Rapid evolution of chemosensory receptor genes in a pair of sibling species of orchid bees (Apidae: Euglossini).

BackgroundInsects rely more on chemical signals (semiochemicals) than on any other sensory modality to find, identify, and choose mates. In most insects, pheromone production is typically regulated through biosynthetic pathways, whereas pheromone sensory detection is controlled by the olfactory system. Orchid bees are exceptional in that their semiochemicals are not produced metabolically, but instead male bees collect odoriferous compounds (perfumes) from the environment and store them in specialized hind-leg pockets to subsequently expose during courtship display. Thus, the olfactory sensory system of orchid bees simultaneously controls male perfume traits (sender components) and female preferences (receiver components). This functional linkage increases the opportunities for parallel evolution of male traits and female preferences, particularly in response to genetic changes of chemosensory detection (e.g. Odorant Receptor genes). To identify whether shifts in pheromone composition among related lineages of orchid bees are associated with divergence in chemosensory genes of the olfactory periphery, we searched for patterns of divergent selection across the antennal transcriptomes of two recently diverged sibling species Euglossa dilemma and E. viridissima.ResultsWe identified 3185 orthologous genes including 94 chemosensory loci from five different gene families (Odorant Receptors, Ionotropic Receptors, Gustatory Receptors, Odorant Binding Proteins, and Chemosensory Proteins). Our results revealed that orthologs with signatures of divergent selection between E. dilemma and E. viridissima were significantly enriched for chemosensory genes. Notably, elevated signals of divergent selection were almost exclusively observed among chemosensory receptors (i.e. Odorant Receptors).ConclusionsOur results suggest that rapid changes in the chemosensory gene family occurred among closely related species of orchid bees. These findings are consistent with the hypothesis that strong divergent selection acting on chemosensory receptor genes plays an important role in the evolution and diversification of insect pheromone systems

Queen execution and caste conflict in the stingless bee Melipona beecheii

Caste conflict theory predicts that worker-destined individuals in insect colonies may try to develop as queens in order to gain greater direct reproduction. In situations where females can determine their own caste fate (`self-determination¿), this is expected to lead to overproduction of queens. Theoretical predictions are supported by patterns of queen production in Melipona stingless bees. In Melipona, queens and workers are similar in size and develop in identical, mass-provisioned, sealed cells, a situation which permits self-determination. In line with predictions, many Melipona females, up to 16%, selfishly develop as queens. Although these observations fit the predictions of caste conflict theory, some of the underlying assumptions remain incompletely tested. In particular, whether immature females can actually determine their own caste fate and whether queen production is really excessive rather than just an insurance against accidental queen loss? Here we test these assumptions. Queens and workers in colonies of Melipona beecheii in Yucatan, Mexico, had the same dry mass, showing that queen development is not conditional on above-average food provisioning. This supports the assumption that individuals can completely control their caste fate. Observations of 30 introduced virgin queens in three colonies showed that queens were killed rapidly and had a life expectancy of just 47 h. A second method, using the number of virgin queens in natural colonies, also indicated a short life span, estimated at 27 h. Rapid and efficient culling of virgin queens supports the hypothesis that queen production is excessive and results from caste fate conflict. In addition, we provide, for the first time, detailed behavioural descriptions of queen killing in this species

Figure 4 in Characterization of the orchid bee Euglossa viridissima (Apidae: Euglossini) and a novel cryptic sibling species, by morphological, chemical, and genetic characters

Figure 4. Results of a PCA of 15 morphological variables measured in male Euglossa viridissima and Euglossa dilemma sp. nov. Components 1 and 3, which showed significant differences between the species, are used for this two-dimensional representation. Note that E. dilemma shows slightly less variability and is essentially nested within E. viridissima morphospace. Centroids of distributions are shown.Published as part of <i>Eltz, Thomas, Fritzsch, Falko, Pech, Jorge Ramírez, Zimmermann, Yvonne, Ramírez, Santiago R., Quezada-Euan, J. Javier G. & Bembé, Benjamin, 2011, Characterization of the orchid bee Euglossa viridissima (Apidae: Euglossini) and a novel cryptic sibling species, by morphological, chemical, and genetic characters, pp. 1064-1076 in Zoological Journal of the Linnean Society 163 (4)</i> on page 1070, DOI: 10.1111/j.1096-3642.2011.00740.x, <a href="http://zenodo.org/record/10114764">http://zenodo.org/record/10114764</a&gt

Figure 7 in Characterization of the orchid bee Euglossa viridissima (Apidae: Euglossini) and a novel cryptic sibling species, by morphological, chemical, and genetic characters

Figure 7. Geographical distribution of tridentate Euglossa dilemma sp. nov. (black) and predominantly bidentate Euglossa viridissima (white) as inferred from recent baiting assays (circles) as well as museum material (diamonds). Note lack of E. viridissima in the south-eastern part (Costa Rica) of the range. Museum material included paratypes of E. dilemma and additional specimens of one or both species in the collections of D. W. Roubik, T. Eltz (CTE), G. Gerlach (CGG), the Zoologische Staatssammlung München (ZSM), the Smithsonian Institution (SI), and the Snow Entomological Collection (SEC). Only unambiguous and non-redundant localities were plotted. Localities of baiting assays are (from west to east): Chamela (Jalisco, Mexico), El Chote (Veracruz, Mexico), Ayozinthepec (Oaxaca, Mexico), Monte Pio and Poza Azul (both Veracruz, Mexico), Tuxtla Gutiérrez, Esquintla, Tapachula, Ocosingo and Palenque (all Chiapas, Mexico), Atasta (Campeche, Mexico), Retalhuleu (Guatemala), Lacanjá (Chiapas, Mexico), Escarcega (Campeche, Mexico), El Remate (Campeche, Mexico), Chablekal, Xmatkuil (Yucatán, Mexico), Tikal (Guatemala), San Crisanto (Yucatán, Mexico), Chetumal and Coba (both Quintana Roo, Mexico), Chinandega, Chacocente, Escameca Grande, Jinotega, Ometepe and Las Pampas (all Nicaragua), and Area de Conservación Guanacaste (Costa Rica).Published as part of <i>Eltz, Thomas, Fritzsch, Falko, Pech, Jorge Ramírez, Zimmermann, Yvonne, Ramírez, Santiago R., Quezada-Euan, J. Javier G. & Bembé, Benjamin, 2011, Characterization of the orchid bee Euglossa viridissima (Apidae: Euglossini) and a novel cryptic sibling species, by morphological, chemical, and genetic characters, pp. 1064-1076 in Zoological Journal of the Linnean Society 163 (4)</i> on page 1074, DOI: 10.1111/j.1096-3642.2011.00740.x, <a href="http://zenodo.org/record/10114764">http://zenodo.org/record/10114764</a&gt

Figure 1 in Characterization of the orchid bee Euglossa viridissima (Apidae: Euglossini) and a novel cryptic sibling species, by morphological, chemical, and genetic characters

Figure 1. Differences in the chemical composition of tibial perfumes between tridentate (black circles) and bidentate (grey circles) Euglossa viridissima-like males as revealed by a multidimensional scaling (MDS) analysis. Only tridentate males contained HNDB. Tridentate males without HNDB are highlighted (red symbols).Published as part of <i>Eltz, Thomas, Fritzsch, Falko, Pech, Jorge Ramírez, Zimmermann, Yvonne, Ramírez, Santiago R., Quezada-Euan, J. Javier G. & Bembé, Benjamin, 2011, Characterization of the orchid bee Euglossa viridissima (Apidae: Euglossini) and a novel cryptic sibling species, by morphological, chemical, and genetic characters, pp. 1064-1076 in Zoological Journal of the Linnean Society 163 (4)</i> on page 1067, DOI: 10.1111/j.1096-3642.2011.00740.x, <a href="http://zenodo.org/record/10114764">http://zenodo.org/record/10114764</a&gt

Figure 6 in Characterization of the orchid bee Euglossa viridissima (Apidae: Euglossini) and a novel cryptic sibling species, by morphological, chemical, and genetic characters

Figure 6. Chronogram showing divergence times and phylogenetic relationships of selected lineages in the genus Euglossa and the sibling species Euglossa dilemma sp. nov. and E. viridissima. The tree topology corresponds to that obtained via Bayesian methods. Bayesian posterior probabilities and parsimony bootstrap values are shown for the sister species only. Divergence times were obtained via penalized likelihood using the fossil-calibrated molecular clock procedures described in Ramírez et al. (2010b). The maximum and minimum age estimates for the MRCA of E. dilemma and E. viridissima correspond to the molecular clock analyses in which the MRCA of the genus Euglossa was assigned a fossil calibration of 20 and 15 Myr, respectively.Published as part of <i>Eltz, Thomas, Fritzsch, Falko, Pech, Jorge Ramírez, Zimmermann, Yvonne, Ramírez, Santiago R., Quezada-Euan, J. Javier G. & Bembé, Benjamin, 2011, Characterization of the orchid bee Euglossa viridissima (Apidae: Euglossini) and a novel cryptic sibling species, by morphological, chemical, and genetic characters, pp. 1064-1076 in Zoological Journal of the Linnean Society 163 (4)</i> on page 1073, DOI: 10.1111/j.1096-3642.2011.00740.x, <a href="http://zenodo.org/record/10114764">http://zenodo.org/record/10114764</a&gt

Figure 2 in Characterization of the orchid bee Euglossa viridissima (Apidae: Euglossini) and a novel cryptic sibling species, by morphological, chemical, and genetic characters

Figure 2. Allele size distribution of Euglossa viridissima- like males from the Yucatán peninsula, Mexico, at the microsatellite locus ann02. Overall, bidentate males (grey bars) had significantly smaller allele sizes than tridentate individuals (black bars), and there was little overlap in allele size. The seven individuals indicated as red circles were also tridentate, but had been clustered with bidentate males in the analysis of perfume similarity (see Fig. 1), lacking HNDB. These seven individuals had the third (central) mandibular tooth significantly displaced towards the tip of the mandible (nearer to the distal tooth, see Fig. 3B), unlike in other tridentate males. Their ann02 allele size suggests that they in fact belong to the bidentate lineage. See text for further explanation.Published as part of <i>Eltz, Thomas, Fritzsch, Falko, Pech, Jorge Ramírez, Zimmermann, Yvonne, Ramírez, Santiago R., Quezada-Euan, J. Javier G. & Bembé, Benjamin, 2011, Characterization of the orchid bee Euglossa viridissima (Apidae: Euglossini) and a novel cryptic sibling species, by morphological, chemical, and genetic characters, pp. 1064-1076 in Zoological Journal of the Linnean Society 163 (4)</i> on page 1068, DOI: 10.1111/j.1096-3642.2011.00740.x, <a href="http://zenodo.org/record/10114764">http://zenodo.org/record/10114764</a&gt