Pollination and late-acting self-incompatibility in Cyrtanthus breviflorus (Amaryllidaceae): implications for seed production

Abstract

Background and aims: Animal pollination is typically an uncertain process that interacts with self-incompatibility status to determine reproductive success. Seed set is often pollen-limited, but species with late-acting self-incompatibility (SI) may be particularly vulnerable, if self-pollen deposition results in ovule discounting. Pollination is examined and the occurrence of late-acting SI and ovule discounting assessed in Cyrtanthus breviflorus.

Methods: The pollination system was characterized by observing floral visitors and assessing nectar production and spectral reflectance of flowers. To assess late-acting SI and ovule discounting, growth of self- and cross-pollen tubes, and seed set following open pollination or hand pollination with varying proportions of self- and cross-pollen, were examined.

Key results: Native honeybees Apis mellifera scutellata pollinated flowers as they actively collected pollen. Most flowers (≥70 %) did not contain nectar, while the rest produced minute volumes of dilute nectar. The flowers which are yellow to humans are visually conspicuous to bees with a strong contrast between UV-reflecting tepals and UV-absorbing anthers and pollen. Plants were self-incompatible, but self-rejection was late-acting and both self- and cross-pollen tubes penetrated ovules. Seed set of open-pollinated flowers was pollen-limited, despite pollen deposition exceeding ovule number by 6-fold. Open-pollinated seed set was similar to that of the cross + self-pollen treatment, but was less than that of the cross-pollen-only treatment.

Conclusions: Flowers of C. breviflorus are pollinated primarily by pollen-collecting bees and possess a late-acting SI system, previously unknown in this clade of the Amaryllidaceae. Pollinators of C. breviflorus deposit mixtures of cross- and self-pollen and, because SI is late-acting, self-pollen disables ovules, reducing female fertility. This study thus contributes to growing evidence that seed production in plants with late-acting SI systems is frequently limited by pollen quality, even when pollinators are abundant.

Fig. 1.

Flowers of Cyrtanthus breviflorus (A) approached by a honeybee and (B) with honeybee contacting anthers. Abbreviation: s, stigma. Scale bars = 5 mm.

Fig. 2.

Reflectance spectra for (A) pollen and anthers and (B) upper and lower tepals in Cyrtanthus breviflorus. Each curve represents the mean spectrum calculated from individual replicates.

Fig. 3.

Mean (± s.e.) percentage of (A) fruit set and (B) seed set following cross-, self- and open-pollination in the Campus and Pelham patches of Cyrtanthus breviflorus, as indicated.

Fig. 4.

Pollen tubes in cross-pollinated (on the left) and self-pollinated (on the right) pistils of Cyrtanthus breviflorus. Fluorescence micrographs show pollen tubes on stigmas (A, B), in mid-styles (C, D), at the top of ovaries (E, F) and associated with individual ovules (G, H).

Fig. 5.

Mean (± s.e.) percentage of fruit set and seed set, as indicated, of Cyrtanthus breviflorus flowers pollinated with cross-pollen, cross + self-pollen and cross + dead pollen, and open-pollinated.