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La hidden flowers1/28/2024 The number of ovules per flower in the pin morph and thrum morph is not significantly different (167.4 ± 29.5 versus 172.3 ± 33.1 t = 0.66, d.f. Like most other distylous species, this species has a strict self- and intra-morph incompatibility system ( Wedderburn & Richards 1990). (2009) for details of the floral morphology), and each flower lasts 8.0 ± 1.4 days. The flowers are pendulous bell-shaped (see Zhu et al. Within this site, individual plants produce 20.0 ± 7.2 (mean ± s.d.) flowers on a single main stem (31.4 ± 7.9 cm), and 3.9 ± 2.0 flowers are open simultaneously (X.-F. Primula secundiflora is a distylous perennial widely distributed in alpine regions of southwestern China, and the study was conducted at the Shangri-la Alpine Botany Garden, Yunnan, China (SABG 27★4′20′′ N, 99☃8′11′′ E altitude, 3260 m a.l.s.). We tested this mechanistic hypothesis using detailed observations and measuring seed production of manipulated flowers that were visited only by robbers. We suggest that nectar robbers touch both high and low sexual organs, and thus pollinate both floral morphs. We found that robbing holes were positioned between high and low sexual organs for both floral morphs ( figure 1). secundiflora are frequently robbed by bumble-bees. However, nectar robbing has been reported only in a limited number of distylous species, and little is known about the effect of nectar robbing on the evolution and ecology of distyly ( Irwin & Adler 2006). 2009), and between different floral types of sexually polymorphic plants ( Zhang et al. Recent studies have shown that the effect of nectar robbing on host plants varies among species with different mating systems ( Burkle et al. Most distylous plants are self- and intra-morph incompatible and distyly has traditionally been viewed as an adaptive trait to promote inter-morph pollen transfer ( Ganders 1979). The height of stigmas in pin flowers corresponds to the height of anthers in thrum flowers, and vice versa. Distylous plants have two floral morphs: pin flowers with long styles and short stamens, and thrum flowers with short styles and long stamens. In this study, we examined the possibility of robber-like pollination in distylous Primula secundiflora. It remains unclear whether nectar robbers could pollinate flowers with sexual organs deeply hidden within corollas. To our knowledge, in all cases of robber-like pollination, the stamens and pistils protruded from corollas, and pollination occurred when nectar robbers (usually bumble-bees or carpenter bees) unintentionally came into contact with these sexual organs. Such robbers were described as robber-like pollinators by Higashi et al. However, some studies have shown that robbers can directly pollinate flowers ( Macior 1966 Koeman-Kwak 1973 Waser 1979 Higashi et al. Nectar robbers usually obtain reward without providing a pollination service ( Inouye 1983 Maloof & Inouye 2000), so they have frequently been described as cheaters in the plant–pollinator mutualism system since Darwin (1872). Nectar robbing occurs commonly in flowering plants, especially those with tubular flowers or flowers with nectar spurs ( Irwin & Maloof 2002). Such asymmetrical pollen flow caused by nectar robbers may act as an important selective agent in floral fitness and evolution of distyly. Our findings suggest that nectar robbers transfer pollen from pin flowers to thrum flowers effectively, and consequently increase male fitness of the pin morph and female fitness of the thrum morph. This resulted in 98 per cent of thrum flowers and 6 per cent of pin flowers setting fruit, and seed number per thrum fruit was also significantly higher than per pin fruit. We manipulated flowers so that only nectar robbers could visit them. We observed that pollen grains of pin flowers were removed while thrum flowers received fresh pollen grains immediately after flowers were robbed. secundiflora were commonly robbed by bumble-bees, and robbing holes were always situated between high and low sexual organs for both floral morphs. Pin flowers have long styles and short stamens, and thrum flowers have short styles and long stamens. In this study, we examined whether robbers pollinate flowers of distylous Primula secundiflora. Nectar robbers are thought rarely to pollinate flowers, especially those with sexual organs hidden within corollas.
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