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Self-sterility in Ipomopsis aggregata (Polemoniaceae) is due to prezygotic ovule degeneration¹

²Department of Botany, University of Toronto, Toronto, Ontario, Canada M5S 3B2 ³Department of Biology, University of California, Riverside, California 92521 USA; and Rocky Mountain Biological Laboratory, Crested Butte, Colorado 81224 USA

ABSTRACT

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.

Key Words: histology • long-distance signaling • ovarian self-incompatibility (OSI) • ovular collapse • Polemoniaceae • Rocky Mountain Biological Laboratory • self-incompatibility