Evidence for Ovarian Self‐incompatibility as a Cause of Self‐sterility in the Relictual Woody Angiosperm Pseudowintera axillaris (Winteraceae)

摘要

Species within the genus Pseudowintera exhibit high rates of self‐sterility. Self‐sterility in the genus has been previously posited—but not confirmed—to be the result of late‐acting ovarian self‐incompatibility (OSI) functioning within nucellar tissue of the ovule to prevent self pollen tubes from entering the embryo sac. Structural and functional aspects of pollen–carpel interactions and early seed development following cross‐ and self‐pollination were investigated in P. axillaris to determine the site, timing and possible mechanisms of self‐sterility. No significant differences were observed between pollen tube growth, ovule penetration and double fertilization following cross‐ and self‐pollination. Pollen tubes exhibited phasic growth in an extracellular matrix composed of proteins and carbohydrates, as well as arabinogalactans/arabinogalactan proteins. A uniform failure in embryo sac development prior to division of the zygote was apparent within 15 d following double fertilization by self gametes. Results indicate that SI mechanisms in P. axillaris do not prevent double fertilization from occurring. Instead, mechanisms of self‐sterility affect post‐zygotic development of the embryo sac. Although self‐sterility may be attributed to inbreeding depression, given the post‐zygotic nature of failure in embryo sac development, the possibility of late‐acting OSI is discussed.