Heritability of walnut seed size and the benefits of large seeds

Japanese walnuts (Juglans mandshurica) are known to be seed-dispersed by the hoarding of two species of rodents, Japanese squirrels (Sciurus lis) and large Japanese field mice (Apodemus speciosus). The previous study showed that the squirrel transported and stored larger walnut seeds farther, whereas the mice transported smaller seeds farther. Divergent selection by different seed dispersers may play an important role in differentiation between local walnut populations, in which understanding seed size heritability is crucial. This study aims to determine how the seed size selected by rodents affects the initial growth of walnuts and to what extent walnut seed size is genetically determined. Sixty walnut seeds were collected from a small seed-producing tree, and 50 walnut seeds from a large seed-producing tree were sown and grown for 18 years. The seed size produced from each tree showed that the heritability of seed size was high (h² = 0.52). When 50 smaller and 50 larger walnut seeds were planted under the same conditions, the dry weight of shoots and roots of the larger seeds was significantly greater than that of the smaller seeds, and root length was also significantly longer in the larger seeds. Seed size is heritable, so if squirrels are inhabiting, larger seeds, which are advantageous for regeneration, will be selected, and walnut trees producing larger seeds will be passed on to the next generation. Because selective hoarding has implications for seed establishment success, inhabiting a suitable seed disperser, the squirrel, is more critical for walnut regeneration than previously assumed.