A scientific note on diploid males in a reproductive event of a eusocial bee
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- dos Santos, C.F., Menezes, C., Imperatriz-Fonseca, V.L. et al. Apidologie (2013) 44: 519. doi:10.1007/s13592-013-0202-0
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In Hymenoptera (ants, wasps, bees), sex determination is controlled by haplodiploidy, with fertilized eggs giving rise to diploid females and unfertilized eggs to haploid males (Whiting 1943; Cook and Crozier 1995). However, sex determination also depends on the combination of alleles at the complementary sex determination locus (CSD). It has been demonstrated that diploid individuals but homozygous at CSD emerge as diploid males, and such individuals are likely sterile (Whiting 1943; Cook and Crozier 1995). However, it has been shown that in several Hymenoptera species, the diploid males are able to mate and generate female offspring, albeit triploid (Liebert et al. 2004; Cournault and Aron 2009; Darvill et al. 2012). Hence, bee populations could be severally affected due the role of diploid male vortex in leading nest and population extinction (Zayed and Packer 2005).
In eusocial stingless bees (Meliponini), for example, diploid male production may be significant, c.a. 50 % of the nest population (Alves et al. 2011; Francini et al. 2012), and queens producing diploid males, as well as themselves, are supposed to be eliminated by the workers (Alves et al. 2011; Francini et al. 2012). Thereby, diploid males in reproductive events, i.e., outside nest, have never been documented in stingless bees (Paxton 2000; Cameron et al. 2004; Kraus et al. 2008). Sexually mature stingless bees males, 2–3 weeks old after emerging, permanently abandon their nests and vague in environment to search for receptive females (van Veen et al. 1997). The rendezvous sites in stingless bee species involve two mating strategies. The strategy that occurs for mostly of the genera (e.g., Tetragonisca) is the congregation of tens to hundreds of males outside the nest entrance (Nogueira-Neto 1954; Kerr et al. 1962; van Veen and Sommeijer 2000). The other known strategy occurs in Melipona species where males aggregate on non-nest associated congregation sites (Sommeijer and Bruijn 1995). In both cases, gynes do one single nuptial flight and as a rule, mating with only one male (Kerr et al. 1962; Peters et al. 1999). Here, we describe the finding of adult diploid males of Tetragonisca angustula (Latreille 1811) in reproductive aggregation beside the nest entrance.
The study was conducted at the campus of Universidade de São Paulo, in Ribeirão Preto, Brazil. The campus possesses a large number of Meliponini nests of several species. A recent inventory registered more than 250 T. angustula nests encompassing feral and managed colonies (Soares, unpublished data). To study male aggregations in nests foundations, several trap nests were distributed around the campus. During a period of 3 weeks, five male aggregations were studied. The aggregations were set near the nests being prepared by workers to receive a new swarm. A total of 378 males from the five aggregations were collected for molecular analysis.
Total DNA was extracted from the thorax of each male through Chelex method (Walsh et al. 1991). Six microsatellite loci were used for genotyping (Brito et al. 2009): Tang03, Tang11, Tang12, Tang57, Tang60, and Tang65. Allele calling was performed in an automatic sequencer ABI 3730 at CEGH-IB-USP, using the program GeneMaker® 2.2. The allele number per locus ranged from 7 to 28. The average allele number per locus was 9.8 ± 0.901 and the effective allele number was 5.2 ± 0.613.
Genotypes of diploid males of T. angustula (Apidae: Meliponini) found in a reproductive aggregation
219/219 or 219
184/184 or 184
219/219 or 219
149/149 or 149
The fact that diploid males were found in reproductive aggregations is an indicative that they escaped from worker recognition. Although there is some evidence that stingless bee workers eliminate diploid males (Francini et al. 2012), there is also evidence that their chemical cuticle profile is very similar to the haploid males, at least soon after emergence (Borges et al. 2012). Whether the diploid males of T. angustula can be chemically recognized is still an opened question to be investigated. Besides, it has already been demonstrated for some species of Meliponini that several unrelated nests contribute to the same male aggregation (Paxton 2000; Cameron et al. 2004; Kraus et al. 2008) thereby decreasing the possibility of the queen mating with related males.
The presence of diploid males in bee populations could largely decrease effective population size through an extinction vortex (Zayed and Packer 2005). More studies must be conducted to evaluate the potential of diploid males to copulate with virgin queens and its consequences for the colony and species population. Hence, our findings demonstrate that diploid males of T. angustula are able to visit mating aggregations, but due to their rarity, likely have a reduced chance to copulate.
We thank the Coordenação de Pessoal de Nível Superior (CAPES-PROAP 2010) for financial support and scholarship through the Graduate Program in Zoology of the Departamento de Zoologia–IBUSP; Ademilson Espencer Egea Soares for kindly providing the inventory data of Meliponini nests at the campus da Universidade de São Paulo/Ribeirão Preto/Brazil; Flávio de Oliveira Francisco and Leandro Rodrigues Santiago for helping with molecular techniques and analysis. Authors gratefully acknowledge Rodolfo Jaffé for his critical reading of the manuscript and suggestions