Ecological Correlates of the Evolution of Pitcher Traits in the Genus Nepenthes (Caryophyllales)
Total Page：16
File Type：pdf, Size：1020Kb
applyparastyle "body/p[1]" parastyle "Text_First" Biological Journal of the Linnean Society, 2018, 123, 321–337. With 5 figures. Keeping an eye on coloration: ecological correlates of the evolution of pitcher traits in the genus Nepenthes (Caryophyllales) KADEEM J. GILBERT1*, JOEL H. NITTA1†, GERARD TALAVERA1,2 and NAOMI E. PIERCE1 1Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford St., Cambridge, MA 02138, USA 2Institut de Biologia Evolutiva (CSIC-Universitat Pompeu Fabra), Passeig Marítim de la Barceloneta, 37, E-08003, Barcelona, Spain †Current address: Department of Botany, National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba, 305-0005, Japan Received 20 August 2017; revised 10 November 2017; accepted for publication 10 November 2017 Nepenthes is a genus of carnivorous pitcher plants with high intra- and interspecific morphological diversity. Many species produce dimorphic pitchers, and the relative production rate of the two morphs varies interspecifically. Despite their probable ecological importance to the plants, little is known about the selective context under which various pitcher traits have evolved. This is especially true of colour-related traits, which have not been examined in a phylogenetic context. Using field observations of one polymorphic species (N. gracilis) and comparative phylogenetic analysis of 85 species across the genus, we investigate correlations between colour polymorphism and ecological factors including altitude, light environment and herbivory. In N. gracilis, colour does not correlate with amount of prey captured, but red pitchers experience less herbivory. Throughout the genus, colour polymorphism with redder lower pitchers appears to be evolutionarily favoured. We found a lack of phylogenetic signal for most traits, either suggesting that most traits are labile or reflecting the uncertainty regarding the underlying tree topology. This study highlights ecological correlates of the vast phenotypic diversity of this group of tropical plants. We point to a need for future study examining herbivores of Nepenthes and experimental investigations on colour polymorphism. ADDITIONAL KEYWORDS: altitude – carnivorous plants – coloration – comparative methods – herbivory – intraspecific diversity – Nepenthes L. – pitcher plants – plant–animal interactions INTRODUCTION For instance, animal pollinators have a prominent role in shaping floral evolution (e.g. Muchhala & Potts, Competition for resources can lead to the divergence of 2007; Kay & Sargent, 2009; Alcantara & Lohmann, a clade into multiple niches and the evolution of novel 2011; van der Niet & Johnson, 2012; Anderson et al., morphological features. This can be seen in many plant 2014; Boberg et al., 2014; Muchhala, Johnsen & Smith, radiations such as the bromeliads, where species that 2014; Lagomarsino et al., 2016). Thus, plant morpho- came to occupy water- and nutrient-limited habitats logical evolution can have multiple, interacting biotic evolved tightly pressed leaf tanks capable of collecting and abiotic drivers. However, disentangling the effects water and nutrient-rich debris (Benzing & Renfrow, of these various drivers is difficult and has not been 1974; Givnish et al., 2011). In addition to resource achieved for many groups. limitation and/or competition, plants must also rou- Pitcher plants are one such group characterized by an tinely respond to a suite of interspecific interactions. adaptation that is subject to multiple interacting driv- ers. Their pitchers are modified leaves used to capture *Corresponding author. Email: [email protected] and digest animal prey – they are nitrogen-acquiring © 2018 The Linnean Society of London, Biological Journal of the Linnean Society, 2018, 123, 321–337 321 Downloaded from https://academic.oup.com/biolinnean/article-abstract/123/2/321/4788353 by Harvard Library user on 24 June 2018 322 K. J. GILBERT ET AL. organs analogous to bromeliad tanks, but are also differences in the symbiont communities of different like flowers in their potential to coevolve with ani- pitcher morphs (Clarke, 1997a), but little is known mal visitors. Pitcher plants are thus a useful system about the evolution or functional significance of to investigate the roles of abiotic and biotic effects on pitcher dimorphism. the diversification of an adaptive trait. Here we exam- In addition to shape, many species also have pitch- ine the carnivorous plant genus Nepenthes L., the most ers that vary from red to green, often with discrete diverse (> 140 species: Cheek & Jebb, 2014) and wide- colour differences between the lower and upper spread family of carnivorous pitcher plants. Its core morphs (Fig. 1 – we hereafter refer to the occur- distribution spans most of Southeast Asia, with a few rence of discrete colour differences between pitcher outlying species in Madagascar to the west and New morphs within a plant as ‘colour polymorphism’). Caledonia to the east, and ranges in altitude from 0 up Despite being a conspicuous feature of Nepenthes, to 3520 m a.s.l. (McPherson, Robinson & Fleischmann, pitcher coloration is poorly understood. A few stud- 2009). The pitchers of different species are used to cap- ies have examined the role of red pigmentation as ture insects, but additionally may be involved in inter- a visual signal in carnivorous plants (Schaefer & actions spanning from commensalism to parasitism Ruxton, 2008; Bennett & Ellison, 2009; Foot, Rice and mutualism (e.g. Beaver, 1979; Adlassnig, Peroutka & Millet, 2014; Jürgens et al., 2015; El-Sayed, Byers & Lendl, 2011; Bazile et al., 2012; Thornham et al., & Suckling, 2016), and several have hypothesized 2012; Scharmann et al., 2013). Pitchers are morpho- that the contrast of red against a green background logically complex, exhibiting an array of traits that are of foliage could be attractive, although many insects under selection by both biotic and abiotic factors that lack red perception (Briscoe and Chittka, 2001). are difficult to tease apart. Red was not found to be a prey attractant in studies Recent work has elucidated the functional signifi- with sundews (Foot et al., 2014; Jürgens et al., 2015; cance of pitcher traits, including the thickness of the El-Sayed et al., 2016), but results have been con- peristome, presence of a waxy layer, viscosity of the flicting in pitcher plants (Schaefer & Ruxton, 2008; fluid and digestive gland structure in relation to prey- Bennett & Ellison, 2009). trapping efficiency (Bonhomme et al., 2011; Renner & As red pigmentation in Nepenthes is due to antho- Specht, 2011; Bauer et al., 2012). Although it has not cyanins (Kováčik, Klejdus & Repčáková, 2012), which yet been well documented whether interspecific dif- are costly to produce (Gould, 2004), the existence of ferences can be explained by niche-partitioning (Chin, intraspecific colour polymorphism in pitcher plants is Chung & Clarke, 2014; but see Peng & Clarke, 2015; particularly puzzling. More generally, the role of plant Gaume et al., 2016), the few known specialist trappers anthocyanins as visual signals in flowers and fruits point to the importance of animal visitors to pheno- is well understood, but the function of anthocyanins typic evolution of pitchers; this includes traits such in leaves is less well resolved. Multiple competing, as the parabolic structure in N. hemsleyana Macfarl. although not necessarily mutually exclusive, hypoth- that functions as an echolocation guide for its mu- eses have been proposed for the role of anthocyanins tualist bat (Schӧner et al., 2015; Schӧner et al., 2017). in leaves (Gould, 2004), the majority of which can be Despite growing knowledge of the significance of dif- divided into two camps: those that argue that anthocy- ferent trapping features to the genus, less than 10% of anins primarily serve a physiological role, vs. those that all species have been the subject of ecological studies posit that they are visual signals and are primarily a detailing their specific prey capture strategies (Clarke result of coevolving with herbivores (Archetti & Brown, & Moran, 2011), and the functional significance of 2004; Archetti et al., 2009). Some potential physio- many pitcher traits has yet to be explored. logical functions of anthocyanins involve protecting One potentially important, yet understudied set leaves from excess light, including UV shielding and of traits in pitcher plants are those related to intra- free radical scavenging (Feild, Lee & Holbrook, 2001; specific polymorphism. Many species produce two Hoch, Zeldin & McCown, 2001; Neill & Gould, 2003), distinct pitcher morphologies (‘morphs’) throughout and facilitating nutrient resorption in the context of the lifespan of an individual plant: lower pitchers deciduous colour-changing leaves (Hoch, Singsaas, & (‘lowers’), which grow gravitropically from plants McCown, 2003). Additionally, abaxial anthocyanins in the rosette phase and possess winged fringes of in understorey plants have been proposed to improve tissue (‘wings’); and upper pitchers (‘uppers’), which photosynthetic efficiency in the ‘back-scatter hypoth- grow from plants in the climbing phase, twine onto esis’, although there is some experimental evidence surrounding vegetation via their tendrils and pos- against this (Hughes, Vogelman, & Smith, 2008). sess a more streamlined form lacking wings (Jebb Considering the coevolution hypothesis, anthocyanins and Cheek, 1997; Fig. 1). There is some evidence of may serve as either direct (Schaefer, Rentzsch & Breuer, prey-partitioning