13 May 2022

From snakes to frogs

Frogs got snake DNA thanks to parasites

Vasilisa Kostina, N+1

Biologists have discovered horizontal gene transfer between reptiles and amphibians. They estimate that Bovine-B transitioned from snakes to frogs at least 54 times between 85 and 1.3 million years ago. The study is published in the journal Molecular Biology and Evolution (Kurabayashi et al., Geography-Dependent Horizontal Gene Transfer from Vertical Predators to Their Prey).

Horizontal transfer is the transfer of genetic material between organisms through a mechanism other than reproduction. The mechanism is well known in prokaryotes, for which it serves as the driving force of evolution, but there is increasing evidence that this process occurs throughout the tree of life. In eukaryotes, horizontal transfer is carried out by transposons — mobile DNA sequences present in all eukaryotic genomes. They can be amplified within the genome and contribute to segmental rearrangements of the genome on both autosomes and sex chromosomes.

A subspecies of transposons are retrotransposons, they spread in the genome by copying and pasting themselves. An example is Bovine-B — it is a long retrotransposon originally found in cattle. Studies have shown that Bovine-B is transmitted vertically in snakes and lizards, and in ruminants it appears as a result of horizontal transfer from reptiles. However, the details of the mechanism of interspecific transfer remained unknown.

Atsushi Kurabayashi from the Nagahama Institute of Biological Sciences and Technology and his colleagues from Australia, America, Germany, Madagascar, South Africa and Japan tested the DNA of 106 species of snakes and 149 species of frogs from around the world for the presence of Bovine-B and plotted its transfer between species.

Scientists have identified nodes where different families diverged, and the topology of the corresponding branches differed from the general phylogenetic consensus. The research team estimates that the retrotransposon switched from snake to frog at least 54 times between 85 and 1.3 million years ago.

The reconstruction of the geographical area for the estimated cases of horizontal gene transfer showed their widespread distribution around the world, but with an uneven geographical distribution. The frequency of cases that occurred during the last 50 million years (that is, with the current continental arrangement formed by the collision of India and Eurasia) in Madagascar was significantly higher than in all other regions.

Most of the Madagascar biota is of continental origin. In particular, two lines of frogs, Heterixalus and Ptychadena mascareniensis, migrated from the continental part of Africa to Madagascar 19-30 and 9.8–22.7 million years ago, respectively. During the geographical analysis, it turned out that by horizontal transfer, Bovine-B overtook these amphibians already on the island. The result is confirmed by the correlation of the estimated age of horizontal transfer with the age of settlement of frogs and the fact that no retrotransposon Bovine-B was found in closely related species living in the continental part of Africa.

Frogs with transposons have been found in various clades. BovB of the frog family Bufonidae is closely related to Afrotyphlops punctatus— a representative of the snake infraorder Scolecophidia. This observation indicates the transfer of Bovine-B from an early divergent group of snakes to a common ancestor of bufonid frogs about 64 million years ago, and since then the transposon has been inherited by modern bufonid lines.

Transposon exchange also took place within both studied classes — Bovine-B Malagasy nematodes isolated from mantellid frogs were found in frogs of the Hyperoliidae family. The researchers also found more than ten snakes in which the structure of Bovine-B did not reflect the phylogeny of the host. So, in two Malagasy boas, whose transposons were supposed to be inherited from the Henophidia clade, Bovine-B was detected from representatives of the family Pseudoxyrhophiidae. These cases are explained by the horizontal transfer of the transposon from snake to snake, in which the newly integrated Bovine-B began to dominate the original one.

Then the scientists decided to find out whether the retrotransposon was transmitted directly or with the participation of intermediate organisms. They collected 97 individuals of 42 species of parasites associated with reptiles and amphibians. Among them were worms, scrapers, intradermal mites specific to frogs, as well as leeches and mosquitoes. Bovine-B was found in PCR products of arthropods, as well as round and annelid worms.

The transposons found in the parasites originated from species other than the final hosts. So, Bovine-B mites from the family Trombiculidae, found in the Madagascar frog Blommersia blommersae, turned out to be more closely related to the Malagasy snake Liophidium torquatum, than with its host (the similarity of sequences is 99.8 and 96.1 percent, respectively).

And the transposon of the nematode Cosmocerca simile found in the Bovine-B-negative Japanese crustacean frog (Buergeria japonica) was included in the clade of snakes of the family Colubridae and turned out to be completely identical to the sample belonging to the Japanese snake Elaphe climacophora.


Scheme of horizontal transfer of Bovine-B by the tick Trombiculidae sp. and the nematode Cosmocerca simile.

The thick and thin arrows show the direction of transfer and the similarity of sequences between taxa, respectively. Drawings from the article by Kurabayashi et al.

Due to this, the aforementioned parasites moving between hosts can be considered potential carriers of Bovine-B snake-type frogs.

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