Why are the offspring of older mothers less apt to live and prosper?

The offspring of older mothers do not fare as well as younger mothers, in humans and in many other species. They are not as healthy, or do not live that long, or have fewer offspring themselves. An old puzzle is why evolution would maintain this maternal effect in so many species, since these late descendants are less apt to survive and reproduce.

In a new study on rotifers (microscopic invertebrates), scientists tested the evolutionary fitness of offspring of older mothers in various real and simulated environments, including the relative luxury of laboratory culture, under threat of predation in the wild or with a supply reduced food. They confirmed that this effect of increased maternal age, called senescence of the maternal effect, reduces the developmental fitness of the offspring in all settings, primarily through reduced fertility during their peak reproductive period. They also suggest an evolutionary mechanism for why this may occur. The study, led by Kristin Gribble of the Marine Biology Laboratory and Christina Hernández of the Woods Hole Oceanographic Institution, is published this week in procedures of the National Academy of Sciences.

“This study is unique in that it combines laboratory data from our previous work with mathematical models to address a longstanding question in the evolution of aging,” says Gribble. “Natural selection should eliminate these less apt offspring of older mothers. So why do we see this phenomenon in so many species?”

To address this, Hernández and his collaborators constructed mathematical models to calculate, for the first time, the strength of the pressure of natural selection on the survival and fertility of descendant populations based on the age of their mothers. They found that this pressure, called the selection gradient, decreases with maternal age.

“Because selection pressure decreases as mothers age, it may not be strong enough to eliminate these adjustment problems [offspring] of the population, “says Hernández.

“Because of this, the senescence of the maternal effect will persist and continue to evolve in the population, even though it results in decreased physical status,” adds Gribble. They still do not fully understand the genetic mechanisms that cause the quality of offspring to decline with maternal age.

The models the team developed can be applied to a wide range of species to assess the physical consequences of senescence from the maternal effect. “As long as you have experimental data, like us, on life expectancy and fertility of offspring of mothers of different ages, you can address this issue in many organisms,” says Gribble.