DISCUSSION
Concurrent developments in evolutionary theory and molecular methodology
over the past two decades have revealed a great deal of evolutionary
dynamism in RNS, including: evolution of cheating in microbes and
countermeasures in host plants (Denison, 2000; Denison et al .,
2003; Denison & Kiers 2004, 2011; Kiers et al ., 2003, 2006;
Sachs et al ., 2004; Simms & Taylor, 2002; Simms et al .,
2006; West et al ., 2002), context-dependence in the mutualistic
outcome of symbiosis (Halloway et al ., 2022; Heath & Tiffin,
2007), influence of quasi-economic forces on the evolution of bacterial
behavior (Hammerstein & Noe, 2016; Noe & Hammerstein, 1995; Werneret al ., 2014), and evolutionary switches in microbial partners
between parasitic, mutualistic, and free-living lifestyles (Sachset al ., 2010, 2014; Sachs & Simms, 2006). Recent controlled
inoculation studies with alder species have demonstrated the ability to
select specific genotypes of Frankia from soil, and also suggest
differential abilities among bacterial genotypes to convert selection by
hosts into increased representation in soil assemblages (Ben Tekayaet al ., 2018; Vemulapally et al . 2022a,b). To date,
however, no field studies that we know of have investigated concurrent
changes in soil and nodule assemblages across environmental gradients at
local spatial scales in any naturally occurring RNS. Our study examined
variation in soil-dwelling Frankia in locations previously
characterized with respect to nodule-dwelling Frankia across
three ecologically-important sources of variation: successional change
across a primary chronosequence, changes in nutrient availability known
to trigger host autoregulation mechanisms, and presence of a specific
host species. We found significant effects of all three ecological
drivers, and also discovered several clades of apparently non-symbioticFrankia .