16.3.3 Facultative Symbionts May Mitigate Damage
to Obligate, Nutritional Symbionts
Insects with bacteriocyte-associated obligate symbionts are frequently also infected
with facultative heritable symbionts, which while not required for host reproduction
often provide conditional benefits (Oliver et al. 2010; Moran et al. 2008; Buchner
1965). In pea aphids, the facultative symbiont Serratia symbiotica protects against
thermal stress, possibly by lysing and releasing protective metabolites that stabilize
Buchnera (Russell and Moran 2006; Burke et al. 2010). Specific strains of other pea
aphid facultative symbionts, including Regiella and Fukatsuia have also been
reported to confer thermal tolerance by supporting Buchnera (Heyworth et al.
2020). That thermal tolerance is also conferred by heritable Rickettsia symbionts
in whiteflies (Brumin et al. 2011) suggests this phenotype may be widespread in
sap-feeding insects. However, more work is needed in understanding the mechanisms underlying thermal tolerance and its importance in natural populations. In
other cases, facultative symbionts are known to supplement the failing functions of
degraded obligate symbionts, often via the provisioning of B vitamins and, in the
process, becoming incipient co-obligate symbionts (Russell et al. 2017; Monnin
et al. 2020; Meseguer et al. 2017). These newly dependent symbionts potentially
provide additional functions, including tolerance to climate-associated stresses.
Genome fragments of functional significance may also move among symbionts or
from symbionts to the host genome (Manzano-Marı n et al. 2020; Husnik and
McCutcheon 2018). If these retain function after transfer, then these too potentially
provide resilience.
16.3.4 Protection Services by Facultative Symbionts May Fail
in a Warmer World
In addition to interacting with obligate symbionts, facultative heritable symbionts are
also associated with mediating diverse ecological interactions. For example, numerous facultative symbionts, inhabiting diverse insect systems, provide protection
against pathogens, parasites, and as noted above, thermal stress (Oliver and Moran
2009). The genomes of facultative symbionts are typical of intermediate size
between obligate symbionts and free-living relatives and retain capabilities for
multiple functions, variable tissue tropism, occasional horizontal transmission,
mediating competition with other microbes, and other features associated with
navigating a host-restricted lifestyle (Moran et al. 2008). Unlike obligate symbionts,
infections are not typically fixed within a population, and specific infections can be
dynamic over space and time. Variable infection frequencies create the potential for
identifying correlations between symbiont prevalence and climate variables, such as
temperature (Morag et al. 2012; Doremus et al. 2018; Charlesworth et al. 2019).
Temperature has also repeatedly been found to impact within-host abundance of
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to Obligate, Nutritional Symbionts
Insects with bacteriocyte-associated obligate symbionts are frequently also infected
with facultative heritable symbionts, which while not required for host reproduction
often provide conditional benefits (Oliver et al. 2010; Moran et al. 2008; Buchner
1965). In pea aphids, the facultative symbiont Serratia symbiotica protects against
thermal stress, possibly by lysing and releasing protective metabolites that stabilize
Buchnera (Russell and Moran 2006; Burke et al. 2010). Specific strains of other pea
aphid facultative symbionts, including Regiella and Fukatsuia have also been
reported to confer thermal tolerance by supporting Buchnera (Heyworth et al.
2020). That thermal tolerance is also conferred by heritable Rickettsia symbionts
in whiteflies (Brumin et al. 2011) suggests this phenotype may be widespread in
sap-feeding insects. However, more work is needed in understanding the mechanisms underlying thermal tolerance and its importance in natural populations. In
other cases, facultative symbionts are known to supplement the failing functions of
degraded obligate symbionts, often via the provisioning of B vitamins and, in the
process, becoming incipient co-obligate symbionts (Russell et al. 2017; Monnin
et al. 2020; Meseguer et al. 2017). These newly dependent symbionts potentially
provide additional functions, including tolerance to climate-associated stresses.
Genome fragments of functional significance may also move among symbionts or
from symbionts to the host genome (Manzano-Marı n et al. 2020; Husnik and
McCutcheon 2018). If these retain function after transfer, then these too potentially
provide resilience.
16.3.4 Protection Services by Facultative Symbionts May Fail
in a Warmer World
In addition to interacting with obligate symbionts, facultative heritable symbionts are
also associated with mediating diverse ecological interactions. For example, numerous facultative symbionts, inhabiting diverse insect systems, provide protection
against pathogens, parasites, and as noted above, thermal stress (Oliver and Moran
2009). The genomes of facultative symbionts are typical of intermediate size
between obligate symbionts and free-living relatives and retain capabilities for
multiple functions, variable tissue tropism, occasional horizontal transmission,
mediating competition with other microbes, and other features associated with
navigating a host-restricted lifestyle (Moran et al. 2008). Unlike obligate symbionts,
infections are not typically fixed within a population, and specific infections can be
dynamic over space and time. Variable infection frequencies create the potential for
identifying correlations between symbiont prevalence and climate variables, such as
temperature (Morag et al. 2012; Doremus et al. 2018; Charlesworth et al. 2019).
Temperature has also repeatedly been found to impact within-host abundance of
274
K. M. Oliver and C. H. V. Higashi