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.