Study on micropyle closure in Platycladus orientalis and its evolutionary significance

[Objective] In order to enrich the data for the study of micropyle closure in gymnosperms and to provide a reference for the phylogenetic study of gymnosperms, micropyle closure of Platycladus orientalis Franco was investigated. [Methods] Female cones of P . orientalis were sampled and examined using morphological techniques, including paraffin section and scanning electron microscopy. In addition, cluster analysis of the mode of micropyle closure was conducted for Bennettitales. [Results] (1) After pollen enters the ovule, the innermost two cell layers of the integument began to grow radially inward until the micropyle was completely closed. (2) During the micropyle closure, the innermost cells of the integument were elongated and became conical in shape, with the cell layers typically increased to 4 to 6 layers. (3) The micropyle closure of gymnosperms could be divided into four clades: One for Cycas and Ginkgo, one for Pinus and Podocarpaceae, a separate group for Bennettitales, and a cluster of P . orientalis and other species. [Conclusion] The micropyle closure of P . orientalis differs from Ginkgo and cycads, which is similar to Pinus and Bennettitales. This study suggests that there is an evolutionary relationship in micropyle closure between Bennettitales and existing conifers, providing novel morphological evidence for the understanding of evolution of the reproductive systems of gymnosperms.