Fungal colonization
First, we analyzed how level of fungal relatedness affected plant growth
and fungal colonization in the whole plant experiment. We found that
level of fungal relatedness affected the intraradical colonization of
host roots: intraradical colonization of the focal plant root was
significantly higher in treatments of non-selfing fungal combinations –
meaning that decreased relatedness of the fungi was associated with
higher colonization of the focal root. Intraradical colonization of the
focal plant connected to a fungal network with high genetic relatedness
(i.e. network containing selfing A5 only) was half of that observed when
the partner plant was inoculated with the non-selfing partner strain
B12, and a fifth of that when the partner plant was inoculated with Agg
(One-way ANOVA: F2,23=5.732, p<0.001) (Fig.
2a). The intraradical colonization of the partner plant roots showed the
opposite pattern, with higher colonization of partner roots inoculated
with A5 versus with B12 or Agg (One-way ANOVA:
F2,21=5.617, p=0.011) (Fig. S1 in Supporting
Information). We then converted these numbers to a ratio of fungal
abundance in focal roots/partner roots so we could compare colonization
dynamics within replicates. A ratio of 1:1 means equal fungal growth in
both roots. As expected, we found a ratio of ~1:1 in the
plants inoculated with only A5. However, as relatedness decreased, this
ratio increased (~10 times higher), meaning a bias in
favor of colonization in the focal root when relatedness decreased
(one-way ANOVA: F2,21=15.812, p<0.0001) (Fig.
2b).
In the in-vitro experiment, we quantified both extraradical and
intraradical fungal abundance. We found a similar trend as in the whole
plant experiment: decreased relatedness (non-selfing) was associated
with increased total fungal abundance. Specifically, we found that the
extraradical abundance of highly related A5-A5 networks were roughly
eight times lower than when the partner plant was inoculated with the
non-selfing partner strain B12 or Agg (one-way ANOVA:
F2,37=9.0833, p<0.001) (Fig. 3a). However, we
found intraradical fungal colonization did not differ statistically
among the treatments (Focal roots: one-way ANOVA:
F2,38=0.418, p= 0.662; partner roots:
F2,38=0.68, p=0.513) (Fig. S2a&b). We also found that
partner roots inoculated with Agg, were also colonized by fungal strain
A5 (7.09·105 ±3.2·104 copy numbers,
Fig. S2c). Since A5 and Agg are not known to fuse, this indicates that
the fungus of the focal root crossed the central compartment and into
the partner root compartment. We then tested whether level of
relatedness affected investment ratio of the fungus, defined as fungal
allocation to intraradical versus extraradical growth. We found that low
fungal relatedness resulted in a bias towards more extraradical growth
(one-way ANOVA: F2,37=12.343, p<0.001) (Fig.
3b).