Why Are Some Inter-species Hybrids Viable and Others Not?

Yeasts, plants, fish and amphibians (in contrast to mammals) can however produce fertile hybrids. According to Professor of Molecular Developmental Biology Gert Jan Veenstra: “In frogs a duplication of chromosomes can appear, in which the whole set of chromosomes from the father and the mother are passed on to the next generation.”

However, there is another problem: some hybrids are not viable, while the crossbreeding the other way around is. The embryos are genetically identical, but there is a difference in viability, depending on the father species and the mother species. As Veenstra explains, “even though it is vital for evolution, the mechanisms of viable and non-viable hybrids are to this date unknown.”
Crossbreeding the African and Western clawed frog

The scientists demonstrated this phenomenon in research on two related frog species: Xenopus tropicalis and Xenopus laevis (also known as the Western and African clawed frogs, respectively). When a female African clawed frog is crossbred with a male Western clawed frog, the embryos are viable. However, the other way around, crossbreeding a male African clawed frog and a female Western clawed frog leads to embryos that die in the early stages of development. The reason for this, remained unclear. 

Credit: Atsushi Suzuki

On 10 January, researchers published what went wrong during this crossbreeding: the maternal molecular machinery of the Western clawed frog cannot fully recognize the paternal chromosomes of the African clawed frog. Two specific pieces of the paternal chromosomes are incompatible with the maternal cell and thus the separation of the chromosomes during cell division is disrupted. These cells now lack a large number of important genes, such as genes for metabolism, and therefore quickly die.

Credit: Atsushi Suzuki

This shows that there is a pronounced asymmetry when it comes to hybrids, depending on the father species and the mother species. “These findings are important, because these type of hybrids are present in nature and in some cases lead to new species. When new species are formed, there seems to be a period of transition: closely related species are able to produce viable offspring, but if the chromosomes are no longer compatible this leads to asymmetric results of crossbreeding. When species further separate, crossbreeding no longer leads to viable offspring. We here show the cellular mechanism behind this phenomenon,” states Gert Jan Veenstra.

The viable hybrid of a male Western clawed frog and a female African clawed frog also revealed a molecular mechanism: parasitic DNA elements (transposons) are activated in one of the genomes. According to Veenstra,“the female’s immune system is not wired to recognise the paternal transposons and hence does not repress them. As a consequence, parasitic DNA elements are now able to fulfill a new role: they can act as regulatory DNA that influences gene activity. This may have a significant influence on formation and characteristics of a new species.” 

Contacts and sources:
 Prof. dr. Gert Jan Veenstra, professor of Molecular Developmental Biology: