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Genomic Study Discover Genetic Reason behind Fig-Wasp Symbiotic System

Researchers from the Fujian Agriculture and Forestry University (FAFU) and the Xishuangbanna Tropical Botanical Garden (XTBG) publish insights to fig-wasp co-evolution through comparative analyses of two Ficus genomes.

The banyan tree, Ficus benghalensis, is one of the many species of fig trees. These trees are epiphytes which would grow on the surface of another plant from which it will derive its necessary nutrients for survival. Each species of fig tree is pollinated only by its own specific species of tiny wasps that breed within the figs of its partner tree.

Particularly for the Ficus species, it possesses aerial roots, which enable them to live as hemi-epiphytes, much like the strangler figs commonly found in tropical forests. Its unique enclosed urn-shaped inflorescence, fig plants depend on fig wasps as its insect pollinators for pollination. In turn the fig trees provide nourishment and shelter for these insects to breed and reproduce. Morphological matching and signalling communication for host location between figs and their pollinators is required for successful pollination and oviposition.

The relationship between the fig trees and fig wasps demonstrates plant-insect co-diversification. In a recent study published in Cell, a team of researchers from Fujian Agriculture and Forestry University (FAFU) and the Xishuangbanna Tropical Botanical Garden (XTBG) provided insights into fig-wasp coevolution through comparative analyses of two Ficus genomes.

One species had aerial roots while the other did not, and one had male and female reproductive organs in the same plant (monecious), while the other did not (dioecious). The genome of a co-evolving wasp was also sequenced.

The researchers sequenced genomes of the monecious Chinese banyan tree, F. microcarpa, and a dioecious species lacking aerial roots, F. hispida, and one wasp genome coevolving with F. microcarpaEupristina verticillata. Comparative analysis of the two Ficus genomes revealed dynamic karyotype variation associated with adaptive evolution.

“We quantified endogenous auxin in F. microcarpa and F. hispida, and proposed that an auxin-dependent pathway promoted by light is associated with aerial root initiation, growth, and pattern formation,” said Wang Gang, associate professor at XTBG and co-first author of the study.

Upon construction of a genetic map and studying sex determination and sex evolution in Ficus plants, the researchers found a budding Y chromosome in F. hispida and a male-specific AGAMOUS paralog, the FhAG2 gene, as a candidate sex determination gene of this fig species.

Data from re-sequencing genomes of 112 Ficus accessions from 62 Ficus species, and understanding of the relationship between the different groups of Ficus and its evolutionary development was established by the researchers. Through this phylogenetic analysis, it was found that monoecy represents the ancestral reproductive system across the genus.

“Population genomic analysis of subgenus Sycomorus figs and their obligate pollinator wasps and electrophysiological testing of pollinators responding to floral scents emitted from three different Ficus species support the important roles of the mevalonate and shikimate pathways in attracting species-specific pollinators and reveal potential molecular mechanisms of co-diversification in this obligate mutualism,” said Associate Professor Wang.

“The work, integrating efforts from several universities and institutes from China and overseas, particularly the expertise on genomic analysis from FAFU and fig biology from XTBG, will enhance our understanding of the species-specific mutualism between figs and fig wasps,” said Chen Jin, one of the corresponding authors of the study. [APBN]