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.