Mathieu Chouteau

The Role of Predators in Maintaining the Geographic Organization of Aposematic Signals

Selective predation of aposematic signals is expected to promote phenotypic uniformity. But while these signals may be uniform within a population, numerous species display impressive variations in warning signals among adjacent populations. Predators from different localities who learn to avoid distinct signals while performing intense selection on others are thus expected to maintain such a geographic organization. We tested this assumption by placing clay frog models, representing distinct color morphs of the Peruvian poison dart frog Ranitomeya imitator and a nonconspicuous frog, reciprocally between adjacent localities. In each locality, avian predators were able to discriminate between warning signals; the adjacent exotic morph experienced up to four times more attacks than the local one and two times more than the nonconspicuous phenotype. Moreover, predation attempts on the exotic morph quickly decreased to almost nil, suggesting rapid learning. This experiment offers direct evidence for the existence of different predator communities performing localized homogenizing selection on distinct aposematic signals.

Wright's Shifting Balance Theory and the Diversification of Aposematic Signals

Despite accumulating evidence for selection within natural systems, the importance of random genetic drift opposing Wrights and Fishers views of evolution continue to be a subject of controversy. The geographical diversification of aposematic signals appears to be a suitable system to assess the factors involved in the process of adaptation since both theories were independently proposed to explain this phenomenon. In the present study, the effects of drift and selection were assessed from population genetics and predation experiments on poison-dart frogs, Ranitomaya imitator, of Northern Peru. We specifically focus on the transient zone between two distinct aposematic signals. In contrast to regions where high predation maintains a monomorphic aposematic signal, the transient zones are characterized by lowered selection and a high phenotypic diversity. As a result, the diversification of phenotypes may occur via genetic drift without a significant loss of fitness. These new phenotypes may then colonize alternative habitats if successfully recognized and avoided by predators. This study highlights the interplay between drift and selection as determinant processes in the adaptive diversification of aposematic signals. Results are consistent with the expectations of the Wrights shifting balance theory and represent, to our knowledge, the first empirical demonstration of this highly contested theory in a natural system.

Advergence in Müllerian mimicry: the case of the poison dart frogs of Northern Peru revisited

Whether the evolution of similar aposematic signals in different unpalatable species (i.e. Müllerian mimicry) is because of phenotypic convergence or advergence continues to puzzle scientists. The poison dart frog Ranitomeya imitator provides a rare example in support of the hypothesis of advergence: this species was believed to mimic numerous distinct model species because of high phenotypic variability and low genetic divergence among populations. In this study, we test the evidence in support of advergence using a population genetic framework in two localities where R. imitator is sympatric with different model species, Ranitomeya ventrimaculata and Ranitomeya variabilis. Genetic analyses revealed incomplete sorting of mitochondrial haplotypes between the two model species. These two species are also less genetically differentiated than R. imitator populations on the basis of both mitochondrial and nuclear DNA comparisons. The genetic similarity between the model species suggests that they have either diverged more recently than R. imitator populations or that they are still connected by gene flow and were misidentified as different species. An analysis of phenotypic variability indicates that the model species are as variable as R. imitator. These results do not support the hypothesis of advergence by R. imitator. Although we cannot rule out phenotypic advergence in the evolution of Müllerian mimicry, this study reopens the discussion regarding the direction of the evolution of mimicry in the R. imitator system.

Pollen-ovule ratios in some Neotropical Araceae and their putative significance

The correlation between pollen-ovule (P/O) ratio and breeding system has generally been analysed with respect either to pollination efficiency, or in terms of sex allocation theory. Pollen/ovule ratios were measured in nine species of Araceae belonging to two genera with bisexual flowers (Anaphyllopsis, Monstera) and three genera with unisexual flowers (Dieffenbachia, Philodendron, Montrichardia). The family Araceae with its unique inflorescence morphology allows the analysis of variations of the P/O ratio with respect to two basal morpho-functional pollination units: the flower or the inflorescence. We found a relationship between the value of the P/O ratio and the breeding system that is partially different from Crudens results (1977). Some facultative xenogamous species have a higher P/O than the obligatory xenogamous species. A link was found between the P/O and the type of inflorescence, the floral cycle, and the mode of growth.

A COMPARATIVE STUDY OF INFLORESCENCE CHARACTERS AND POLLEN-OVULE RATIOS AMONG THE GENERA PHILODENDRON AND ANTHURIUM (ARACEAE)

Floral characters in angiosperms may be involved in different relationships in order to ensure and maximize pollination. To assess these relationships, which may provide insight to understand floral evolution, we analyzed 14 floral characters in 23 species of Philodendron and 20 species of Anthurium, which are tropical long‐living plants bearing spadiciform inflorescences. Contrary to what has been reported in the literature, no correlations were found between the pollen volume and either the style depth or the stigma depth. The trade‐off between pollen size and number normally explained by limited resources was found only in Philodendron. Instead, pollen number was positively correlated with the inflorescence peduncle diameter. The higher range of variation of inflorescence peduncle diameters in Anthurium may explain the lack of correlation between pollen size and number. These results suggest that adaptive constraints driving pollen size and number could differ in the Philodendron and Anthurium genera from what is found for temperate angiosperms. Also, the stigma area and the pollen quantity were positively correlated with respect to the inflorescence flowering cycle and the flower morphology. Finally, the pollen‐ovule ratio is not linked to the breeding system in the studied genera. Our data show that the aroid inflorescence, which behaves as a single flower, is the main pollination unit.

Pollination ecology of Monstera obliqua (Araceae) in French Guiana

Data on pollination ecology of Araceae are still scarce and most concern species belonging to the subfamily Aroideae (Garcia-Robledo et al. 2004, Gibernau 2003, Ivancic et al. 2004, 2005; Maia & Schlindwein 2006). In this subfamily, inflorescences consist of unisexual flowers: female flowers are located in the lower portion and the male flowers are in the upper portion of the inflorescence (Mayo et al . 1997). In the documented neotropical Aroideae, pollinators are nocturnal beetles and pollination mechanisms take place within a floral chamber during a short flowering cycle (generally 24–48 h) with floral rewards (sterile flowers rich in proteins and/or lipids) for the beetle pollinators, the secretion of resin to secure pollen on the pollinator, and the production of heat and odours (Chouteau et al. 2007, Garcia-Robledo et al. 2004, Gibernau & Barabe 2002, Gibernau et al. 1999, 2000, 2003; Maia & Schlindwein 2006, Young 1986).

Territorial aggressiveness on the arboreal ant Azteca alfari by Camponotus blandus in French Guiana due to behavioural constraints.

This study reports new information on interactions between two sympatric ant species, the plant-ant Azteca alfari (Dolichoderinae) living in association with the myrmecophyte Cecropia obtusa (Cecropiaceae) and Camponotus blandus (Formicinae), a ground-nesting, arboreal-foraging species. Workers of A. alfari forage only on the foliage and the upper parts of the trunk of their host Cecropia, while C. blandus nests in the ground but frequently forages and patrols pioneer tree foliage, including Cecropia. The activity pattern of A. alfari and the number of C. blandus on Cecropia obtusa was monitored hourly during a two-day period in a disturbed area in French Guiana. The maximum activity of C. blandus occurred between 8:30 and 12:30, at which time A. alfari had retreated within the domatia and were least present on the trunks. Even though aggressive confrontations were observed, C. blandus workers often initiate confrontations but do not prey on A. alfari nor exploit food bodies produced by Cecropia, the principal food source of A. alfari. Hence hostility appears to be the result of territoriality. Differences in their foraging rhythms are proposed as promoting resource and territory partitioning in this ant assemblage.

Development of microsatellite loci from a reference genome for the Neotropical butterfly Heliconius numata and its close relatives

The Neotropical butterfly Heliconius numata (Lepidoptera: Nymphalidae: Heliconiinae) is known for its striking diversity of wing color patterns driven by the Müllerian mimicry of multiple local models and controlled by a single supergene locus. Such fine‐scale variation of traits under strong selection offers a unique opportunity for the study of the ecology and genetics of adaptation. However, little is still known of the population processes driving geographical variation in wing‐pattern phenotypes. We report the characterization of 26 microsatellite markers for the butterfly H. numata, including six located inside the wing color‐pattern supergene region. All markers are polymorphic, with allele numbers ranging from 2 to 21 per locus, an observed heterozygosity of 0.111 to 0.848 and an expected heterozygosity of 0.126 to 0.942. A subset of 18 of these markers was tested on five closely related sympatric Heliconius species with an amplification success ranging from 88% to 94%. The obtained set of microsatellite markers provides a new and useful set of tools to investigate patterns of differentiation and selection in populations of mimetic Heliconius butterflies. Moreover, markers developed within the color‐pattern supergene will facilitate characterization of the association between the genetic architecture and the functional diversity of wing patterns. Finally, the cross‐species amplification success of the described markers extends their utility to also encompass comparative population genetic studies of closely related species within a clade of rapidly diversifying species.

The development and characterization of polymorphic microsatellite loci for the genus Melinaea (Nymphalidae, Ithomiini)

AbstractDue to their preference for undisturbed habitats, the butterflies of the genus Melinaea are promising indicators of ecological conditions. Here we describe 12 polymorphic microsatellite markers, with 3–26 alleles per locus, an observed heterozygosity of 0.138–0.889, and an expected heterozygosity of 0.400–0.970. These markers will prove useful in investigating patterns of differentiation in this clade.