In vitro resynthesis of lichenization reveals the genetic background of symbiosis-specific fungal-algal interaction in Usnea hakonensis

Mieko Kono,Yohey Terai,Yoko Satta,Yoshiaki Kon,Yoshihito Ohmura

doi:10.1186/s12864-020-07086-9

Abstract

BackgroundSymbiosis is central to ecosystems and has been an important driving force of the diversity of life. Close and long-term interactions are known to develop cooperative molecular mechanisms between the symbiotic partners and have often given them new functions as symbiotic entities. In lichen symbiosis, mutualistic relationships between lichen-forming fungi and algae and/or cyanobacteria produce unique features that make lichens adaptive to a wide range of environments. Although the morphological, physiological, and ecological uniqueness of lichens has been described for more than a century, the genetic mechanisms underlying this symbiosis are still poorly known.ResultsThis study investigated the fungal-algal interaction specific to the lichen symbiosis using Usnea hakonensis as a model system. The whole genome of U. hakonensis, the fungal partner, was sequenced by using a culture isolated from a natural lichen thallus. Isolated cultures of the fungal and the algal partners were co-cultured in vitro for 3 months, and thalli were successfully resynthesized as visible protrusions. Transcriptomes of resynthesized and natural thalli (symbiotic states) were compared to that of isolated cultures (non-symbiotic state). Sets of fungal and algal genes up-regulated in both symbiotic states were identified as symbiosis-related genes.ConclusionFrom predicted functions of these genes, we identified genetic association with two key features fundamental to the symbiotic lifestyle in lichens. The first is establishment of a fungal symbiotic interface: (a) modification of cell walls at fungal-algal contact sites; and (b) production of a hydrophobic layer that ensheaths fungal and algal cells;. The second is symbiosis-specific nutrient flow: (a) the algal supply of photosynthetic product to the fungus; and (b) the fungal supply of phosphorous and nitrogen compounds to the alga. Since both features are widespread among lichens, our result may indicate important facets of the genetic basis of the lichen symbiosis.

Highlights

Symbiosis is central to ecosystems and has been an important driving force of the diversity of life
We examined the transcriptomes of U. hakonensis when: 1) the mycobiont U. hakonensis, and the photobiont Trebouxia sp. isolates are cultured independently; 2) the symbiotic phenotype is resynthesized in vitro; and 3) U. hakonensis is in
Among the significant Differentially expressed gene (DEG), we focused on the genes consistently up-regulated in both symbiotic states as ‘symbiosis-related genes’

Summary

Introduction

Symbiosis is central to ecosystems and has been an important driving force of the diversity of life. Recent findings have brought up controversial issues of the members involved in lichen symbioses [4,5,6], several experiments have shown that in vitro co-culturing of the mycobiont and the photobiont can initiate development of symbiotic phenotypes to a limited extent [7,8,9,10]. Primarily early stages of lichenization have been investigated and genes of mycobionts and photobionts have been reported as those likely to function in lichen symbioses [11,12,13]. Later stages, especially thallus differentiation, are more likely to reflect unique characters of lichens, and investigation into genetic mechanisms that control the development of those stages is essential to understanding the fundamental nature of lichen symbiosis

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