Symbionts
We initially expected that symbiont gene expression would primarily
depend on its own abundance within the host. Instead, in bothCladocopium and Durusdinium , gene expression was similar
when the genus was either the overwhelming majority or minority, but was
highly distinct when the relative proportion of the two genera were
close to equal (codominant state, Fig 1). Functional analysis of the
codominant state revealed upregulation of translation and photosynthesis
machinery in both symbiont genera (Fig. 2, 3). Association between high
growth rate and high concentrations of ribosomes has been demonstrated
in a variety of organisms, including multicellular plants, green algae
(Giordiano
et al., 2015), insects, crustaceans
(Elser
et al., 2003), bacteria
(Bosdriez
et al., 2015), and yeast
(López-Maury
et al., 2008). We therefore believe that the observed
functional signal indicates higher growth rate in both symbiont genera
in the codominant state.