Data from: Outcrossing mating system of the early-divergent moonwort fern (Botrychium lunaria, Ophioglossaceae) revealed in the European Alps

Premise of the Research. Vascular plants depend on sexual
recombination for generating new genetic variability to meet environmental
needs. Nevertheless, members of the early-divergent fern
genus Botrychium (Ophioglossaceae) typically maintain
gametophytic selfing and show strong inbreeding within populations. To
explain this evolutionary anomaly, the existence of previous or current
but undiscovered outcrossing, genetically rich, precursors of the existing
genetically depauperate taxa has been hypothesized. 
Methodology. Using allele expression at thirteen independently
assorting enzyme loci, we compared allelic diversity and levels of
heterozygosity in 471 Botrychium lunaria individuals
across sixteen populations in the Alps and Jura Mountains of Switzerland.
We examined habitat characteristics influencing mating systems and
investigated population genetic structure based on a discriminant analysis
of principal components and a graph-theoretic framework. We tested the
pattern of isolation by distance and explored the biological processes
favoring or limiting spore dispersal at a regional scale. Pivotal
Results. We found high genetic diversity within and among
populations, and similar observed (HO) and expected heterozygosity (HS)
across loci (HO = 0.020-0.450 and HS =
0.040-0.590, respectively). Estimates of HO(0.050-0.272) were
lower than HS (0.144-0.305) across individuals,
indicating a weak deficit in heterozygotes. Mean values of departure (D)
from expected heterozygosity were close to what would be expected with
random mating (DL = 0.061 and DP = 0.059 across loci and
populations, respectively). The mean inbreeding coefficient was low
(FIS = 0.247) and the overall genetic differentiation was
moderate (global FST = 0.083).
Conclusions. Together, the discriminant analysis of principal
components and the Population Graph revealed a complex population genetic
structure and a weak genetic differentiation with substantial gene flow
among most populations. One population (Arosa), sampled from a site
recently released from permafrost, demonstrated probable founder effect
not correlated with geographical isolation. The topology of
the Population Graph identified three subgroups with several key
populations (BGU, CHL, GSB, SBE, and VRO) that maintained genetic
connectivity of populations from distant locations.