Data from: Vertical and horizontal photobiont transmission within populations of a lichen symbiosis

Lichens are widespread symbioses and play important roles in many
terrestrial ecosystems. The genetic structure of lichens is the result of
the association between fungal and algal populations constituting the
lichen thallus. Using eight fungus- and seven alga-specific highly
variable microsatellite markers on within-population spatial genetic data
from 62 replicate populations across Europe, North America, Asia and
Africa, we investigated the contributions of vertical and horizontal
transmission of the photobiont to the genetic structure of the epiphytic
lichen Lobaria pulmonaria. Based on pairwise comparisons of multi-locus
genotypes defined separately for the mycobiont and for the photobiont, we
inferred the transmission mode of the photobiont and the relative
contribution of somatic mutation and recombination. After constraining the
analysis of one symbiont to pairs of individuals with genetically
identical symbiotic partners, we found that 77 % of fungal and 70 % of
algal pairs were represented by clones. Thus, the predominant dispersal
mode was by means of symbiotic vegetative propagules (vertical
transmission), which dispersed fungal and algal clones co-dependently over
a short distance, thus shaping the spatial genetic structure up to
distances of 20 m. Evidence for somatic mutation generating genetic
diversity was found in both symbionts, accounting for 30 % of pairwise
comparisons in the alga and 15 % in the fungus. While the alga did not
show statistically significant evidence of recombination, recombination
accounted for 7.7 % of fungal pairs with identical algae. This implies
that, even in a mostly vegetatively reproducing species, horizontal
transmission plays a role in shaping the symbiotic association, as shown
in many coral and other symbioses in nature.