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Developmental stability is the ability of an organism to buffer nonadaptive phenotypic variation resulting from stochastic perturbations during development (developmental noise) (Waddington, 1957; Palmer & Strobeck, 1986; Debat & David, 2001; Nijhout & Davidowitz, 2003). Variation in developmental stability will affect the ability of organisms to reach their target phenotype (the phenotype that would be reached from a given genetic and environmental background without noise of any kind: Nijhout & Davidowitz, 2003). Therefore developmental stability is an important variational property related to fitness (Tracy et al., 2003; see Clarke, 2003 for discussion). However, measuring developmental stability remains difficult because it consists of estimating within-individual variance around an optimal value, which is unknown in most cases. Because both sides of bilateral characters depend on the same genes and share the same environment, the measure of small, directionally random differences between the two sides, referred to as fluctuating asymmetry, has been suggested to reflect the composite effect of developmental noise and developmental stability (Van valen, 1962; Palmer & Strobeck, 1986).
Although fluctuating asymmetry, as a measure of developmental stability, has been studied intensively over the past two decades, both theoretically and empirically (see Polak, 2003 for review), the relationship between developmental stability and fluctuating asymmetry remains poorly understood. Part of the problem resides in a lack of understanding of the processes that control the development of both sides of bilateral characters, and whether these processes have organism-wide or more localized effects.
Analysis of the correlation pattern between fluctuating asymmetry measurements on different characters or repeated modules, such as leaves or flowers, provides valuable information on the level at which fluctuating asymmetry, and possibly developmental stability, are regulated. Indeed, correlation in unsigned fluctuating asymmetry (independent of the direction of asymmetry) among traits should reflect the existence of an organism-wide regulation of developmental stability (Leamy, 1993; Polak et al., 2003). However, correlation in signed asymmetry among traits might also result from the effects of developmental noise shared among structurally or developmentally related traits (Van Dongen et al., 1999; Klingenberg et al., 2001; Polak et al., 2003).
Another series of questions underlying the relationship between fluctuating asymmetry and developmental stability concerns the origin and possible control of fluctuating asymmetry during ontogeny. Several models have been put forward to describe the ontogeny of fluctuating asymmetry (Emlen et al., 1993; Swaddle & Witter, 1997; Aparicio, 1998; see Kellner & Alford, 2003; Klingenberg, 2003 for reviews). Distinction between these different models depends on whether asymmetry is regulated during ontogeny, and whether this regulation involves feedback mechanisms between the two sides (Kellner & Alford, 2003). Data concerning the ontogeny of fluctuating asymmetry, such as the level of changes in magnitude and direction of asymmetry that occur during trait development, are therefore crucial for a better understanding of the mechanisms that control/affect developmental stability.
Plants are particularly useful for studying the ontogeny of fluctuating asymmetry and correlation patterns in the level of fluctuating asymmetry across traits. Their modular structure provides additional organizational levels at which fluctuating asymmetry can be measured and compared. Additionally, measurements on leaves and flowers provide the opportunity to test for correlation in fluctuating asymmetry among homologous traits within individuals.
Using observations in a twining vine, Dalechampia scandens (Euphorbiaceae), we address questions related to the developmental origin of fluctuating asymmetry. We first tested whether the observed asymmetry resulted from the twining behaviour of the vine, or from the phenotypic expression of developmental noise. We further tested whether fluctuating asymmetry reveals perturbations in developmental processes at the level of the leaf, shoot or plant. To answer this question, we analysed the pattern of correlation in signed and unsigned asymmetry at these different levels of organization. We then analysed the asymmetry pattern during leaf expansion to test whether cell expansion amplified already existing asymmetries produced during cell division, or whether regulatory processes might decrease the level of asymmetry. Finally, we analysed the relationship between fluctuating asymmetry and the rate of leaf expansion, to test whether fast-expanding leaves are less asymmetrical, as predicted if fluctuating asymmetry is negatively related to fitness (fitness-indicator model: Teather, 1996; Valkama & Kozlov, 2001); or more asymmetrical, as predicted if a trade-off occurs between growth rate and the ability to correct developmental errors (developmental-homeostasis model, Martel et al., 1999; Lempa et al., 2000).