Hybridisation, pollen–pistil interactions and hybrid seed set among Casuarina species

Context Casuarina trees are extensively planted in the tropics for wood production and environmental services. The breeding program’s current focus is on developing hybrids that combine the specific characteristics of different species and are suitable for various planting environments and end uses. Aim Breeding programs aim to develop interspecific hybrids possessing desirable combinations of species-specific characters of Casuarina for increasing plantation productivity. Methods Controlled-pollination studies were conducted involving C. equisetifolia, C. junghuhniana and C. cristata to determine the possibility of hybridisation among them and to evaluate the hybridisation success rate in different cross-combinations. The pollen-tube growth in the pistil was studied to understand the role of pollen–pistil interactions in hybridisation success. Key result Fourteen cross-combinations of the three species resulted in 2–57% fruit set and 75.1–93.8% seed set. C. equisetifolia and C. junghuhniana hybridised reciprocally, whereas C. cristata hybridised with other species only as pollen parent. The larger flower size of C. cristata acted as a barrier preventing hybridisation with species having smaller-sized flowers. Variation in seed germination was large (3.6–91.3%), highest for C. junghuhniana and lowest for C. equisetifolia mothers. Fertilisation occurred 45 days after pollination in C. equisetifolia and C. junghuhniana and after 50 days in C. cristata. The ovary was immature at pollination and pollen tubes showed discontinuous growth in the pistil marked by periods of arrest for the ovules to develop. The stigma showed 3–5.7 pollen tubes, and invariably a single tube entered the ovary. The absence of major pre-fertilisation barriers but varying levels of hybridisation success indicates post-zygotic selection through embryo abortion. Conclusion Increasing the parental combinations of interspecific crosses where fruit set is limiting and pollinating more flowers per cross where germination is low may enhance hybridisation success. Implications The cross compatibility among different species of Casuarina provides scope to produce site and end-use specific hybrids to meet the commercial and livelihood needs.