Abstract
To gain a mechanistic understanding of color pattern formation, including molecular and genetic mechanisms, it is necessary to not only describe adult phenotypes, but also to understand color production and pattern formation during embryonic and postembryonic stages. Here, we review the pigment cell mechanisms underlying color production and pattern formation during embryonic development in lizards and snakes to provide a comprehensive pigment-cell–based understanding of color pattern formation. We first focus on different color production mechanisms in terms of epidermal and dermal pigment cell architectures and then discuss the genetic determinants of pattern formation considering both biologically relevant theoretical models and the processes of pigment cell specification, migration, and architecture differentiation. Clarifying the contributions of pigment cells and genetic factors improves our general understanding of reptilian color pattern evolution.
Highlights
Why do we study color pattern production mechanisms in squamate reptiles
Assuming that the final transformation to mature pigment cells occurs after NC progenitors migrate to certain regions, gene regulatory networks (GRNs) responsible for color pattern formation will be those involved in the chemotactic parameter (αʣ miotic rate (γ) timing and speed of motility, sensitivity to chemical signals, cell population growth and death, and the strength of cell–cell and cell–environment interactions during migration from the NC to correct sites
According to the new perspective of the molecular mechanisms that regulate gene expression, we are increasingly appreciating that mutations affecting cis-regulatory sequences are the most frequent cause of phenotypic divergence (Wray, 2007; Carroll, 2008; Stern and Orgogozo, 2008; Jones et al, 2012; Wittkopp and Kalay, 2012)
Summary
Why do we study color pattern production mechanisms in squamate reptiles. Basically, animal color pattern is a longstanding and important topic in evolutionary biology in general, and squamates in particular, which exhibit a wide variety of color patterns such as warning signal of venomous coral snakes and its mimicry by the distantly related non-venomous snakes (Figure 1A), and convergent evolution of stripes and vivid blue tails by small lizards (Figure 1B), are important target of several agents of natural selection for effective visual signals to conspecifics for reproduction, to predators for avoidance, and to sometimes prey for feeding (e.g., Cooper and Greenberg, 1992; Pianka and Vitt, 2003; Stuart-Fox et al, 2008; McKinnon and Pierotti, 2010; Kronforst et al, 2012; Allen et al, 2013; Olsson et al, 2013). Due to our little understandings of physiological mechanisms controlling environmentally derived pigments such as carotenoids (Kikuchi et al, 2014), this review did little consideration of phenotypic plasticity in color patterns of squamates, admitting that there exist important topic of behavioral ecology and physiology in squamate reptiles (Pérez i de Lanuzaa et al, 2014).
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