This week’s Evolution 101 blog post is by MSU graduate student Emily Weigel.
Mothers have more of an effect on their offspring than one might first think. In addition to the DNA, both mitochondrial and nuclear, that a mother provides to her children, mothers also influence the development of their children through “maternal effects.” Maternal effects act on the expressed characteristics of the organism (the organism’s “phenotype”) so that the organism is influenced not just by its own environment and genes, but also by the environment and phenotype of its mother.
What are maternal effects, and how do they work?
Maternal effects can be seen in the way a mother provisions her eggs with mRNA, proteins, hormones, or antibodies, which can control the size, sex, growth, or behavior of her offspring. Mothers can also directly influence their offspring through their own behavioral traits; behaviors like nursing, grooming, predator defense, and “decisions” on when and where to lay eggs can all affect the offspring and its survival.
For example, in many species, including frogs, fish, and mites, traits delaying a mother’s reproduction can result in offspring that hatch later, develop more slowly, and mature at larger body sizes, all of which can promote offspring survival and reproductive success. In this same way, maternal effects may not always be adaptive, as is the case with mayfly mothers who mistake the sheen of an asphalt road for a good, wet place to deposit eggs. Those offspring are not likely to survive not because of their inherent characteristics, but because of their mother’s egg-laying behavior.
Although offspring-and maternal traits obviously interact (for example, feeding behavior by a mother and begging behavior by her offspring), maternal effects are generally thought to be a direct result of the mother’s actions. Considering this definition, indirect effects resulting from the mates a mother chooses, for example, are not maternal effects, but simply yet another way mothers impact their offspring.
Why do maternal effects matter?
Maternal effects can produce meaningful variation in a population upon which selection can act. Selection acting on these traits will naturally affect the fitness of such organisms and therefore the evolutionary dynamics of the population, particularly with respect to the evolution of offspring traits subject to maternal effects and the maternal traits themselves. Additionally, maternal effects may be important for the evolution of adaptive responses (“phenotypic plasticity”) to rapidly changing environments, as they allow for immediate adjustments in phenotype based on the environment. Because those changes are often in both the offspring and mother, multi-generational effects under certain conditions may spread through the population much quicker than favored genetic mutations alone. Conversely, under perhaps less-rapidly changing conditions, time lags in the response to selection could be shown in traits subject to maternal effects because selection in one generation depends on selection of both the current generation and that of the previous. Understanding how and the speed at which populations may respond to environmental change informs our predictions of how populations may change in the future and to understand the interplay between evolutionary and ecological dynamics.
Because mothers impact their offspring through countless other ways (maternal cytoplasmic inheritance, genomic imprinting, and mitochondrial DNA, to name a few), perhaps today is a day to thank your mom for making the majority of you, you.