case of gynogenesis like the ambystoma complex. Good to know!!😉

  2. In water frogs the situation is way more complex than you describe here. Including triploid x diploid populations, interspecies gene flow, and strong sex-bias. It is a great system, but very complex.

  3. Thanks for the artical.
    Can this hybread frog forward his father’s spicy’s gens back and help in the breeding of the “orignal” kind? even if he just return what he took ^_^

    1. Well, the different kleptons of european water frogs usually only keep the genetic material of the marsh frog in their gametes, as they usually only reproduce with the other parental species (thus creating the hybrids). So, if a regular diploid hybrid were to reproduce with a marsh frog, it will produce a regular marsh frog.

      1. Thanks for the rapid replay.
        It’s intresting,
        The kleptons usually reproduce with the other parental species just like the regular marsh frog usualy reproduce with his own species.
        If it’s about same rate of exceptions in both cases it’s even keep on natural balance.
        Is there studies about the benefits and evolutionary reasons of this phenomenon?

      2. Well, the author named Bogart has made many different articles about the gynogenesis in Ambystoma in which he discusses about the evolutionary advantadges of these kind of systems. You have some links at the bottom of this article.

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