Acta Botanica Brasilica - 32(3): 349-358. July-September 2018. doi: 10.1590/0102-33062017abb0358 Anther specializations related to the division of labor in Microlicia cordata (Spreng.) Cham. (Melastomataceae) Mariana de Souza Carvalho Velloso1* , Vinícius Lourenço Garcia de Brito2 , Ana Paula Souza Caetano3 and Rosana Romero2 Received: October 11, 2017 Accepted: January 31, 2018 . ABSTRACT In heterantherous flowers, some anthers have an attraction and feeding function, while others are related to plant reproduction. Microlicia cordata has dimorphic stamens organized in two cycles: the antesepalous one with large stamens and pink anthers, and the antepetalous one with small stamens and yellow anthers. Division of labor was analyzed using the parameters of structure, composition, amount and estimated viability of pollen, and the color of floral parts. The anthers of the pollinating stamens were larger than the anthers of the feeding stamens, although anatomically similar. There was a difference in the amount of pollen produced by the anthers of the two cycles, but no difference was found in pollen viability. Considering a bee color vision model, the color of the anthers of the pollinating stamens contrasted less with that of the corolla, and thus is probably less attractive to visitors. Conversely, the anthers of the feeding stamens and the ventral appendage of the connective of the pollinating stamens contrasted more with the corolla, presenting the same color to the pollinators. These results are in accordance with the idea of division of labor among anthers of heterantherous flowers, especially regarding the quantity of pollen and the color of the floral parts. Keywords: color vision, division of labor, heteranthery, Microlicia, pollen dilemma, poricidal anthers conflict known as the “pollen dilemma” (Westerkamp 1996; Introduction Luo et al. 2009; Vallejo-Marín et al. 2009; Lunau et al. 2014; Barônio et al. 2016). Among the Angiosperms, approximately 20,000 species The occurrence of two or more types of stamens contain pollen as the only reward for their pollinators in in the same flower, or heteranthery, is understood as a the so-called pollen flowers (Vogel 1978; Buchmann 1983; mechanism for the solution of this conflict since it generates Luo et al. 2009). Among them, the buzz-pollinated flowers specialization of stamen types with different and specific are adapted to pollination by bees and their pollen grains functions in the pollination process (Luo et al. 2008; Vallejo- not only participate in the transport of male gametes, Marín et al. 2009; Barrett 2010). Usually, these stamens but are also consumed by these insects (Buchmann 1983; differ in shape, position, size and/or color, a condition that Vallejo-Marín et al. 2009). In this respect, the utilization has arisen in at least 16 angiosperm families (Buchmann of pollen for feeding potentially reduces the number of 1983; Luo et al. 2009; Vallejo-Marín et al. 2009; 2010; viable gametes for reproduction, leading to an evolutionary Barrett 2010). Such differences might also be associated 1 Programa de Pós-Graduação em Biologia Vegetal, Instituto de Biologia, Universidade Federal de Uberlândia, 38400-902, Uberlândia, MG, Brazil 2 Instituto de Biologia, Universidade Federal de Uberlândia, 38400-902, Uberlândia, MG, Brazil 3 Programa de Pós-Graduação em Ecologia e Conservação dos Recursos Naturais, Instituto de Biologia, Universidade Federal de Uberlândia, 38400-902, Uberlândia, MG, Brazil * Corresponding author: [email protected] Diagramação e XML SciELO Publishing Schema: www.editoraletra1.com.br Mariana de Souza Carvalho Velloso, Vinícius Lourenço Garcia de Brito, Ana Paula Souza Caetano and Rosana Romero with differences in the amount of pollen grains as well corresponds to the position of the stigma. This permits as pollen exine ornamentation and cytoplasmic content the pollen to be deposited in strategic places on the body (Hrycan & Davis 2005; Luo et al. 2009). Heteranthery was of the bee that are not easily reached during the grooming first described more than 150 years ago, when Darwin, in behavior (Forbes 1882; Almeda 1977; Renner 1989; Luo et 1862, stated that he was intrigueded by the phenomenon. al. 2009; Ferreira & Araújo 2016). A few years later, Müller (1881; 1882; 1883) suggested the In addition to the morphology of the different types of division of labor between the different stamens, whereby stamens, the division of labor may be cryptic and involve one set would serve to feed and the other would act on the the pollen grains from each type of anther, which may pollination process, as supported by current studies (Luo vary in structure, content, quantity and release dynamics et al. 2008; 2009; Vallejo-Marín et al. 2009; Paulino et al. (Mori et al. 1980; Nepi et al. 2003; Luo et al. 2008; Leite 2016; Papaj et al. 2017). 2016; Paulino et al. 2016). Moreover, since heteranthery In Melastomataceae, heteranthery has been described in may be related to the color patterns of the floral parts, species of different tribes such as Bertolonieae, Melastomeae, especially the corolla, stamens and ventral appendage of Merianieae, Microlicieae, Rhexieae and Sonerileae (Müller the connective (Fig. 1), it is essential to understand how the 1881; Cogniaux 1891; Renner 1989; Mendoza & Ramírez colors of these different structures are potentially perceived 2006; Luo et al. 2008). This family, although notoriously and discriminated by the pollinators, and if they also have diverse, with more than 5.400 species (Goldenberg et al. some function in the process of division of labor (Luo et 2015), typically possesses falciform anthers with poricidal al. 2009; Ferreira & Araújo 2016). Until now, the color dehiscence (Renner 1989; Santos et al. 2009). These patterns of such structures (i.e. conspicuous feeding anthers characteristics cause pollen grains to be available only to and cryptic pollinating anthers) were only considered in bees that are able to vibrate their wing muscles during terms of humam-based color vision, but not in terms of their visits to flowers, with pollen grains usually being the bee color vision models. Once confirmed that the ventral only reward offered by the flower (Renner 1989). Thus, the appendage of the connective has the same color as the evolutionary history of the family is closely associated with a feeding anthers in the pollinator color vision, this structure pollination system known as “buzz pollination” (Buchmann would be considered a strategy increasing the attractiveness 1983; Renner 1989; Luo et al. 2008; Luca et al. 2013). of the flowers, which would signal more stamens than the In species of this family, the androecium commonly flower actually has (Dafni 1984; Lunau 2006). Within this consists of two stamen cycles (Cogniaux 1891; Mendoza context, the species of Microlicia (Melastomataceae) are good & Ramírez 2006; Luo et al. 2009). In the species with models for testing the division of labor between anthers heteranthery, the set of “feeding” stamens (antepetalous) at different levels, since they present two stamen cycles of consists of smaller and more conspicuous elements and different size, shape and color (Romero 2003), reflecting is located in an accessible place in the flower. In contrast, the extensive occurrence of heteranthery in the family. the remaining stamens (antesepalous), considered to be The objective of the present study was to analyze the “pollinating” stamens, are larger, have a more discrete color different levels of differentiation and speciallization of or a color similar to that of the corolla and their location each type of anther in the flowers of Microlicia cordata and Figure 1. Microlicia cordata. A. Front view of a flower showing the presence of two morphological types of stamens;B. Detail of the morphology of the feeding stamen on the left, and of the pollinating stamen on the right. Symbols: 1: anther; 2: pedoconnective; 3: filament; 4: ventral connective appendage. Scale bars: A: 1.5 mm; B: 1 mm. 350 Acta Botanica Brasilica - 32(3): 349-358. July-September 2018 Diagramação e XML SciELO Publishing Schema: www.editoraletra1.com.br Anther specializations related to the division of labor in Microlicia cordata (Spreng.) Cham. (Melastomataceae) to compare morphological and anatomical data between removed, numbered, and photographed in order to measure anthers and pollen grains from the pollinating and feeding the size of the anthers, the pedoconnective and ventral stamens that may promote division of labor. The following appendage of the connective of both cycles, with the aid questions were considered: 1) Are there differences in of ImageJ software (Hartig 2013). Structural measures amount and viability of pollen grains, and in morphology were obtained from the photographs. Analysis of variance and in cytoplasmic content between the feeding and (ANOVA) was applied considering length as the response pollinating anthers? 2) Are the anthers of the pollinating and structure (anther of the feeding stamen, anther of the stamens less conspicuous than the anthers of the feeding pollinating stamen and ventral appendage of the connective stamens when compared to the corolla, considering bee of the pollinating stamen) as the independent variable. The color vision? 3) Do the anthers of the feeding stamens and lengths of the structures were then compared a posteriori the ventral appendage of the connective of the pollinating by a pairwise t-test with Bonferroni corrections. stamens have the same color? According to the division of For the analysis of anatomy and pollen grain content, labor hypothesis, we expect to find differences in pollen flower buds and freshy opened flowers of five M. cordata content and structure, as well as in pollen quantity and individuals were collected and fixed in 2 % glutaraldehyde quality produced by different anthers. We also expect the and 4 % formaldehyde in 0.1 M sodium phospate buffer, pH pollination anthers to be more cryptic, whereas the feeding 6.8 (McDowell & Trump 1976), dehydrated in an ethanol anthers and the ventral appendage of the connective of series and embedded in plastic resin (Gerrits & Smid the pollinating stamens to be similar in color and more 1983).