When it comes to the kids, mom and dad don’t always agree, waging an age-old battle of the sexes that starts in the genes. The struggle is universal enough that it extends even into the plant kingdom, where Harvard researchers say they’ve found a rare zone of calm — and where mom always wins.
That botanical Switzerland is in the ancient lineage of water lilies, specifically in a species that used to live in the runoff of hot springs and is now extinct in the wild. These lilies have figured out how to bypass the father’s input into feeding the next generation by starving a part of the seed holding the father’s genes and instead nourishing the kids solely from maternal tissue.
“It looks like what mom has basically done is locked dad out of the kitchen,” said Rebecca Povilus, who conducted the research while a graduate student working at Harvard’s Arnold Arboretum. “This is just a fundamentally different way of negotiating resources than what’s been seen in any other angiosperms so far.”
Angiosperms — the scientific term for flowering plants — are all around us, and their seeds are critically important to feeding the world. Some two-thirds of human calories come from angiosperm seeds, in particular a part called the endosperm, which nourishes the embryonic plant, according to Arnold Arboretum Director William Friedman, senior author of a paper on the work that was published last month in the journal Proceedings of the Royal Society B.
This tasty endosperm is what we grind to make bread, roll up in seaweed for sushi, or grab by the handful from the popcorn bucket at the movies.
The mother plant bulks up the endosperm with her own resources to nourish the baby plant, much like a yolk feeds a developing chick. But unlike in chickens, humans, or any animal, flowering plants reproduce through not just one fertilization, but two.
The first is the familiar blending of maternal and paternal genes spawning the embryo. The second fertilization creates the endosperm. The presence of paternal DNA means that the father can influence how the endosperm is used. This happens through a process called imprinting, in which different genes are activated depending on whether they come from mom or dad. (Imprinting also occurs in humans.)
Researchers have exploited this phenomenon to explore tension between paternal and maternal interests. Mom’s best strategy is to apportion resources to the healthiest seeds, those that have the best chance of carrying her genes to the next generation regardless of who the father is. Dad’s strategy, by contrast, is to maximize resources to the single seed that his pollen grain fertilized — regardless of its fitness — since pollen from many fathers has likely fertilized the plant’s other flowers.
In previous research, scientists sought answers by breeding plants with extra sets of chromosomes and crossing them with genetically normal plants, which gave rise to seeds with twice the number of paternal or maternal genes, magnifying parental strategies and making them easier to observe. In seeds with additional doses of paternal genes, the endosperm grew bigger — and more abnormal seeds developed — while the endosperms with a larger maternal contribution were smaller, close to normal size.
The current research, conducted by Povilus, Friedman (who is also the Arnold Professor of Organismic and Evolutionary Biology), and Pamela Diggle of the University of Connecticut, replicated that earlier work, but did so in an ancient lineage of plants to see if the parental struggle looked the same at the dawn of the evolution of flowering plants. For Friedman, the research is the latest in a nearly 25-year quest to understand how mothers and fathers interact within the confines of a flowering plant seed.
The findings echoed the earlier work when it came to the endosperm. But the researchers also discovered that the water lilies had developed a way for the mother to gain control over what happens in the seed by bypassing the endosperm almost entirely.
