All the biodiversity on our planet has originated through a series of speciation events. Speciation occurs when two lineages cease interbreeding, for one reason or another, but even after speciation the new species commonly retain the ability to hybridise for a while. Soon after speciation, the species genomes may be similar enough to allow for gene exchange (introgression) but over time, as the species gradually accumulate mutations and become less and less similar and compatible, the hybrids tend to become less fertile and also to form less often. Eventually the parental species will no longer be able to hybridise. But how for how long do they retain the ability to form hybrids?

Here I have collected a few examples of pairs of species and lineages that have retained the ability to hybridise for an extraordinary length of time.

In sturgeons, the Russian sturgeon (Acipenser gueldenstaedtii) and the American paddlefish (Polyodon spathula) can be hybridised and form offspring in captivity, despite the parental species being reported to have diverged in the Jurassic, some 184 million years (Ma) ago! (Káldy et al., 2020).

In ferns, naturally occurring hybrids between two species (Cystopteris fragilis and Gymnocarpium dryopteris) that diverged ca 60 Ma ago are known from the Pyrenees (Rothfels et al., 2015).

In flowering plants, to my knowledge, the hybrid between the most divergent parents occurs in Horse-chestnut, Aesculus. In this genus, the European Ae. hippocastanum and North American Ae. pavia diverged some 47—67 Ma ago, but hybridise in cultivation (Du et al., 2020; Upcott, 1936; Xiang et al., 1998). Their hybrid has since undergone a whole-genome duplication and thereby become a separate, fertile species, Ae. carnea, which is frequently cultivated as an ornamental tree.

What are common for these lineages? Two things stand out:

Firstly, and most fundamentally, these species have retained the ability to hybridise for a very long time because their genomes evolve extremely slowly. Both sturgeon and ferns are commonly referred to as “living fossils”, lineages that have retained the same morphology for millions of years, judging from the fossil record. The same can be said about Aesculus, which are mostly trees with long generation time and also exceptionally slow nucleotide substitution rates; at least the European species is considered a Tertiary relic where it grows in the wild in the Balkans.

Secondly, in two of these examples , hybridisation has occurred because humans have brought together in captivity species that do not meet in nature. Because they never occur together, they also never evolved barriers against hybridisation.

Literature:

Du, Z.-Y., Harris, A. J., & Xiang, Q.-Y. (2020, 2020/04/01/). Phylogenomics, co-evolution of ecological niche and morphology, and historical biogeography of buckeyes, horsechestnuts, and their relatives (Hippocastaneae, Sapindaceae) and the value of RAD-Seq for deep evolutionary inferences back to the Late Cretaceous. Molecular Phylogenetics and Evolution, 145, 106726. https://doi.org/10.1016/j.ympev.2019.106726

Káldy, J., Mozsár, A., Fazekas, G., Farkas, M., Fazekas, D. L., Fazekas, G. L., Goda, K., Gyöngy, Z., Kovács, B., Semmens, K., Bercsényi, M., Molnár, M., & Patakiné Várkonyi, E. (2020). Hybridization of Russian Sturgeon (Acipenser gueldenstaedtii, Brandt and Ratzeberg, 1833) and American Paddlefish (Polyodon spathula, Walbaum 1792) and Evaluation of Their Progeny. Genes, 11(7), 753. https://www.mdpi.com/2073-4425/11/7/753

Rothfels, C. J., Johnson, A. K., Hovenkamp, P. H., Swofford, D. L., Roskam, H. C., Fraser-Jenkins, C. R., Windham, M. D., & Pryer, K. M. (2015). Natural hybridization between genera that diverged from each other approximately 60 million years ago. The American Naturalist, 185(3), 433–442. https://doi.org/https://doi.org/10.1086/679662

Upcott, M. (1936, 1936/08/01). The parents and progeny of Aesculus carnea. Journal of Genetics, 33(1), 135-149. https://doi.org/10.1007/BF03027607

Xiang, Q.-Y., Crawford, D. J., Wolfe, A. D., Tang, Y.-C., & DePamphilis, C. W. (1998). Origin and biogeography of Aesculus L. (Hippocastanaceae): A molecular phylogenetic perspective. Evolution, 52(4), 988-997. https://doi.org/10.2307/2411231