Current Biology 24, 984–987, May 5, 2014 ª2014 Elsevier Ltd All rights reserved
http://dx.doi.org/10.1016/j.cub.2014.03.010
Report Leaf Mimicry in a Climbing Plant Protects against Herbivory Ernesto Gianoli1,2,* and Fernando Carrasco-Urra2 1Departamento de Biologı´a, Universidad de La Serena, Casilla 554, La Serena, Chile 2Departamento de Bota ´ nica, Universidad de Concepcio´n, Casilla 160-C, Concepcio´n, Chile
Summary Mimicry refers to adaptive similarity between a mimic organism and a model. Mimicry in animals is rather common, whereas documented cases in plants are rare, and the associated benefits are seldom elucidated [1, 2]. We show the occurrence of leaf mimicry in a climbing plant endemic to a temperate rainforest. The woody vine Boquila trifoliolata mimics the leaves of its supporting trees in terms of size, shape, color, orientation, petiole length, and/or tip spininess. Moreover, sequential leaf mimicry occurs when a single individual vine is associated with different tree species. Leaves of unsupported vines differed from leaves of climbing plants closely associated with tree foliage but did not differ from those of vines climbing onto leafless trunks. Consistent with an herbivory-avoidance hypothesis, leaf herbivory on unsupported vines was greater than that on vines climbing on trees but was greatest on vines climbing onto leafless trunks. Thus, B. trifoliolata gains protection against herbivory not merely by climbing and thus avoiding ground herbivores [3] but also by climbing onto trees whose leaves are mimicked. Unlike earlier cases of plant mimicry or crypsis, in which the plant roughly resembles a background or color pattern [4–7] or mimics a single host [8, 9], B. trifoliolata is able to mimic several hosts. Results and Discussion Mimicry cases in plants are not common, and their adaptive value is rarely reported [1, 2]. The most known example of mimicry in plants occurs in Australian mistletoes, a group of hemiparasitic plants whose leaves mimic those of their respective host tree species [8]. The associated benefits or ecological agents involved in this case of leaf mimicry are not clearly discerned [9]. Floral mimicry in which pollinators are attracted and deceived [10, 11] has also been reported (mainly describing the resemblance between two species). Other examples of mimicry or crypsis in plants include leaf variegation, which is a whitish mottling that resembles leaf damage by mining larvae and may deter herbivores that avoid feeding or ovipositing on previously attacked leaves [4], succulent Lithops plants that resemble stones in arid regions of Southern Africa [7], and leaves [6] or bracts [5] that may make a plant cryptic against a leaf litter background. Even though evidence of mimicry in plants has accumulated recently, it remains a rather contentious issue [1]. The climbing plant Boquila trifoliolata (Lardizabalaceae) is endemic to the temperate rainforest of southern South America [12]. Leaves of this twining vine are very variable in
*Correspondence: egianoli@userena.cl
size and shape and are composed of three leaflets that are pulvinated and therefore may change their orientation. Field observations indicate that B. trifoliolata often mimics the leaves of its supporting trees in terms of size, shape, color, orientation, and vein conspicuousness, among other features (Figure 1). This phenomenon includes the display of a mucronate leaf apex (a small spine at the leaf tip) when twining around a tree with such mucronate leaves (Figure 1); the botanical description of B. trifoliolata does not include this feature [14]. Unlike earlier mimicry reports, leaf mimicry by this climbing plant is confined not to a single species but to several host trees. Moreover, when traversing different hosts, the same individual vine changes its leaf morphology accordingly (Figure S1 available online). To quantify this phenomenon, we compared 11 leaf traits from both B. trifoliolata individuals and the tree species with which they were associated in a mature forest (45 vine individuals associated with 12 host tree species). We further evaluated whether leaf mimicry by this vine was related to herbivore avoidance, in analogy to cryptic behavior against predators in animals. The statistical analysis (a mixed generalized linear model [GLM] with observations of tree leaf phenotype nested in species, which was a random factor) showed a significant association between the leaf phenotype of B. trifoliolata and that of the supporting trees in 9 of the 11 leaf traits measured, including leaf and leaflet angle, leaf area and perimeter, leaflet petiole length, and leaf color (Table 1). These patterns can hardly be explained by covariation of leaf phenotype with light availability because (1) the light environment of sampling sites was rather homogeneous (4%–8% light availability), and (2) the host tree species, with contrasting leaf phenotypes, are not segregated across the light gradient [15]. Furthermore, leaves of prostrate individuals of B. trifoliolata (i.e., those vines growing on the ground) did not differ from those of vines that were climbing onto leafless stems or trunks (multivariate analysis of variance [MANOVA]; Table 2) but did differ from those climbing onto leafed individuals of the analyzed tree species (7 of 8 species, MANOVA; Table 2; Figure S2). Therefore, when there is no leaf to mimic, climbing plants are not different from plants growing unsupported, which show the ‘‘standard’’ leaf phenotype of the species. We also verified that individuals growing on bare tree trunks did differ from those growing on leafed tree hosts (6 of 8 species, MANOVA; data not shown; Figure S2). We found some field evidence supporting the hypothesis that leaf mimicry in climbing individuals of B. trifoliolata is related to herbivore avoidance. First, following the premise that indistinguishable phenotypes should lead to similar levels of leaf damage [9], we found in paired comparisons that herbivory did not differ between climbing vines and the supporting host trees (t138 = 21.712, p = 0.09; mean 6 SE of an herbivory index: vines 1.91 6 0.04 and trees 2.01 6 0.04). Second, leaf herbivory was significantly higher in creeping, unsupported individuals than in those climbing on trees (Figure 2). Third, leaf herbivory on vine individuals climbing onto leafless supports—on which there is no leaf model to mimic—was higher than leaf herbivory on unsupported individuals (Figure 2). Given that leafless stems conferred no protection, these results suggest that B. trifoliolata gains
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Figure 1. Leaf Mimicry in the Climbing Plant Boquila trifoliolata Pictures of the twining vine B. trifoliolata co-occurring with woody species in the temperate rainforest of southern Chile, where leaf mimicry in terms of size, color, and/or shape is evident. White arrows point to the vine (V) and to the host tree (T). Leaf length of the tree species is shown in parentheses [13]; this may help to estimate leaf size variation in the vine. (A) Myrceugenia planipes (3.5–8 cm). (B) Rhaphithamnus spinosus (1–2 cm). (C) Eucryphia cordifolia (5–7 cm). Notably smaller leaves of B. trifoliolata appear to the left of the focus leaf. (D) Mitraria coccinea (a woody vine; 1.5–3.5 cm). Both here and in (F), the serrated leaf margin of the model cannot be mimicked, but the vine shows one or two indents. (E) Aextoxicon punctatum (5–9 cm). (F) Aristotelia chilensis (3–8 cm). (G) Rhaphithamnus spinosus (1–2 cm). (H) Luma apiculata (1–2.5 cm). The inset shows more clearly how B. trifoliolata has a spiny tip, like the supporting treelet and unlike all the other pictures (and the botanical description) of this vine. See also Figure S1 for pictures showing different leaves of the same individual of B. trifoliolata mimicking different host trees.
protection against herbivores not merely by climbing and thus avoiding ground herbivores or by benefiting from associational resistance, as has been shown for other twining vines [3, 16, 17], but also by climbing onto host trees whose leaves are mimicked. For crypsis to be a viable defensive strategy in plants, herbivores to be avoided should be visually orientated [2]. The main herbivores in the study site are small gastropods, weevils, and leaf beetles [18], whose foraging behavior relies on both visual and olfactory cues [19–22]. A case of leaf mimicry in which its defensive role has been documented is that of leaf variegation. Thus, variegated leaves were less attacked by herbivores than plain green leaves were in both a subcanopy liana [4] and an understory herb [23] from
tropical rainforests. Likewise, when vegetative bracts that resemble leaf litter and cover reproductive stems in an Appalachian herb were experimentally removed, herbivory increased and fruit set decreased [5]. In the case of Australian mistletoes that mimic their hosts, protection against arboreal herbivores (possums) had been hypothesized [8], but no benefit in terms of reduced herbivory has been found [9]. Nonetheless, mistletoe species mimicking host leaves have higher nitrogen levels (a proxy for palatability to herbivores) than their host trees, whereas nonmimicking mistletoes show lower leaf nitrogen content than their hosts [24]. This suggests that mistletoe mimicry might have arisen as an adaptive response against herbivory risk [9, 24].
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Table 1. Relationships among Leaf Traits of the Climbing Plant Boquila trifoliolata and Those of Its Supporting Trees Leaf Traits
Adjusted r2
p
Maximum width Maximum length Area Perimeter Area:perimeter Thickness Leaf petiole length Leaflet petiole length Leaf angle Leaflet angle Color
0.49 0.54 0.63 0.60 0.07 0.15 0.58 0.25 0.23 0.62 0.32
<0.001 <0.001 <0.001 <0.001 0.272 0.130 <0.001 0.037 0.045 <0.001 0.012
Results of mixed GLM analyses are shown. The predictor variable was tree leaf phenotype (individual observations) nested in species, which was a random factor. Adjusted r2 corresponds to the whole model, with degrees of freedom (df) = 12, 32. In all cases, the multiple r coefficients were >0. n = 45 Boquila individuals in 12 host tree species.
Unlike the Australian mistletoes, which are hemiparasites with physiological connections to the host tree [9], B. trifoliolata is merely a climbing plant whose stems are in contact with the trunks or branches of host trees. In fact, we have observed that leaf mimicry may occur even when there is no contact at all between the vine and the mimicked tree. The Australian mistletoes typically show specific associations with only one or two host trees [8, 25], thus allowing a long-term evolutionary process of mimicry, and show little phenotypic variation when found on hosts other than their specific hosts [8, 26]. In contrast, mimicry by B. trifoliolata involves phenotypic plasticity because the same individual is able to resemble several host trees simultaneously. Furthermore, unlike other reported cases of mimicry or crypsis in plants [4–7], in which the mimetic plant roughly resembles an undetermined background or color pattern, B. trifoliolata is able to mimic several hosts. The imitation of several distinct models by a single species has been deemed particularly advantageous in light of frequency-dependent selection [27], but evidence of such mimetic polymorphisms is rare and limited to butterfly species [27]. Moreover, these phenomena are explained not by plastic responses but by population divergence. We currently lack a mechanistic explanation for this unique phenomenon. Because leaf mimicry is observed even when
Table 2. Comparison of the Leaf Phenotype of Prostrate Vines versus Vines Climbing on Leafless Trunks and on Tree Hosts Prostrate Boquila versus Boquila Climbing on:
Wilks l
F
df
p
Leafless trunks Aristotelia chilensis Amomyrtus luma Azara lanceolata Aextoxicon punctatum Caldcluvia paniculata Eucryphia cordifolia Myrceugenia planipes Rhaphithamnus spinosus
0.712 0.107 0.393 0.534 0.158 0.141 0.157 0.219 0.364
1.469 15.879 3.087 1.268 8.209 8.875 9.264 7.139 3.172
11, 40 11, 21 11, 22 11, 16 11, 17 11, 16 11, 19 11, 22 11, 20
0.182 <0.001 0.012 0.324 <0.001 <0.001 <0.001 <0.001 0.012
Results of paired MANOVAs in which the leaf phenotype (all leaf traits taken together) of prostrate vines was compared against the leaf phenotype of vines climbing on leafless trunks and against the leaf phenotype of vines climbing on tree hosts (see Table 1 for trait list). See also Figure S2 for a scatterplot of canonical scores for the leaf phenotype of the three vine groups included in this table.
Figure 2. Herbivory on Leaves of Boquila trifoliolata in Different Scenarios Herbivory index (mean 6 SE) of leaves of Boquila individuals that were either creeping unsupported on the forest understory (no support), climbing on supporting trees that had their leaves close to those of the vines (host trees), or climbing onto leafless supports (leafless stems). Different letters above bars indicate significant differences between groups (p < 0.05, Tukey honestly significant difference test, following a one-way ANOVA in which F2,102 = 42.01; p < 0.001).
the model is not directly in contact with the vine, we might hypothesize that host plant volatiles trigger specific phenotypic changes in neighboring vine leaves. Volatile organic compounds have been shown to elicit specific responses in neighboring plants, including induction of secondary metabolites, increased expression of defense-related genes, and, overall, numerous changes of the plant transcriptome [28–30]. However, research on airborne plant-plant signaling has not reported specific morphological changes in leaves, and nothing is known about the involvement of volatiles of the model in leaf mimicry via induced reprogramming of genes in the mimic. An alternative hypothesis, but perhaps less plausible, would consider horizontal gene transfer between plants, a phenomenon that is increasingly being reported [31–36]. These cases include both single and multiple transfer events per species, which are hypothesized to be mediated by a vector or result from plant-plant parasitism or natural grafts [31–36]. The plasticity in leaf mimicry in B. trifoliolata could involve horizontal gene transfer on an ecological timescale and might be mediated by airborne microorganisms. The latter speculation is based on the fact that mimicry is observed with respect to the foliage to which the vine is nearest, irrespective of whether this foliage belongs to the host tree that the vine has climbed. Further research on leaf mimicry by B. trifoliolata might lead to the identification of the host tree volatiles or vector-mediated gene transfers that trigger differential gene expression in this singular climbing plant. Supplemental Information Supplemental Information includes Supplemental Experimental Procedures and two figures and can be found with this article online at http://dx.doi.org/ 10.1016/j.cub.2014.03.010. Acknowledgments This study was supported by FONDECYT (Chilean Agency of Research Fund) grant 1100585 to E.G. We are grateful to A. Saldan˜a, C. SalgadoLuarte, O. Godoy, M. Cisternas, and D. Gianoli for their help in the field and their contribution of pictures. We thank K.C. Burns and three anonymous reviewers for thoughtful comments that improved the manuscript and CONAF (Chilean Agency of Protected Areas) for granting us permits to work in Puyehue National Park.
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