The banyan tree looks odd for a reason

The banyan tree (Ficus macrocarpa) is known for its rather odd appearance, thanks to the ability to sprout roots from branches that eventually reach the soil.

Perhaps less well known is that it, like others in the Ficus species, has co-evolved with the wasp that pollinates it. The size and shape of wasps correspond exactly to those of the fig fruits, and each fig species creates a unique perfume to attract that wasp.

Now researchers in the US and China think they know how and why this happens. Perhaps not surprisingly, it’s all about the genome.

In a paper in the journal Cell, a team led by Ray Ming form the University of Illinois and Jin Chen, of the Chinese Academy of Sciences, describes its analysis of the banyan tree, its pollinating wasp, and a related fig tree, Ficus hispida.

Unlike F. microcarpa, which produces aerial roots and bears male and female flowers on the same tree, F. hispida produces distinct male and female trees and no aerial roots.

“When we sequenced the trees’ genomes, we found more segmental duplications in the genome of the banyan tree than in F. hispida, the fig without the aerial roots,” Ming says. “Those duplicated regions account for about 27% of the genome.”

The duplications were found to increase the number of genes involved in the synthesis and transport of auxins, a class of hormones that promote plant growth. 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,” Ming says, adding that 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 they turned to the fig wasp, and compared it with related wasps, the researchers observed that it was retaining and preserving genes for odourant receptors that detect the same smelly compounds the fig trees produce – a likely signal of co-evolution.

They also report discovering a Y chromosome-specific gene that is expressed only in male plants of F. 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. But Ficus species that have male and female flowers together on one plant have only one copy of this gene,” Ming says. “This strongly suggests that this gene is a dominant factor affecting sex determination.”

Understanding this evolutionary history is important, the researchers says, because the ability to produce large fruits in a variety of habitats makes Ficus a keystone species in most tropical forests.