known as the extended phenotype (Dawkins 1982; Hunter 2018). Central to this new
attention is microbial communities associated with hosts, where the extended phenotype can be broadly defined to include many of the microbial functions described
above, namely, access to new sources of nutrients, defenses against disease, and
complexities of life histories (Hunter 2018). I will use a narrower definition here to
categorize functions that extend the phenotype in other, and more radical directions.
I include here the Bobtail Squid-Vibrio fisheri association, the coral–symbodinium
symbiosis and the increased tolerance to abiotic stressors that endophytes provide for
host plants, yet many others could have been featured here.
The benefits of light production are key to the survival of the bobtail squid in its
natural habitat. Vibrio fisheri’s bioluminescence provides the bobtail squid
(Euprymna scolopes) with the ability to use counterillumination to avoid predators
(Nyholm and McFall-Ngai 2004). This association is unique in its horizontal but
tightly selective transfer, due to its chemical (mucus, peptide secretion) and physical
(active cilia creating countercurrents) constraints that favor V. fischeri over all other
bacterial colonists (Nawroth et al. 2017). The squid obtains the symbiont from the
seawater, and the initial contact triggers the development of the light organ, where
the bacteria are housed (Koropatnick et al. 2004; McFall-Ngai 2015). Therefore, in
this system, both partners shape each other’s biology in a wonderful display of
animal–microbe dialog (Schwartzman and Ruby 2016).
Coral associations with zooxanthellae are key to a multitude of marine fish and
invertebrates that find resources and refuge among the coral reef. The coral–zooxanthellae association is based on the host providing shelter and nutrients like
ammonium, while the zooxanthellae are primary producers and provide a carbon
source in areas of the ocean that are typically nutrient poor. The resulting ecosystem
is an oasis and refuge for marine life and one of the most beautiful sights on planet
Earth. In contrast to the tight V. fisherii–Bobtail Squid association, different species
of corals associate with many different species of zooxanthellae in the genus
Symbodinium (Toller et al. 2001). Patterns of association indicate zooxanthellae
tolerance to heat and depth stress, as well as coral taxonomic preferences (Kennedy
et al. 2016).
Plant growth-promoting bacteria are those that stimulate growth and facilitate
strong stress responses. Microorganisms associated with roots have been shown to
increase drought resistance and tolerance to temperature extremes (Stone et al.
2018), facilitating the range expansion of otherwise susceptible species (Geisen
et al. 2017; Ramirez et al. 2019) and the survival of endangered species in changing
habitats (West et al. 2019; David et al. 2019). Some of the proposed mechanisms
behind these functions include biofilm EPS secretion that resists the diffusion of
water oxygen and nutrients and secretion of heat shock proteins (McLellan et al.
2007; Sandhya et al. 2009; Grover et al. 2011; Kumar Meena et al. 2015; Ortiz et al.
2015). Phyllosphere bacteria can also confer drought and heat resistance (Costerton
et al. 1994; Auerbach et al. 2000) with the additional benefits of UV (Jansen et al.
1998; Jacobs and Sundin 2001) and frost protection (Lindow et al. 1982; Wisniewski
et al. 1997; Attard et al. 2012). Physiological studies on symbionts suggest that most
of these properties are provided by few species (Lindow et al. 1982; Auerbach et al.
304
C. Cuellar-Gempeler
attention is microbial communities associated with hosts, where the extended phenotype can be broadly defined to include many of the microbial functions described
above, namely, access to new sources of nutrients, defenses against disease, and
complexities of life histories (Hunter 2018). I will use a narrower definition here to
categorize functions that extend the phenotype in other, and more radical directions.
I include here the Bobtail Squid-Vibrio fisheri association, the coral–symbodinium
symbiosis and the increased tolerance to abiotic stressors that endophytes provide for
host plants, yet many others could have been featured here.
The benefits of light production are key to the survival of the bobtail squid in its
natural habitat. Vibrio fisheri’s bioluminescence provides the bobtail squid
(Euprymna scolopes) with the ability to use counterillumination to avoid predators
(Nyholm and McFall-Ngai 2004). This association is unique in its horizontal but
tightly selective transfer, due to its chemical (mucus, peptide secretion) and physical
(active cilia creating countercurrents) constraints that favor V. fischeri over all other
bacterial colonists (Nawroth et al. 2017). The squid obtains the symbiont from the
seawater, and the initial contact triggers the development of the light organ, where
the bacteria are housed (Koropatnick et al. 2004; McFall-Ngai 2015). Therefore, in
this system, both partners shape each other’s biology in a wonderful display of
animal–microbe dialog (Schwartzman and Ruby 2016).
Coral associations with zooxanthellae are key to a multitude of marine fish and
invertebrates that find resources and refuge among the coral reef. The coral–zooxanthellae association is based on the host providing shelter and nutrients like
ammonium, while the zooxanthellae are primary producers and provide a carbon
source in areas of the ocean that are typically nutrient poor. The resulting ecosystem
is an oasis and refuge for marine life and one of the most beautiful sights on planet
Earth. In contrast to the tight V. fisherii–Bobtail Squid association, different species
of corals associate with many different species of zooxanthellae in the genus
Symbodinium (Toller et al. 2001). Patterns of association indicate zooxanthellae
tolerance to heat and depth stress, as well as coral taxonomic preferences (Kennedy
et al. 2016).
Plant growth-promoting bacteria are those that stimulate growth and facilitate
strong stress responses. Microorganisms associated with roots have been shown to
increase drought resistance and tolerance to temperature extremes (Stone et al.
2018), facilitating the range expansion of otherwise susceptible species (Geisen
et al. 2017; Ramirez et al. 2019) and the survival of endangered species in changing
habitats (West et al. 2019; David et al. 2019). Some of the proposed mechanisms
behind these functions include biofilm EPS secretion that resists the diffusion of
water oxygen and nutrients and secretion of heat shock proteins (McLellan et al.
2007; Sandhya et al. 2009; Grover et al. 2011; Kumar Meena et al. 2015; Ortiz et al.
2015). Phyllosphere bacteria can also confer drought and heat resistance (Costerton
et al. 1994; Auerbach et al. 2000) with the additional benefits of UV (Jansen et al.
1998; Jacobs and Sundin 2001) and frost protection (Lindow et al. 1982; Wisniewski
et al. 1997; Attard et al. 2012). Physiological studies on symbionts suggest that most
of these properties are provided by few species (Lindow et al. 1982; Auerbach et al.
304
C. Cuellar-Gempeler