Banyan trees make up more than 700 species of fig trees and are well-known for their aerial roots. A genomic study on the fig tree Ficus macrocarpa reveals its species-specific mutualism system with wasp pollinators.
The study by the University of Illinois, Fujian Agriculture and Forestry University, the Chinese Academy of Sciences, Taiwan University, and East China Normal University was published in the journal Cell. The researchers also observed the difference between several Ficus species.
For example, F. macrocarpa produces aerial roots with male and female flowers on the same tree. On the other hand, F. hipida produces separate male and female trees without aerial roots.
The evolution of the Ficus species has intrigued researchers due to their relationship with wasp pollinators. The wasps contribute to the fig tree's large fruits as well.
Banyan Trees
Figs are part of the diet of nearly 1,200 birds and mammals. Moreover, figs are commonly found in Mediterranean and Middle Eastern diets and are sacred symbols in several spiritual traditions such as Hinduism and Buddhism.
Several wasp species have a relationship with the banyan tree. For example, the fig's male phase will ripen after the male Courtella wardi wasp mates with a female wasp, which carries volatiles from receptive figs and chews a hole in the fig wall. The hole allows the female pollen to leave the fig.
The Otiteselline fig wasp plays a role during the fig's Interflora phase. Each fig species also produce a unique odor to attract specific pollinators.
During the study, plant biologist Ray Ming led the team to study two fig species and their corresponding wasp pollinators. Sequencing the banyan tree genomes led to discovering 'segmental duplications in the genome" of F. macrocarpa, which has aerial roots that were not found in F. hispid. The segmental duplications comprised nearly one-third of the fig tree's entire genome.
Genome Segmental Duplications
The repeated genome segments were responsible for the hormones involved in plant growth called auxins. The segmental duplications also contained genes that are part of the banyan tree's immune system, nutrition, and production of compounds that attract wasp pollinators.
Ming explained that auxins in aerial rooted fig trees "are five times higher than in the leaves of trees with or without aerial roots. Auxin production is also accelerated by light receptors triggered by genes in the segmental duplications.
The genome of the fig wasp was also compared to other pollinator wasps. Fig wasps had specific genes for odorant receptors that respond specifically to banyan trees' organic compounds.
Observations also revealed dioecy, a condition where a Y chromosome-specific gene is unique to male fig trees of F. hispida and some other fig species that produce separate male and females trees. On the other hand, F. macrocarpa has both male and female genes on the same tree.
Ming explained that Y chromosome-specific gene was duplicated two times in dioecy fig trees where the plants had three copies of the gene. The Ficus species that have both male and female flowers on the same tree only had one copy of the gene. "This strongly suggests that this gene is a dominant factor affecting sex determination."
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