Section 1: A Long-Standing Mystery Decoded
Biologists at the University of California, Davis, have unveiled the genetic mechanism behind the alternating flowering patterns of walnut trees. Published in Science, the study reveals that walnuts alternate between male and female flowering within a single season, with individual trees consistently beginning as either “male-first” or “female-first.” This phenomenon, first observed by Charles Darwin in 1877, has remained genetically stable for 40 million years and shares surprising parallels with sex determination in humans and other animals.
Jeff Groh, a graduate student in population biology and lead author of the study, explained, “Walnuts and pecans exhibit temporal dimorphism, alternating between male and female flowering during the season. While this has been known since the 1800s, the molecular basis was unclear until now.” The team discovered that this unique flowering system helps walnuts avoid self-pollination, an evolutionary strategy common among flowering plants to enhance genetic mechanism.
Section 2: Tracking the Genetics of Flowering Patterns
The research drew on extensive data from UC Davis’s walnut breeding program and native Northern California black walnut trees. By monitoring the trees’ flowering sequences and assigning them to “male-first” or “female-first” categories, Groh and his doctoral advisor, Professor Graham Coop, sequenced the genomes of these trees. They identified two genetic variants associated with the flowering trait in walnuts, which have persisted in at least nine species for millions of years.
This genetic variation is maintained through a balancing mechanism: when one flowering type becomes more prevalent, the other type gains a reproductive advantage, pushing the population back to an equilibrium ratio of nearly 1:1. “It’s unusual to see such stable genetic variation over such a long period,” noted Groh.
Interestingly, the same mechanism operates in pecans, another member of the walnut family. However, in pecans, the genetic polymorphism governing flowering order is located in a different region of the genome and is believed to be over 50 million years old, predating the walnut’s genetic system.
Section 3: Evolutionary Parallels with Animal Sex Chromosomes
The study raises intriguing questions about how related species like walnuts and pecans evolved similar flowering systems using distinct genetic pathways. Researchers suggest two possibilities: either their ancestors independently developed similar solutions to temporal flowering, or the system emerged even earlier—about 70 million years ago—and evolved through different genetic mechanisms over time.
What’s particularly striking is the resemblance between these flowering systems and animal sex chromosomes, such as the X and Y chromosomes in humans, which also maintain a balanced variation for sex determination. Groh pointed out, “There’s a clear parallel to a common mode of sex determination in animals.”
This groundbreaking research not only sheds light on the genetic intricacies of walnuts but also deepens our understanding of how plants and animals have converged on similar evolutionary strategies to ensure reproductive success over millions of years.
