Abstract: It is important to study hybrid seeds because it will illuminate hybrid inviability and reproductive isolation mechanism between plant species. Morphological characteristics and viability of seeds derived from self-pollination, cross-pollination and hybridization were analyzed in three sympatric Cypripedium species. Hybrid seeds with high viability were successfully obtained from sympatric plants C.flavum, C.tibeticum and C.calcicola. They were not significantly different in seed shape, surface sculpture compared with seeds from self-pollination and cross-pollination. The width, length, embryo ratio and pigmentation ratio of hybrid seeds were not significantly lower than those of seeds from self-pollination and cross-pollination. The results suggested that three sympatric Cypripedium species could be hybrid compatible and that hybrid inviability could not be the main reason of isolation among them. It was not necessary to produce other isolation mechanisms because pollinators with different sizes functioned as the main isolation mechanism between C.flavum and C.tibeticum or C.calcicola. However, with the same pollinators and hybrid viability, C.tibeticum or C.calcicola would produce other isolation mechanisms or they should be combined into one species.

Key words: Cypripedium, sympatric species, hybrid inviability, reproductive isolation