Host sanctions and the legume– rhizobium mutualism
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Date
2003Author
Kiers, E Toby
Rousseau, Robert A
West, Stuart A
Denison, R Ford
Metadata
Abstract
Explaining mutualistic cooperation between species remains one
of the greatest problems for evolutionary biology1–4. Why do
symbionts provide costly services to a host, indirectly benefiting
competitors sharing the same individual host? Host monitoring
of symbiont performance and the imposition of sanctions on
‘cheats’ could stabilize mutualism5,6. Here we show that soybeans penalize rhizobia that fail to fix N2 inside their root nodules. We
prevented a normally mutualistic rhizobium strain from cooperating
(fixing N2) by replacing air with anN2-free atmosphere
(Ar:O2). A series of experiments at three spatial scales (whole
plants, half root systems and individual nodules) demonstrated
that forcing non-cooperation (analogous to cheating) decreased
the reproductive success of rhizobia by about 50%. Non-invasive
monitoring implicated decreased O2 supply as a possible mechanism
for sanctions against cheating rhizobia. More generally,
such sanctions by one or both partners may be important in
stabilizing a wide range of mutualistic symbioses.