Microbe-mediated adaptation and speciation
Perhaps the most extreme potential outcome of microbe-mediated
adaptation is speciation of plant hosts. Although there are examples in
animals of microbes leading to speciation (Shropshire & Bordenstein
2016), there is minimal evidence in plants. Given that microbes can
influence plant reproductive traits including plant phenology (Lau &
Lennon 2012; Wagner et al. 2014, 2020; Panke-Buisse et al.2017; Metcalf et al. 2019) and pollinator preferences (Vannette
& Fukami 2016; Rebolleda-Gómez et al. 2019), reproductive
isolation and eventually speciation could potentially be affected by
microbes. In one recent example, arbuscular mycorrhizal fungi may have
facilitated sympatric speciation in two species of Howea palms in
Australia (Osborne et al. 2018). The authors propose that
colonization of a new habitat by ancestral Howea was facilitated
by developing a mutualistic relationship with endemic mycorrhizal
communities in the novel calcareous soils. This led to evolutionary
divergence into two sister species, H. forsteriana and H.
belmoreana. Howea forsteriana lost the ability to affiliate with
ancestral mycorrhizal communities and therefore to survive on the
ancestral volcanic soils, therefore leading to reproductive isolation.
Microbes also may be correlated with host speciation at deeper
evolutionary time scales. For example, plants in the coffee family
(Rubiaceae ) form symbiotic associations with leaf fungal
endophytes. These endophytes produce secondary metabolites that protect
hosts from pathogens and herbivores. Verstraete et al. (2017)
proposed endophyte associations affect macro-evolutionary patterns in
the host, observing that plant lineages hosting leaf endophytes have
higher rates of speciation compared with those without endophytes.