Filtering of mutualistic partnerships in the field
Many theoretical and empirical studies have shown that stabilizing
mechanisms maintain mutualistic interactions in environments with a high
genetic diversity of mutualistic partners, including cheaters (Archetti
& Scheuring, 2013; Kiers & Denison, 2008; Kiers et al., 2007, 2003;
West et al., 2002ab). However, to our knowledge, this is the first study
to reveal host filtering for genetic variants under field conditions
through explicit comparison between colonized symbionts and the regional
community pool. In our findings, Frankia communities in host
individuals were constructed by smaller numbers of Frankia OTUs
than the number of Frankia OTUs in the surrounding environments
(Fig. S1, 2, S1). Moreover, the comparison with the null models showed
that the Frankia communities in the host plant were not chosen
from soils by chance (Fig. 2b). As a result, symbiont communities in
nodules were relatively constant across host plant individuals. SeveralFrankia OTUs were detected from both rhizosphere soils and root
nodules (e.g., OTU01, 02, 03, 25, 26, and 28; Fig. 1) and, while these
OTUs should have infection ability, OTU 25, 26, and 28 infected only a
few host individuals. In addition, even though the occurrences of OTU01,
02, and 03 were not frequent in rhizosphere, it was constantly found
from nodules of most trees. Thus, the differences in bacterial assembly
between roots and rhizospheres would not depend on infective or
non-infective strains, but filtering mediated in determining symbiont
members from the genetically diverse Frankia pool.
While filtering forces are likely to regulate symbiont assembly into a
host plant in a natural system, most alders interacted with
phylogenetically different OTUs (i.e., OTU01, 02, and 03; Fig. 1, S1).
This may be a result of genetic variation in multiple bacterial
functions. Nitrogen fixation is the fundamental function of nodulating
bacteria including Frankia bacteria, promoting not only the
growth of host plants but also survival and herbivory defense. Dean et
al. (2014) found that nitrogen resources from nitrogen-fixing symbionts
were needed for induced herbivory resistance and artificial fertilizer
did not induce the resistance. This suggests that herbivore resistance
does not depend on the amount of total nitrogen supply but on specific
nitrogen forms, such as amides and ureides, provided by nitrogen-fixing
symbionts. Nitrogen fixed by associated rhizobacteria, includingFrankia , can be stored in nodules and transported to aerial parts
as these specific forms (Berry et al., 2011; Dean et al, 2009; Schubert,
1986). Furthermore, Frankia bacteria have some other plant growth
promoting functions, such as solubilization of inorganic phosphate
(Sayed et al., 2002) and the synthesis of plant hormones (Péret et al.,
2007) and siderophores (Tisa et al., 2016). Nouioui et al. (2019)
discovered genes encoding these functions from some Frankiastrains in their in silico genome analyses and detected variation
in the genes among the Frankia strains. The symbioses with
genetically diverse microsymbionts could be explained by the
complimentary effects of genetic variation in these multiple functions
of Frankia strains on host performance.
In addition, some of the Frankia strains that were obtained in a
previous study, which focused on host seedlings (Kagiya and Utsumi,
2020), were not detected in the root nodule samples of most host trees
in this study (Fig. S1). This may be explained by the following: (1) the
above strains might be extremely rare genotypes, (2) genetic structure
of Frankia might be differentiated in small spatial scales, and
(3) the different communities of mutualistic partners might be
constructed among host ages.