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.