Pollen and stamen mimicry: the alpine flora as a case study

Arthropod-Plant Interactions, Apr 2017

Many melittophilous flowers display yellow and UV-absorbing floral guides that resemble the most common colour of pollen and anthers. The yellow coloured anthers and pollen and the similarly coloured flower guides are described as key features of a pollen and stamen mimicry system. In this study, we investigated the entire angiosperm flora of the Alps with regard to visually displayed pollen and floral guides. All species were checked for the presence of pollen- and stamen-imitating structures using colour photographs. Most flowering plants of the Alps display yellow pollen and at least 28% of the species display pollen- or stamen-imitating structures. The most frequent types of pollen and stamen imitations were (mostly yellow and UV-absorbing) colour patches on petals (65% of species displaying imitations), patterns of inflorescences (18%), stamen-like pistils (10%), and staminodes (6%), as well as three-dimensional structures such as convex lower lips and filamental hairs (<5%). Dichogamous and diclinous species display pollen- and stamen-imitating structures more often than non-dichogamous and non-diclinous species, respectively. The visual similarity between the androecium and other floral organs is attributed to mimicry, i.e. deception caused by the flower visitor’s inability to discriminate between model and mimic, sensory exploitation, and signal standardisation among floral morphs, flowering phases, and co-flowering species. We critically discuss deviant pollen and stamen mimicry concepts and evaluate the frequent evolution of pollen-imitating structures in view of the conflicting use of pollen for pollination in flowering plants and provision of pollen for offspring in bees.

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Pollen and stamen mimicry: the alpine flora as a case study

Arthropod-Plant Interactions Pollen and stamen mimicry: the alpine flora as a case study Klaus Lunau 0 1 2 Sabine Konzmann 0 1 2 Lena Winter 0 1 2 Vanessa Kamphausen 0 1 2 Zong-Xin Ren 0 1 2 Anther Stamen Sensory 0 1 2 0 Key Laboratory for Plant Diversity and Biogeography of East Asia, Kunming Institute of Botany, Chinese Academy of Sciences , Kunming, Yunnan 650201 , People's Republic of China 1 Institute of Sensory Ecology, Department Biology, Heinrich- Heine-University Du ̈sseldorf, Universita ̈tsstr. 1, 40225 Du ̈sseldorf , Germany 2 & Klaus Lunau Many melittophilous flowers display yellow and UV-absorbing floral guides that resemble the most common colour of pollen and anthers. The yellow coloured anthers and pollen and the similarly coloured flower guides are described as key features of a pollen and stamen mimicry system. In this study, we investigated the entire angiosperm flora of the Alps with regard to visually displayed pollen and floral guides. All species were checked for the presence of pollen- and stamen-imitating structures using colour photographs. Most flowering plants of the Alps display yellow pollen and at least 28% of the species display pollen- or stamen-imitating structures. The most frequent types of pollen and stamen imitations were (mostly yellow and UV-absorbing) colour patches on petals (65% of species displaying imitations), patterns of inflorescences (18%), stamen-like pistils (10%), and staminodes (6%), as well as three-dimensional structures such as convex lower lips and filamental hairs (\5%). Mimicry; Pollen exploitation; Pollination - In honour of Charles Michener and Stefan Vogel. Handling editor: Isabel Alves dos Santos and Isabel Machado. Introduction Melittophilous flowers display a great diversity of visual signals to attract pollinators (Wester and Lunau 2017) . For flower-visiting bees, floral colour patterns are of outstanding importance for long-distance perception and orientation at close range (Lunau et al. 1996) . Whereas largescale components of flowers and inflorescences serve as distant attractants, small-sized features function as floral guides directing flower visitors to distinct areas of the flower facilitating access to floral reward and/or pollen transfer (Lunau et al. 1996; Wilmsen et al. 2017) . Several morphological structures contribute to the development of small-sized components of floral colour patterns such as pollen, anthers, stamens, receptacles, styles, staminodes, colour patches, or the arrangement of florets within an inflorescence (Lunau 2000, 2007) . To understand signalling of flowers and inflorescences, it is helpful to assume that various structures might contribute to a uniform signalling colour pattern. By contrast, in many studies only colour markings of petals are regarded as floral colour patterns. In this study, the neutral term floral guide is preferred to nectar guide, pollen guide, or honey guide, which are terms that imply a specific purpose. Furthermore, the non-interpretive term imitation is used to indicate the similarity of structures, whereas the term mimicry is used to denominate structures thought or known to deceive flower visitors owing to their inability to discriminate between the mimic and a model signal (Dafni 1984; Roy and Widmer 1999; Johnson and Schiestl 2016; Lunau and Wester 2017) . Floral colour patterns consist of at least two different colours, one usually large-sized, peripheral colour component and one generally small-sized, central colour element. In the 1970s, two researchers highlighted—largely independently of one another—one common particular aspect of floral colour patterns, namely signalling structures visually resembling pollen and anthers in colour and shape. The zoologist Gu¨ nther Osche focused on the evolution of these stamen-imitating floral guides (Osche 1979, 1983a) . He regarded the colour of pollen as the very first floral colour signal in angiosperm evolution and a cue for flower visitors predating the evolution of conspicuous petals (Osche 1986) . He also argued that primarily anemophilous flowering plants developed yellow and UV-absorbing pollen by embedding flavonoid pigments as protection against bacteria, fungi, and ultraviolet radiation. Flavonoids like quercetin thus represent a preadaptation and a reliable cue for the very first flower visitors at a time when flowers had neither petals nor other signalling structures (Osche 1983a) . Osche (1979, 1983a, b, 1986) distinguished various forms of mimicry such as pollen, anther, stamen, and androecium mimicry, independently of the way of deception of the signal receivers. The botanist Stefan Vogel discussed the deceptive nature of floral guides that possess the same colour as pollen and stamens (Vogel 1978) and categorised various types of deception (Vogel 1993) . Contrary to Osche, Vogel did not agree that pollen- and stamen-mimicking floral guides might function as nectar guides and thus deceive ne (...truncated)


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Klaus Lunau, Sabine Konzmann, Lena Winter, Vanessa Kamphausen, Zong-Xin Ren. Pollen and stamen mimicry: the alpine flora as a case study, Arthropod-Plant Interactions, 2017, pp. 427-447, Volume 11, Issue 3, DOI: 10.1007/s11829-017-9525-5