In the Light of Evolution Volume V: Cooperation and Conflict (2011) / Chapter Skim
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2 Evolutionary Transitions in Bacterial Symbiosis--JOEL L. SACHS, RYAN G. SKOPHAMMER, and JOHN U. REGUS
2 Evolutionary Transitions in Bacterial Symbiosis--JOEL L. SACHS, RYAN G. SKOPHAMMER, and JOHN U. REGUS
Pages 27-48
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From page 27... ...
Five evolutionary transitions are investigated, including the origins of bacterial associations with eukaryotes, the origins and subsequent stable maintenance of bacte rial mutualism with hosts, the capture of beneficial symbionts via the evolu tion of strict vertical transmission within host lineages, and the evolutionary breakdown of bacterial mutualism. Each of these transitions has occurred many times in the history of bacterial–eukaryote symbiosis.
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From page 28... ...
Whereas some bacterialderived organelles and endosymbionts cannot live independently of hosts, most bacterial mutualists retain extensive environmental phases and form infections that are facultative for the bacterium (Szathmáry and Smith, 1995; Nyholm and McFall-Ngai, 2004; Sachs et al., 2011)
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From page 29... ...
can modulate evolutionary transitions in bacterial symbiosis and can explain how and why transitions occur. Here, we investigate evolutionary transitions that have occurred in the history of bacterial mutualism.
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From page 30... ...
TABLE 2.1. Fourteen Focal Bacterial-Host Mutualisms Analyzed Benefits Provided by Transmission Among Symbiont, Host Bacteria to Host Host Localization Hosts Rhizobia [e.g., Nitrogen Nodules (Sprent Horizontal Sawada et al.
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From page 31... ...
, partner fidelity feedback (Wilkinson, 1999; Turnbaugh et al., 2009) , partner choice (Vaishnava et a l., 2008)
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From page 32... ...
"Host-Association Origins" specifies the inferred ancestral condition at the origin of host association in the described lineage(s)
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From page 33... ...
Breakdown" specifies evidence of evolutionary transitions in bacterial lineages from mutualism to other lifestyles, with "abandonment" referring to transitions from mutualism to an environmental lifestyle. "Forces Stabilizing Bacterial Mutualism" specifies potential forces stabilizing cooperation in a bacterial mutualist lineage, divided into the three model classes [byproduct cooperation, partner choice, and partner fidelity feedback (Sachs et al., 2004)
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From page 34... ...
28 s i c u te 32 29 36 er 30 41 n Te Ac 39 * 31 ti n 37 ob 33 40 34 ac te 35 a ri 38 es flexi ut ic Cy rm Chloro n Fi o b c t e ria a a FIGURE 2.1 Inferred evolutionary history of bacterial host association.
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From page 35... ...
To evolve host association, bacteria must be able to compete with other microbes on host surfaces, evade negative host responses, uptake novel resources on or inside the host, and ultimately gain transmission to new hosts. Considering these potential hurdles, one unanswered question is whether origins of host association are rare in bacterial lineages.
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From page 36... ...
ORIGINS OF BACTERIAL MUTUALISM Fundamental questions about the origins of bacterial mutualisms remain unresolved. Do bacterial mutualists evolve from parasitic ancestors or do they represent independent origins of host association (Ewald, 1987; Szathmáry and Smith, 1995; Corsaro et al., 1999; Moran and Wernegreen, 2000; Medina and Sachs, 2010)
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From page 37... ...
However, as we observed earlier for the origins of host association, nitrogen fixation loci are also prone to horizontal transfer as parts of genome islands. This creates a scenario in which bacterial mutualists can evolve de novo from environmental ancestors via the gain of a core set of symbiosis loci (Sullivan et al., 1995; Sachs et al., 2010a)
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From page 38... ...
. Three classes of models have been proposed for the maintenance of cooperation between species -- byproduct cooperation, partner fidelity feedback, and partner choice (Axelrod and Hamilton, 1981; Bull and Rice, 1991; Sachs et al., 2004; Foster and Wenseleers, 2006)
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From page 39... ...
. There is vigorous debate over the relative importance of partner fidelity feedback versus partner choice (Bull and Rice, 1991; Simms and Taylor, 2002; West et al., 2002a,b; Weyl et al., 2010; Archetti et al., 2011)
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From page 40... ...
The degree to which hosts can spatially separate symbiont genotypes is a key prerequisite for partner choice mechanisms (Denison, 2000; West et al., 2002a,b; Sachs et al., 2004) , but should have no bearing on partner fidelity feedback.
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From page 41... ...
(E) Juvenile squid host Euprymna scolopes during colonization by symbiont Vibrio fischeri, housed in structured deep crypts [dc; adapted from Visick and Ruby (2006)
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From page 42... ...
partner-fidelity feedback and partner choice as the key selective forces that maintain bacterial mutualisms (Bull and Rice, 1991; Simms and Taylor, 2002; West et al., 2002a,b; Weyl et al., 2010; Archetti et al., 2011)
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From page 43... ...
. However, bacterial mutualists can also promote their own vertical transmission.
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From page 44... ...
. Ancient bacterial mutualisms (Sagan, 1967; Moran, 2003; Keeling, 2010; Ran et al., 2010)
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From page 45... ...
The origin of host association appears to be a readily surmountable step for bacteria. The commonness and near universality of this transition suggests that it is selectively advantageous and might be rarely affected
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From page 46... ...
Bacterial symbiosis first evolved with horizontal transmission, and several bacterial lineages have subsequently evolved strict vertical transmission. Some of the most ancient cases of bacterial mutualism exhibit vertical transmission, so this transition can promote the evo lutionary stability of symbioses.
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From page 47... ...
For focal symbiont taxa, we analyzed phylogenies containing the lineages of interest to assess whether host association originated from parasitic ancestors or free-living ancestors and to search for evidence of mutualism breakdown. Ancestral states for symbiotic lineage and evidence of mutualism breakdown were inferred by using parsimony on the available phylogenies (Normand et al., 1996; Turner et al., 1999; Nishiguchi and Nair, 2003; Sawada et al., 2003; Ruby et al., 2005; Svenning et al., 2005; Kaltenpoth et al., 2006, 2009; Kikuchi et al., 2007, 2011; Münchhoff et al., 2007; Williams et al., 2007, 2010; Sachs et al., 2009, 2010a; Mueller et al., 2010)
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