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Based on previous studies, extreme (>99%) self‐sterility in scarlet gilia (Ipomopsis aggregata) appears to be involved in late‐acting ovarian self‐incompatibility (OSI). Here, we confirm this suggestion by comparing structural events that follow from cross‐ vs. self‐pollinations of I. aggregata. Growth of cross‐ and self‐pollen tubes in the style at 11 h and growth in the ovary at 24 h was equivalent. Nonetheless, by 24 h, cross‐pollen effected a significantly higher percentage of both ovule penetration and fertilization. Ovules in self‐pollinated flowers showed pronounced changes, including an absence of embryo sac expansion and reduced starch in the integument, by 11 h post‐pollination, well before pollen tube entry into the ovary. In addition, the integumentary tapetum and adjacent 1–3 cell layers exhibited abnormal cell division, pronounced deposition of thick, pectin‐rich cell walls, and cellular collapse. Ovules and embryo sacs from cross‐pollinated flowers rarely showed such features. Developmental changes in ovules from self‐pollinated flowers eventually resulted in integument and embryo sac collapse, a process not observed in ovules of unpollinated flowers. We suggest that OSI involves long‐distance signaling between self‐pollen or self‐pollen tubes and carpel tissue that reduces availability of receptive ovules for fertilization before pollen tubes arrive in the ovary.
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