1. Introduction
Organisms can change their phenotypic traits (morphology, behavior, and physiology) and adapt to environmental variations. The ability of a single genome to produce a range of phenotypes in response to environmental conditions is called phenotypic plasticity (Agrawal 2001; Fordyce 2006). In general, the degree of phenotypic plasticity has a direct effect on fitness and therefore represents an important feature of the organism’s adaptation.
The change in traits observed in phenotypic plasticity may not be binary (high and low) or represented by an on/off reaction but rather a continuous process in individuals (Auld et al. 2010; Forsman 2015). Owing to this variation, individual organisms differ in cost and/or adaptive status relative to that of the optimal phenotype in a giving environment. Costs of inducible phenotypes are a central component of the evolution of plasticity (DeWitt et al. 1998; Auld et al. 2010) but have proven difficult to measure empirically. Variation in phenotypic plasticity can produce several adaptive states (i.e., adaptive, maladaptive, or neutral); therefore, studies of phenotypic plasticity tend to focus on cost detection and adaptation status (Auld et al. 2010; Murren et al. 2015). Because even trait variation of phenotypic plasticity is linked to evolution (Bolnick et al. 2011), it is important to clarify why variance in plasticity traits occurs and is maintained in the environment.
Predation is an important factor driving natural selection, and defensive traits are expressed against predators in a plastic or constitutive manner. Daphnia (Arthropoda Crustacea) is an excellent model system for studying predator-induced plasticity (Tollrian and Dodson 1999; Lass and Spaak 2003), with alterations in their phenotype against predators including changes in body size, head shape, tail length, number of eggs, reproduction status, and distribution depth (Lass and Spaak 2003). To express predator-induced plasticity, Daphnia need to perceive predatory kairomone (chemical substance) and/or other factors besides predators; the former is called primary factor and the latter secondary factor (Riessen & Gilbert 2018). Riessen and Gilbert (2018) suggested in a review that secondary factors are related to increases or decreases in the degree of plasticity. This suggests that predator-induced plasticity displays different trait values among individuals owing to the interaction between primary and secondary factors. Therefore, a wide range of factors can induce predator-induced plasticity. Considering variations in predator-induced plasticity, it is important to consider how secondary factors as well as the essential triggers work. There are numerous studies focusing on the predator-induced plasticity ofDaphnia , making it potentially feasible to target and synthesize the various secondary factors affecting variations in this plasticity.Daphnia are tractable in various experimental settings and can be analyzed with modern genomic tools (Miner et al. 2012) and large-scale gene expression technology (Colbourne et al. 2011). Specifically,Daphnia pulex is the first crustacean to have its whole genome sequenced (Colbourne et al. 2011). Moreover, multiple studies ofDaphnia have identified the neural mechanisms associated with predator-induced defenses (Miyakawa et al. 2015; Weiss et al. 2015; Weiss and Tollrian 2018). It can also argued that, based on the predator-prey system, the elucidate secondary factors regulating variations in Daphnia plasticity could lead to a deeper understanding of phenotypic plasticity.
The goal of this review is to clarify variations in predator-induced plasticity in Daphnia and summarize the secondary factors influencing those variations. We begin with a brief overview of variations of inducible defenses in Daphnia and then examine the relationship between plasticity variation and the various secondary factors involved. Recent theoretical work indicate that intraspecific trait (non plasticity) variation can have significant ecological effect (Bolnik et al. 2011), the variation of degree of expression in inducible defense might have likewise significant relationship ecological and evolutionary context. Exploring such variations associated with inducible defense is a critical step in clarifying how changes in traits occur and are maintained according to the environment.