The Anna Karenina Principle
Under optimal thermal conditions, epibiota associated with G.
vermiculophylla native and non-native disperse equally, but at an
elevated temperature, natives disperse substantially more than
non-natives. This indicates that specifically under stress, epibota
associated with non-native holobionts are more stable, or in other
words, more prone to stochastic processes. While we note that our
experiment only included two native and two non-native populations these
results support a difference in thermal stress tolerance between the
distribution ranges and suggest that the degree of influence of the host
on its epibiota is higher among the non-natives. Sotka et al. (Sotka et
al., 2018) found evidence of increased heat tolerance and salinity
stress tolerance in non-native G. vermiculophylla in another
common garden experiment and argued that the invasion process has
selected for more stress tolerant genotypes. The higher epibiota
dispersion in native populations we observed beta diversity could
indicate that such tolerance may be linked to interactions between host
and epibiota, specifically mechanisms from the host. Especially in the
light of the invasion process, where the invader undergoes disturbance
and must reassemble new harmless or beneficial epibiota while
acclimating to a new environment, a selective pressure for mechanisms
with which hosts influence epibiota could be beneficial. Ultimately,
such adaptations would expand the range of environments where the host
can successfully reassemble functional microbiota.
The role of host traits influencing epibiota in the
invasion process
While the here observed beta diversity pattern is noteworthy, this study
does not provide insight in putative traits that are differentially
expressed in native and non-native populations which could explain
differences in host influence. Further studies are required to better
characterize the mechanisms by which G. vermiculophyllainfluences its epibiota and to identify if these traits vary between the
native and non-native ranges. However, these data join the list of
studies suggesting that changes in the interaction between host and
microbiota in the G. vermiculophylla holobiont have played an
important role in the invasion process (Bonthond et al., 2020, 2021;
Saha & Weinberger, 2019; Saha et al., 2016; S. Wang, Wang, et al.,
2017; S. Wang, Weinberger, et al., 2017). Most likely the observations
made in these different studies have a common basis. For example,
changes in traits related to defense against fouling (Saha et al.,
2016), could promote promiscuity toward potential symbionts (Bonthond et
al., 2021) and at the same time facilitate host influence under
stressful conditions. Similarly, adaptations in traits related to the
acquisition of vitamins (for some of which seaweeds are auxothroph;
Croft et al., 2006; Kazamia et al., 2012) or other resources (e.g.,
Gerard et al., 1990) might enable the host to become more promiscuous
toward symbionts that can potentially provide them and therewith
indirectly support the synthesis of defense related metabolites,
enhancing defense capacity in general and under stressful conditions.
For now, we can only speculate and it remains a challenge for future
experimental studies to identify the host mechanism(s) explaining host
influence.