Elaborated Pollen Packaging and Dispensing Mechanism Induced by Petal Architecture from a Papaveraceae Species
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Elaborated pollen packaging and dispensing mechanism induced by petal architecture from a Papaveraceae species Shanlin Yang1, Guangming Chu1, Xiang Shi1 and Shaoming Wang2 1 College of Agriculture, Shihezi University, Shihezi, China 2 College of Life Sciences, Shihezi University, Shihezi, China ABSTRACT Secondary pollen presentation (SPP) is a reproductive strategy that enhances the efficiency of pollen transfer, which has been explored for more than 200 years, resulting in 10 identified types of SPP. The ephemeral plant Hypecoum erectum L. (Papaveraceae) has an elaborate petal structure. The middle lobe is a key functional organ in SPP. To explore the importance of the middle lobe structure, we measured the flowering process, the curling movement and growth of the middle lobe, pollination characteristics, pollination efficiency, and the mating system in H. erectum in the field. The yellow middle lobe structure had an important role in attractingpollinators.Themiddlelobesontheinnerpetalsfunctionasaredundant cucullate structure and wrapped about 84%ofthetotalpollengrainsassoonas the anthers dehisced. These then grew upward and gradually presented pollen to pollinators via the roll out of themiddlelobes.Onebeespecies,Colletes vestitus from Colletidae, was the only effective pollinator of H. erectum.TheSPP mechanism increased the efficiency of pollen transfer by C. vestitus.Themiddle lobes, which wrapped pollen and grew upward, contacted the stigma and provided an advantage for self-pollination and outcrossing by growing upward higher than the corolla. Hypecoum erectum L. has a mixed mating system with selfing and outcrossing. Thus, the SPP mechanism plays a key role during the pollination process and is necessary for improving pollination efficiency and promoting Submitted 29 November 2018 reproductive success. Accepted 30 April 2019 Published 11 June 2019 Subjects Ecology, Plant Science Corresponding author Keywords Pollination process, Secondary pollen presentation, Middle lobe, Hypecoum erectum L., Xiang Shi, [email protected] Elaborate petals, Packaging and dispensing mechanism Academic editor Jasmine Janes INTRODUCTION Additional Information and Declarations can be found on Most flowers are hermaphrodites, obtaining fitness by both dispersing pollen (male page 15 function) and receiving pollen (female function) (Fan, Kress & Li, 2015). For some DOI 10.7717/peerj.7066 animal-pollinated flowers, pollen reception can be sufficient with only a few visits by Copyright pollinators, however, the reproductive success by pollen dispersal usually continues to 2019 Yang et al. increase with numerous visits by pollinators (Bell, 1985). Accordingly, there are stronger Distributed under selective pressures for the male function than the female function in the flower’s Creative Commons CC-BY 4.0 characteristics. In order to improve the male function, the pollen presentation mechanisms How to cite this article Yang S, Chu G, Shi X, Wang S. 2019. Elaborated pollen packaging and dispensing mechanism induced by petal architecture from a Papaveraceae species. PeerJ 7:e7066 DOI 10.7717/peerj.7066 which can promote pollen transfer will be favored (Harder & Wilson, 1994; Wilson et al., 1994). Plants use dispersing mechanisms or pollen packaging mechanisms in order to disperse pollen to pollinators (Lloyd & Yates, 1982). Packaging mechanisms allow the total pollen production of a plant to be distributed into separate parts in order to continually become available to pollinators. Plants can control the presentation speed and presentation time of pollen with the packaging mechanism (Lu & Tan, 2007). Dispensing mechanisms limit the available pollen that can be removed by a pollinator from the package in a single visit (Lloyd & Yates, 1982). The effective packaging and/or dispensing strategies could allow all of the pollinators visiting a flower to take part in pollen dispersal, while ensuring the removal of most of the pollen produced by the time flowering is complete (Harder & Thomson, 1989). Petals are a characteristic feature of eudicots and the most noticeable organs of the flower. They have evolved into a great variety across diverse clades. Color, morphology, size, structure, and the function of petals reveal a broad diversity over evolutionary time and the elaborate petals mainly present as tubular, bilabial, cup-shaped, flat, marginal lobes, and ventral lobes of various shapes (Irish, 1998; Endress & Matthews, 2006; Zhao, 2016; Thien et al., 2009). A variety of lobation patterns can play a crucial part in strengthening the floral structure (Schönenberger, 1999), attracting pollinators (Thien & Marcks, 1972), guiding pollinators to the nectar (Rogers, 2004), promoting self-pollination (Liu et al., 2008), and secondary pollen presentations (SPPs) (Yeo, 1993). The middle lobes, as part of the lobation pattern, can be adapted to the lateral lobes and become important petal structures (Moody & Hufford, 2000).ThemiddlelobeinChenorchis has a plane structure across which pollinators can moveandwhichisalsoanimportantstructure for self-pollination in some plants (Liu et al., 2008). Endress & Matthews (2006) analyzed 58 species of plants with special petal characteristics in 30 families and found that the lobation patterns in 11 families were closely related to the evolution of floral characteristics. However, there is limited research on the evolution of lobation patterns, especially the middle lobes (Riviere, Clayson & Dockstader, 2013). Secondary pollen presentation (SPP) is the phenomenon where pollen is presented to pollinators on floral structures (other than the anthers) or via particular mechanisms of expulsion where pollen contacts other floral structures (Yeo, 1993). Secondary pollen presentation was first described by Sprengel (1793) in Campanula. However, for a long time afterwards, this phenomenon has been neglected in the study of floral characteristics. International studies of pollen presentation schedules in the evolution of angiosperms have mainly focused on the past 30 years. Concurrently, some researchers have explored the evolution of floral characteristics from the perspective of the pollinator’s adaptations, so that the pollen presentation theory can be formed and continuously improved (Wang & Tan, 2011). The presentation of pollen on a surface other than the primary pollen presentation organ (anthers) may protect pollen from desiccation or exploitation, aid in cross-pollination or self-pollination, and increase the efficiency of pollen delivery (Yeo, 1993). Attracted pollinators and the enhanced male function of the flowers of SPP plants is obvious via the extension of the male phase by the protection, controlled release, and clear placement and receipt of the pollen (Howell, Slater & Knox, 1993). This phenomenon has only been Yang et al. (2019), PeerJ, DOI 10.7717/peerj.7066 2/18 recorded in 25 families, including five monocotyledonous and 20 dicotyledonous families (Fan, Kress & Li, 2015). There are many interesting SPP mechanisms not yet discovered in nature. As one of the main plant families with elaborate petal structures, the Papaveraceae have been studied since the 19th century. Elaborate floral characteristics mainly occur in some genera of Dicentra, Corydalis, and Hypecoum. Hypecoum species, however, have received very little research attention. The SPP function in Hypecoum, which is enhanced by the middle lobe structure, was first described and illustrated by Hildebrand in 1869. Hildebrand (1869), however, rarely observed pollinators visiting flowers and did not record stigma receptivity. Dahl (1989) found that median anthers in a single flower in Hypecoum are vascularized by two bundles and the transversal anthers by one. Although the above scholars have made many groundbreaking discoveries on the Hypecoum, many details related to its pollination characteristics and breeding system have not been discussed. Hypecoum erectum L. is a typical ephemeral plant and is widely distributed throughout the Gurbantunggut Desert in Xinjiang, China. The H. erectum flower is bisymmetric and characterized by the elaborate shape of the two inner petals. The inner petals are deeply trilobate with yellow middle lobes. The aim of this study was to research the floral characteristics and explore how the middle lobes improve male fitness of H. erectum.We proposed the following hypotheses: Hypecoum erectum L. may have a SPP mechanism because of the relative position of the middle lobes and stamens, and the process of gradual pollen presentation in the middle lobes plays an important role in increased pollination efficiency because the length of the pollen presentation time and visitation frequencies will be increased. In light of the above hypotheses, we investigated the floral characteristics and pollination adaptability of H. erectum. The study of such elaborate petal structures in H. erectum will be helpful in understanding the adaptations of SPP and the role of reproductive success in floral evolution. MATERIALS AND METHODS Study site and species All experiments were carried out in 2017 at the Cainan oilfield (4502′22″ N, 8823′26″ E; 728 m a.s.l; 100 m  100 m), which is on the southeastern edge of the Gurbantunggut Desert in the Xinjiang Uyghur Autonomous Region, China. Periodically, steady snow in this region creates an opportunity for species to germinate. In 2017, there was steady snow cover for 100–160 days, which was 20–30 cm deep in the winter. According to the data from the nearest weather station, the Fukang National Field Scientific Observation and Research