Chemosensory reception in the stingless bee Tetragonisca angustula
Graphical abstract
Introduction
In social insects such as bees, ants and termites, division of labor plays a key role in colony organization (Ratnieks and Anderson, 1999, Hölldobler and Wilson, 2009, Duarte et al., 2011). Task division is often associated with physiological (e.g., changes in juvenile hormone titers) and/or morphological differences, and with changes in behavior throughout the lifespan of individuals (Robinson, 1992, Seeley, 1995, Hölldobler and Wilson, 2009, Duarte et al., 2011, Hammel et al., 2016). Thus, insect colonies are characterized by the presence of specialized individuals that perform different tasks simultaneously, i.e., workers (Baracchi and Cini, 2014).
Tetragonisca angustula (Latreille) is one of the main native eusocial bees of the Neotropical region of the American continent (Vit et al., 2013). It is the most widespread species in this area and is also important in agroecosystems due to its highly valued honey and pollination of commercial crops, such as canola and strawberry (Slaa et al., 2006, Velez-Ruiz et al., 2013, Vit et al., 2013, Zamudio and Alvarez, 2016). This small, 4 to 5 mm, stingless bee is found from Mexico to Central Argentina (Camargo and Pedro, 1992, Moure, 2008, Yáñez-Ordónez et al., 2008, Zamudio and Alvarez, 2016). Colonies consist of thousands of individuals (2000–5000 bees, Zamudio and Alvarez, 2016), with a worker caste of bees that perform diverse tasks inside and outside the nest during their lives (Hammel et al., 2016). Besides age-related task division, workers from this species also present a peculiar morphological guard subcaste. These guard bees are heavier and have larger legs than foragers (Grüter et al., 2012), and transition faster from one task to the next (Hammel et al., 2016). Additionally, T. angustula guards can be divided in two types according to two conspicuous behaviors they perform that are complementary for nest defense (Wittman, 1985, Kärcher and Ratnieks, 2009, Grüter et al., 2011). Hovering guards maintain sustained flights in front of the nest entrance and standing guards stand on the nest entrance tube. Hence, this stingless bee species shows a labor division based on both temporal and morphological castes.
In the honey bee Apis mellifera, worker size variation is minor and division of labor has been related to age, changes in juvenile hormone titers, and different response thresholds to task-specific stimuli (Robinson, 1987, Robinson, 1992, Seeley, 1989). Moreover, differences in foraging-related behaviors correlate with changes in the sucrose response thresholds (SRTs) of individuals involved in the exploitation of different resources (e.g., pollen, nectar and water; Page et al., 1998).
Just as gustatory stimuli (e.g., sugar concentration) have a key role in the foraging context, the ability to detect and discriminate odors is essential for bees. During foraging, bees collect nectar and pollen from scented flowers and they can learn to associate the odor with the presence and quality of the reward (von Frisch, 1954). Furthermore, odor detection and discrimination are not only important in the foraging context but are also crucial in nestmate recognition and nest defense (e.g., pheromones, cuticular hydrocarbons; Breed et al., 2004, Howard and Blomquist, 2005).
Even though it is possible to measure the SRTs of stingless bees (Mc Cabe et al., 2007, Mc Cabe et al., 2017, Henske et al., 2015), it remains unknown whether individuals involved in distinct tasks such as nectar and pollen foraging perceive gustatory stimuli differentially, as honey bees do. Furthermore, there are no studies comparing odor detection thresholds among stingless bee workers. Hence, we used a behavioral, physiological and morphological approach to evaluate chemosensory detection and processing of different T. angustula worker subcastes. Specifically, we compared gustatory and olfactory responses of pollen and nonpollen foragers, and standing and hovering guards. We used the proboscis extension response to evaluate differences in sucrose response thresholds. To study if guards and foragers antennae respond differently to odor concentrations and if that response is task-related, we recorded the detection of different odors by antennae through electroantennography (EAG). This technique measures the voltage fluctuation caused by electrical depolarization of the olfactory neurons (Syntech, 2015). In addition, we described the sensilla present on the antennae of foragers and guards using scanning electron microscopy.
Section snippets
Study site and animals
Two colonies of T. angustula with a queen, brood and food reserves were used (C1 and C2). Wooden boxes contained about 4000 worker bees each and were open to the field so bees could forage freely outside. Experiments were performed from December 2016 to March 2017 at the Experimental Field of the Universidad de Buenos Aires (Argentina, 34°32́S, 58°26́W).
Experimental procedure
Four groups of worker bees were captured from each colony. We identified each group according to their behavior. Foragers were captured
Gustatory responsiveness
We found that most nonpollen foragers responded to 30 and 50% w/w sucrose solutions, showing the highest SRT (mean GRS = 2). By contrast, most pollen foragers and guards responded to concentrations from 3% w/w onwards (mean GRS = 4. Fig. 1B. GLM model: score ~ worker + colony, N = 433; worker term: X2(0,05, 3) = 0.352, p < 0.001, see Table S1 for all Tukey HSD contrasts. colony term: X2(0,05, 1) = 0.004, p = 0.6165). This indicates that nonpollen foragers are less responsive to low sucrose
Gustatory responsiveness
Our results show that nonpollen foragers present the lowest gustatory responsiveness since they showed the lowest GRS. We found that nonpollen foragers started responding at 30% w/w sucrose solution while pollen foragers started at 3% w/w, suggesting that foragers involved in nectar collection could be mainly engaged in finding high quality-nectar sources. Indeed, our findings are in line with what was reported for honey bees, where pollen foragers present lower SRTs compared to nonpollen
CRediT authorship contribution statement
María Sol Balbuena: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, and Writing - original draft, Funding acquisition, Resources, Writing - review & editing. Walter M. Farina: Funding acquisition, Resources, Writing - review & editing.
Acknowledgments
We thank two anonymous reviewers for their constructive comments and suggestions, Diego E. Vázquez for helping us with the statistical analysis, and Romina Barrozo for her help with the SEM images. We are also grateful to Denise Nery and Lucila Herbert for their help with grammar. This study was partly supported by grants from Agencia Nacional de Promoción Científica y Tecnológica (ANPCYT, PICT 2016 2084), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET, PIP 112-201501-00633
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