maenas (intertidal zone) and Segonzacia mesatlantica - Station ...

198 CHAPITRE 5. ADAPTATIONS RESPIRATOIRES DE S. MESATLANTICA
Since the number of thawings could have had an effect on complex proportions or lactate and urate
contents but not on the subunit composition, this variable was not used for the forward selection. Only
sex and exp3 explained significant amounts of variance of the response variables (p-values lower than
0.1 ; 0.09 for both) and were kept as covariables in the following analyses. Experimental conditions
were described with the same method as above : a variable Control with mean 0 separated the 13
control crabs (coding value = 1/13) from the 10 acclimated animals (coding value = -1/10) ; a variable
Oxygenation with mean 0 represented the 6 crabs submitted to hypoxia (coding value = -1/6), the
13 control crabs (coding value = 0), and the 4 animals subjected to normoxia (coding value = 1/4) ;
and a variable Temperature with mean 0 represented the 3 crabs subjected to 10°C (coding value =
-1/3), the 13 control crabs (coding value = 0), and the 7 animals subjected to 20°C (coding value =
1/7). The Interaction variable was created by multiplying the corresponding values of the oxygen and
temperature treatment variables. There was also a small lack of orthogonality because of the imbalance
of the experimental design (r(Oxygen, Interaction) = 0.04, r(Temperature, Interaction) = -0.08,
r(Oxygenation, Temperature) = 0.09).
The interaction was tested by performing an RDA with the 16 subunit abundances as response
variables (Y), the Interaction term as the explanatory variable (X) and sex, exp3, Oxygenation and
Temperature as covariables (W). No significant interaction was found (R2=0.03, p-value=0.53). The
oxygenation and temperature effects were tested separately, with Oxygenation and Control or Temperature
and Control as explanatory variables and sex and exp3 as covariables. No significant effect
of either factor on subunit abundances in the native samples was found (R2=0.11, p-value=0.19 for
oxygenation ; R2=0.12, p-value=0.09 for temperature). Results were not significant either when only
the 8 more abundant subunits were considered.
The next step was to determine whether the subunit composition was different between hexamers
and dodecamers, in addition to the two dodecamer-specific subunits. For this, the 24 experiments on
purified complexes were associated into a single data set with 48 observations (24 rows for dodecamer
and 24 rows for hexamer composition) and a dummy binary variable accounting for the type of complex
analyzed was created (Dodecamer = 1 for dodecamer or 0 for hexamer). Using the 16 subunits in
a univariate permutation test against the complex type, without covariables, gave a significant p-value
of 0.003. For clarity, the analysis was rerun with only the 8 more abundant subunits and gave a p-value