The mix of genes in the cells of an organism isn’t just an instruction book on how to build the cells. It’s also a history book created of all the crossings of DNA between parents. Sometimes the parents are different species, and novel DNA enters the genome. Yao Li and colleagues believed that the likelihood of hybridization varies according to climate. They examined the genomes of two East Asian Cerris oaks (Quercus acutissima and Q. chenii) to test the hypothesis that the mid-Pliocene warm climate promoted hybridization, while the Pleistocene cool climate limited cross-pollination.
The botanists wanted to examine how hybridization zones move over time. As climactic conditions vary, the ranges of plants should also change. While the retreating plants would become locally extinct, some of their genes would remain behind in their hybrid offspring. Studying the right plants could reveal a record of past cross-pollination.
Li and colleagues chose oaks for the experiment. They state in the article that oaks are helpful for studying natural hybridization because they often exchange genes without disrupting species integrity. There are also plenty of oak species in East Asia, with over 100 species. The team chose Q. acutissima and Q. chenii to study as they’re sister species that diverged between ten and thirty million years ago. This length of time means they have been interacting since at least the Miocene.
Li and colleagues set out to examine the interaction between the two species from the mid-Pliocene, around three million years ago. The team examined nuclear microsatellites that plants inherited from their ancestors’ paternal and maternal sides. They also examined chloroplast DNA, inherited only down the maternal route. They combined this with fossil evidence, niche modelling and Bayesian clustering analysis to reconstruct gene flow in past eras.
The scientists found that the current genomic mix of East Asian Cerris oaks is due to effects from both Pliocene and Pleistocene climates. This is different from the results found in Europe, where white oaks were limited in their hybridization by the last glacial-interglacial cycle. Li and colleagues conclude: “Our results indicate that the hybridization history between sister species occurring in areas less affected by glaciations is more complex than that for species seriously threatened by Pleistocene climate change. The impact of pre-Quaternary climate should be taken into account especially when exploring ancient gene flow among species native to East Asia.”
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Li, Y., Zhang, X., Wang, L., Sork, V.L., Mao, L. and Fang, Y. (2021) “Influence of Pliocene and Pleistocene climates on hybridization patterns between two closely related oak species in China,” Annals of Botany. https://doi.org/10.1093/aob/mcab140