Photobiont diversity within populations of a vegetatively reproducing lichen, Parmotrema tinctorum, can be generated by photobiont switching

Abstract

Photobiont diversity within populations of a vegetatively reproducing lichen can be generated by photobiont switching between the original lichen photobiont and the compatible algal partners on its surrounding substrate. The hypothesis was tested using Denaturing Gradient Gel Electrophoresis (DGGE) with a partial rbcL sequence amplified from thalli of Parmotrema tinctorum and from the substrate immediately adjacent to each thallus. On the surface of tombstones where P. tinctorum was growing, only various haplotypes of Trebouxia corticola (s. lat.) that is characterized by having distinct starch sheaths surrounding the pyrenoid were detected. DGGE could detect one to five bands of T. corticola (s. lat.) haplotypes on each substrate, and one (or rarely two) of them was often identical with the photobiont haplotype of P. tinctorum growing on the same tombstone. Through PCR screening directed at a fungal rDNA fragment, many substrate samples were found to be free of microscopic contamination from P. tinctorum. Individual algal haplotypes from the substrate were identified by sequencing of the DGGE rbcL bands and compared to the corresponding sequences of the P. tinctorum photobionts. The presence of compatible algae on the lichen substrate and the genetic identity between some of the substrate algae and those in the lichen suggest the possibility of photobiont switching in vegetatively reproducing lichens like P. tinctorum. The following observations also support the phenomenon of photobiont switching: 1) high genetic diversity of photobiont in small populations; 2) multiple photobionts in a single thallus; 3) incomplete correspondence in co-phylogenetic analyses between mycobiont and photobiont; and 4) clear selectivity for photobiont in diverse lichens.