Comparative Analysis of Male and Female Floral Development in Actinidia chinensis and Actinidia eriantha

Members belonging to the genus Actinidia are perennial dioecious fruit trees. Variation in male and female floral organs during growth is crucial in sex determination. Previous studies have been limited to individual kiwifruit species ( Actinidia chinensis and Actinidia deliciosa ), and although two sex determination genes have been identified, their differential expression has not been linked to differences in pistil development. In the present study, we investigated male and female flower development in two kiwifruit species, namely, A. chinensis and Actinidia eriantha . Kiwifruit flower development was divided into seven stages. The first four stages are common to both male and female flower development in kiwifruits, and the primary stage of male and female flower development in kiwifruits begins at stage 5, which is largely reflected by the absence of development of ovule primordia in male kiwifruit, whereas differences in flower development between the two kiwifruit species are primarily in the timing of the initiation of the squaring stage, time required for the later stages of flower development (stage 6), and size of flower buds. Quantitative results demonstrated that SyGl was persistently and highly expressed after stage 4 in male kiwifruit flowers, possibly causing abnormal development of the pistil and ovary malformation in male kiwifruit flowers. In contrast, FrBy was up-regulated in male flowers of A. chinensis and A. eriantha during late developmental stages (stage 5 or stage 6), coinciding with pollen maturation. We propose that this peaked expression is linked to the programmed degradation of the anther chorionic layer, facilitating timely dehiscence and pollen maturation. Altogether, the findings of this study link the stages of kiwifruit flower development in the two species with their phenological periods and flower bud morphology, thus providing a reference for future comparison of flower development stages and gene expression during development in kiwifruits.