Correlations among canalization, integration and plasticity
Our results showed an increase in among-individual variations (mean
coefficient of variation, CV) for all traits by high vs. low density in
fertile soil only, indicating a decreased level of canalization in
traits, consistent with other results
(Imasheva, Loeschcke et al. , 1997;
Ramler et al. , 2014;
Teder et al. , 2008;
Woods et al. , 1999). At 50 d,
plants in infertile soil were smaller, and displayed stronger responses
to density than in fertile soil (Wanget al. , 2017). The decrease in canalization or increase in
among-individual variations might be complementary to the increase in
plasticity. However, there were also more positive correlations between
the degree of plasticity (ABS RDPIs) and CV with higher
densities, suggesting some shared mechanisms of the two process in
response to environmental stress (Debat &
David, 2001; Meiklejohn & Hartl, 2002).
This was consistent with that plasticity and within-environment
variation are favored by environmental stresses in the unpredictable
environments of Mediterranean ecosystem
(Valladares et al. , 2002). As
sources of phenotypic variation, canalization and plasticity are
hypothesized to under the control of some common mechanisms
(Debat & David, 2001;
Meiklejohn & Hartl, 2002), but proved by
little evidence (Debat et al. ,
2009; Dworkin, 2005). Phenotypic
plasticity actually depicts an ability of an organism to react to
external environmental changes via self-regulation, rather than simply a
phenomenon of phenotypic difference or change. To the opposite,
developmental canalization is a mechanism of an organism in buffering
against environmental or genetic disturbances to minimize phenotypic
variations (Waddington, 1952,
1956). The correlations between the two
processes were not significant at low density, indicating both the
adaptability to a novel environment and the robustness are compatible
when a certain degree of phenotypic fluctuations exists due to noise
(Kaneko, 2012). However, the presence of
environmental stresses may favor stronger plasticity over canalization
in traits. It seems the loss of canalization in some circumstances may
not necessarily be harmful, but facilitate the production of plasticity.
In spite of a decrease in mean NC by high density at 70 d, it had
positive correlations with the magnitude of trait plasticity (ABS
RDPIs), which intensified with higher densities. It
suggested in the evolution of traits within an individual organism, the
traits of greater integration were more able to respond to environmental
stress flexibly, or traits of greater plasticity are more likely affect
the variations of other traits. The mechanisms behind differences of
organismal systems in their capacity to buffer or accommodate
stress-induced variation are poorly understood
(Badyaev et al., 2005). On the one hand,
buffering of a stressor might be a consequence of developmental
complexity rather than an evolved resistance mechanism for resilience to
stressors (Meiklejohn & Hartl, 2002;
Rice, 1998;
Siegal & Bergman, 2002;
Waxman & Peck, 1998). Under this
scenario, the complexes of traits that share the greatest number of
developmental interactions (i.e., the most developmentally integrated)
should be the most able to maintain functionality and to accommodate the
effects of stress during ontogeny. On the other hand, an organism’s
ability to function in different environments requires the ability to
track and respond to environmental change
(Eshel & Matessi, 1998;
Carl D. Schlichting & Smith, 2002;
Waddington, 1941;
Wagner et al. , 1997). The traits
of stronger integration have more connections with other traits, thus
are more able to dominate the phenotypic variation, and enhance the
whole-plant fitness through plastic responses. Because integration can
alleviate the constraints to trait plasticity by environmental signal
amplification or inhibition through developmental interaction among
trait plasticity (Lande, 2019).
Inversely, traits of greater plasticity are more likely to affect other
traits through phenotypic integration, leading to stronger correlations
among traits.
Both phenotypic integration and among-individual variation positively
correlated with the degree of plasticity, suggesting some cooperation
mechanisms among the three processes. Greater flexibility of individual
systems is hypothesized to be produced by lessening their homeostatic
integration (West-Eberhard, 2003). Such a
decrease might enhance the range of performance of individual organismal
systems and ultimately increase organismal capacity to adapt to changing
conditions (Badyaev et al. , 2005;
Rutherford, 2003). Meanwhile, mechanisms
for phenotypic plasticity and developmental canalization may be always
functioning in an organism. Organisms can maintain functionality in
stressful environments by channeling stress-induced developmental
variation through buffering some organismal functions while increasing
the flexibility of others (Alberch, 1980;
Nijhout, 2002). And the relaxation of
canalization to some extent may assist the production of plasticity in
traits.