Evolution of Polyketide Synthase Genes in Cladonia species

Abstract

Lichens are well known for producing a great variety of secondary metabolites including polyketide chemicals. Polyketides are involved in stress responses such as drought, UV, microbial infection, and so on. Biosynthesis of polyketide chemicals are carried out by polyketide syntheses (PKS). Most of the lichens contain multiple copies of PKS genes and it is believed that each copy of the PKS gene is involved in biosyntheses of different polyketide chemicals. As it is regarded that each chemical has unique biological role in stress responses, it is important to study the evolution of the gene to understand its role in environmental adaptation of lichen species. In the current study, we amplified and sequenced KS domains of PKS genes from thirty two samples, which belonged to ten Cladonia species, collected from King George Island, Chile, and Svalbard. Sixty-two KS domain sequences were obtained by direct sequencing and cloning of the PCR amplicons. Phylogenetic analyses revealed that they belonged to non-reducing PKS gene and grouped into twelve clades by the criterion of monophyly and 90% similarity cut off. We examined presence of two of the most frequently found clades (clade 6 and clade 7) and one of the rare clades (clade 4) by specific amplification method. Clade 7 was detected from all of the samples, but clade 4 and clade 6 were detected only from fifteen and thirty samples, respectively. Reconstruction of character change revealed that current distribution of clade 4 PKS genes can be explained by eight acquisition and loss events. Comparison of PKS phylogeny and rDNA phylogeny revealed that clade 7 PKS genes evolved non-orthologously but we could not find concrete evidence to contradict orthologous evolution of clade 6 PKS genes.