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.