Walnuts are monoecious and cross-pollinating fruit trees, that is, on the same tree, there are both male and female flowers, male flowers grow in the middle and lower parts of the branches, and female flowers grow on the top of the new branches. Although the male and female flowers of walnuts are on the same tree, the time when male and female flowers bloom is inconsistent. Some male flowers bloom first, and some female flowers bloom first. In botany, this is called "heterodichogamy".
This heterodichogamy directly affects the sexual reproduction process such as flowering and pollination, and affects the quality and yield of walnut kernels. Therefore, exploring the key genes for the formation of walnut heterodichogamy is extremely important for improving the quality and yield of kernels.
In January 2025, Graham Coop's research team at the University of California, Davis, published a research paper titled "Ancient structural variants control sex-specific flowering time morphs in walnuts and hickories" in Science. The study found that ancient genetic structural variations control sex-specific flowering time morphs in walnuts and hickories.
It is known that heterodichogamy is genetically controlled by a single locus in the Juglandaceae family, with dominant alleles for female-preferential flowering. These patterns suggest that heterodichogamy may have evolved in their common ancestor. This article studies the molecular basis of the inheritance of heterodichogamy in these genera, its evolutionary origins and maintenance over deep time scales, and its genetic similarities to dioecy.
This study first identified a narrow (~20-kb) genomic region in Juglans hindsii that controls flowering type and further found that this region is associated with flowering type throughout the Juglans genus, spanning ~40 million years of evolutionary history. This region contains a clear candidate gene (TPPD1) related to trehalose-6-phosphate metabolism, a known flowering time pathway.
The shared polymorphism pattern in Juglans is localized to the 3' untranslated region (UTR) of TPPD1 and adjacent tandem sequences of 8 to 12 inverted 3' UTR paralogs found only in the dominant haplotype.
In addition, the study identified a group of small RNAs transcribed from this tandem array at the same developmental stage and suggested that these RNAs may play a role in regulating TPPD1.
In Carya illinoinensis, the study identified a single 200 to 445 kb region controlling heterodichogamy. This region shows strong linkage disequilibrium across ~20 protein-coding genes, several of which are involved in flower development. These haplotypes are isolated throughout the Carya genus, and the authors estimate that they diverged >50 million years ago.
Figure 1. Balanced polymorphism for flowering order in walnuts and hickories is controlled by multiple genetic mechanisms. (Groh, et al., 2025)
In summary, the two different mechanisms are highly conserved, and Juglans and Carya derive the same flowering mechanism from completely different genes, reflecting that the ancestors of walnuts and hickories converged on similar solutions during evolution. But it is also possible that this temporally separated flowering system appeared in this family even earlier, about 70 million years ago, but the exact genetic mechanism that implements it has changed over time.
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