The Secrets Behind Walnut Sexes: Unraveling the Genetic Code

Flowering plants utilize multiple strategies to prevent self-pollination. Some achieve this by shaping their flowers to hinder self-pollination; certain species exist as separate “male” and “female” plants. Others time their male and female flowering separately. Trees in the family that encompasses walnut, hickory, and pecan take this concept a step further. A walnut tree produces flowers of one sex followed by the other in the same growing season, but the order varies among different trees. Specific individual trees are consistently “male-first” or “female-first” in their flowering pattern, a phenomenon recognized by Charles Darwin in 1877. In the 1980s, Scott Gleeson, a graduate student at UC Davis, observed that this characteristic was governed by a single genetic locus.

“Walnuts and pecans exhibit a temporal dimorphism whereby they alternate between male and female flowering throughout the growing season,” stated Jeff Groh, a graduate student in population biology at UC Davis and the primary author of the study. “This has been known since the 1800s but has not been understood at the molecular level until now.”

This phenomenon occurs in both cultivated walnuts and their wild relatives, such as the Northern California black walnut. Among wild types, the ratio of male-first to female-first trees is nearly equal at 1:1.

Groh, alongside his doctoral advisor, Professor Graham Coop of the Department of Evolution and Ecology, utilized data from UC Davis’ walnut breeding program and also monitored flowering in native Northern California black walnut trees located around the UC Davis campus. By categorizing them into male-first or female-first groups, the researchers sequenced their genomes and pinpointed sequences related to the trait.

Same process, different genes

In walnuts, they identified two variants of a gene corresponding with female-first or male-first flowering. This DNA polymorphism is present in at least nine walnut species and has demonstrated stability for close to 40 million years.

“It’s quite unusual to maintain diversity over such an extended period,” Groh remarked. In this scenario, the two flowering types offset one another. If one flowering type becomes more prevalent in the population, the less common type gains a reproductive advantage, thereby increasing its frequency. This pushes the system towards a 50:50 balance, preserving genetic diversity.

Pecans, Groh discovered, also possess a balanced genetic polymorphism that dictates flowering sequence, but this occurs in a different region of the genome compared to walnuts. The pecan polymorphism appears to be older than that in walnuts, at over 50 million years.

How did walnuts and pecans, being related, evolve to exhibit the same flowering mechanism through notably different genes?

It may be that the ancestors of walnuts and pecans converged on similar solutions as part of their evolutionary journey. However, it is also plausible that this temporally separated flowering system originated even longer ago in this family, approximately 70 million years back, with variations in the exact genetic mechanisms employed to achieve it over time.

Interestingly, this bears similarity to the function of animal sex chromosomes, with two structural variants (X and Y chromosomes in humans and other mammals) maintained in a state of equilibrium.

“There’s a clear connection to a common mode of sex determination,” Groh commented.

Additional co-authors of the study include: Diane Vik, Matthew Davis, J. Grey Monroe, Kristian Stevens, Patrick Brown, and Charles Langley, all affiliated with UC Davis. Financial support was provided by grants from the U.S. Department of Agriculture, National Institutes of Health, National Science Foundation, the Davis Botanical Society, and the American Society of Plant Taxonomists. This research utilized trees from the UC Davis Putah Creek Riparian Reserve; Gene Cripe from Turlock, California; USDA Wolfskill Experimental Orchard; Sonoma Botanical Garden and the UC Botanical Garden at Berkeley.