Abstract
BackgroundLichen is a classic mutualistic organism and the lichenization is one of the fungal symbioses. The lichen-forming fungus Endocarpon pusillum is living in symbiosis with the green alga Diplosphaera chodatii Bialsuknia as a lichen in the arid regions.Results454 and Illumina technologies were used to sequence the genome of E. pusillum. A total of 9,285 genes were annotated in the 37.5 Mb genome of E. pusillum. Analyses of the genes provided direct molecular evidence for certain natural characteristics, such as homothallic reproduction and drought-tolerance. Comparative genomics analysis indicated that the expansion and contraction of some protein families in the E. pusillum genome reflect the specific relationship with its photosynthetic partner (D. chodatii). Co-culture experiments using the lichen-forming fungus E. pusillum and its algal partner allowed the functional identification of genes involved in the nitrogen and carbon transfer between both symbionts, and three lectins without signal peptide domains were found to be essential for the symbiotic recognition in the lichen; interestingly, the ratio of the biomass of both lichen-forming fungus and its photosynthetic partner and their contact time were found to be important for the interaction between these two symbionts.ConclusionsThe present study lays a genomic analysis of the lichen-forming fungus E. pusillum for demonstrating its general biological features and the traits of the interaction between this fungus and its photosynthetic partner D. chodatii, and will provide research basis for investigating the nature of its drought resistance and symbiosis.
Highlights
Lichen is a classic mutualistic organism and the lichenization is one of the fungal symbioses
All sequences were assembled into 908 scaffolds (> 2 kb; N50, 178 kb) containing 1,731 contigs, with a genome size of 37.5 Mb (Table 1), which was almost identical to the result calculated by real-time polymerase chain
Differences in expression levels of genes involved in symbiosis under co-culture conditions From the results presented in Figure 7, the expression levels of most genes increased only under the condition for experimental group IV (Table 5), which indicated that the contact time and biomass ratio of both symbionts affects their recognition and nutrient transfer significantly
Summary
Lichen is a classic mutualistic organism and the lichenization is one of the fungal symbioses. Most lichens and isolated lichen-forming fungi grow extremely slow, but the lichen-symbiosis is a very successful association as lichens can survive in almost all adverse terrestrial conditions [3]. They are famous for their particular secondary products, which are frequently used as antibacterial and antiviral compounds [4,5]. The lichen-forming fungi differ from non-lichenized fungi by their adaptations to symbiosis with photobiont [6] This mutualistic association, as called lichenization, is one of the most important fungal lifestyles and the lichenization, considered by some researchers, has evolved many times in the phylogeny of fungi [7,8], and some major fungal lineages may have derived from lichen symbiotic ancestors [9]
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