An international team of scientists from China, Taiwan and the United States has successfully sequenced and analyzed the genomes of the Chinese banyan tree (Ficus microcarpa), which is famous for its extraordinary aerial roots; Ficus hispida, a related fig species lacking aerial roots; and Eupristina verticillata, a wasp coevolving with the banyan tree. They’ve identified regions in the banyan tree’s genome that promote the development of its unusual aerial roots and enhance its ability to signal its wasp pollinator, and found a sex-determining region in Ficus hispida.
“Understanding the evolutionary history of Ficus species and their wasp pollinators is important because their ability to produce large fruits in a variety of habitats makes them a keystone species in most tropical forests,” said Professor Ray Ming, a researcher at the University of Illinois, Urbana-Champaign.
To better understand their evolutionary developments, Professor Ming and colleagues analyzed the genomes of the two fig species, Ficus microcarpa and Ficus hispida, along with that of Eupristina verticillata, a wasp that pollinates Ficus microcarpa.
“When we sequenced the trees’ genomes, we found more segmental duplications in the genome of the banyan tree than in Ficus hispida, the fig without the aerial roots,” Professor Ming said.
“Those duplicated regions account for about 27% of the genome.”
The duplicated regions also contained genes involved in plant immunity, nutrition and the production of volatile organic compounds that signal pollinators.
“The levels of auxin in the aerial roots are five times higher than in the leaves of trees with or without aerial roots,” Professor Ming said.
“The elevated auxin levels appear to have triggered aerial root production. The duplicated regions also include genes that code for a light receptor that accelerates auxin production.”
When the researchers studied the genome of the Eupristina verticillata wasp and compared it with those of other related wasps, they observed that the wasps were retaining and preserving genes for odorant receptors that detect the same smelly compounds the fig trees produce.
These genomic signatures are a signal of coevolution between the fig trees and the wasps.
The authors also discovered a Y chromosome-specific gene that is expressed only in male plants of Ficus hispida and three other fig species that produce separate male and female plants, a condition known as dioecy.
“This gene had been duplicated twice in the dioecious genomes, giving the plants three copies of the gene,” Professor Ming said.
“But Ficus species that have male and female flowers together on one plant have only one copy of this gene.”
“This strongly suggests that this gene is a dominant factor affecting sex determination.”
The results were published in the journal Cell.
Xingtan Zhang et al. Genomes of the Banyan Tree and Pollinator Wasp Provide Insights into Fig-Wasp Coevolution. Cell, published online October 8, 2020; doi: 10.1016/j.cell.2020.09.043