Introduction
Spatially and temporally heterogeneous environments promote phenotypic
plasticity, the propensity of an organism to change its phenotype in
response to changes in the environment (West‐Eberhard, 2003). Under
natural selection, adaptive phenotypic plasticity evolves such that the
resulting reaction norm gives higher fitness across the changing
environment (Lande, 2009). Phenotypic plasticity can also be transferred
from mother to offspring such that the maternal response to the
environment induces changes to the offspring reaction norm (Uller,
2008). Maternal effects can then impact offspring fitness and ultimately
population dynamics (Benton et al., 2008). Hence, expression of adaptive
reaction norms is essential to maintain high fitness as well as
population viability in heterogeneous environments. This is especially
the case for reaction norms that are expressed in response to food
abundance, as changes in these can have strong effects on different
components of life history (Boggs, 2009). In short-lived species for
example, resource allocation to somatic maintenance (including survival)
increases at the cost of growth and reproduction when food is limited
(Lynch, 1989; Martínez-Jerónimo et al., 1994). Resulting changes in
maternal resource allocation to offspring can ultimately influence
offspring survival and reproduction (Enserink et al., 1995; Hafer et
al., 2011; Saastamoinen et al., 2013).
At the physiological level, the neurotransmitter dopamine plays an
important role as mediator of trait responses to food (see Barron et
al., 2010 for a review on dopamine-mediated behavioural and
morphological responses to food across taxa). Issa et al. (2020) showed
that in addition to influencing morphological and behavioural traits,
dopamine can regulate life-history responses to food abundance.
Specifically, in the zooplankton species Daphnia magna, exposure
to dopamine caused life-history reaction norms (age at maturation,
fecundity) to change in a way that resulted in higher population growth
rates (calculated from maternal life history traits) when food was
limited (Issa et al., 2020). This happened without any apparent fitness
costs at high food abundance. These observations raise the question of
why endogenous dopamine levels do not evolve towards higher values. Issa
et al. (2020) suggested this may be due to costs of high dopamine levels
being paid by the offspring generation, which was not quantified in
their study. Negative maternal effects on offspring size from dopamine
treatments were detected. A smaller offspring size may have detrimental
effects on offspring survival since body size in Daphnia is
positively associated with filtering rates (Porter et al., 1983) and
offspring survival under food limitation (Gliwicz & Guisande, 1992).
Because offspring survival is a crucial component of maternal fitness,
investigation of costs to offspring from enhanced maternal dopamine
levels could give better insight into the selective forces shaping the
evolution of the dopamine system.
In this study, we experimentally tested for effects of maternal dopamine
exposure on offspring fitness. We exposed one generation
(F0) of D. magna to dopamine at high versus
restricted food ration and starved the offspring (F1) to
measure their starvation resistance. Based on the previous findings of
Issa et al. (2020), we predicted changes to the slopes of the
life-history reaction norms under dopamine exposure that result
in faster somatic growth and
smaller offspring when food is limited. Moreover, we hypothesized that
the smaller offspring originating from dopamine-exposed mothers would
experience reduced survival under starvation.