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Written by Maria Juliana
A few species of stingless bees have fooled human observers. The queen, often seen on brood combs and exhibiting active egg laying, ceases her posture and hides herself between the food pots during an event known as reproductive diapause. Diapause is considered an adaptation that allows the queen (and colony’s) survival in adverse environmental conditions. This event is mostly common in temperate zones, but it also occurs in the tropics as an adaptive response to diminishing resources in the cold and dry season.
Reproductive diapause is common among queens in the stingless bee genus Plebeia, occurring as an obligatory condition in some species. Reports on the occurrence of reproductive diapause in other stingless bee genera in Brazil are scarce. In our study, we described for the first time the occurrence of reproductive diapause in Melipona marginata in Southeast region of Brazil, comparing this event with events observed in South Brazil by Borges and Blochtein (2006) in Melipona obscurior, a closely related species. In the study described here, we compared the photoperiod and temperature in both localities to understand the factors that trigger the reproductive diapause in eusocial bees. In addition, we compared the queen’s chemical profile before and during reproductive diapause to verify the occurrence of chemical changes in the signaling of fertility.
Fig1. A Melipona marginata queen in regular egg laying activity. Photo: M. J. Ferreira-Caliman.
We observed that Melipona marginata queens gradually declined the frequency of oviposition in early May, and in the cold and dry months (to May from July) they ceased egg laying completely. Five out of six colonies we observed entered the reproductive diapause, suggesting that this event is facultative in Melipona bees and that this variation is determined by internal factors of the nests, such as the ratio of adults to brood and food stores.
The environmental factors involved in reproductive diapause are commonly associated with photoperiod and temperature (Derlinger, 2002). The photoperiod and temperature seem to be the triggering factor of reproductive diapause in M. marginata in Southeast Brazil, as well as Melipona obscurior in South Brazil. In these two species, the reproductive diapause period coincided with the months of shorter day length and low temperatures, occurring between the months of March and August, suggesting that the reproductive diapause is a mechanism used by Melipona bees to overcome the diminishing resources in the cold and dry season.
The workers did not stop their activities and all behaviors related to colony maintenance were performed, such as queen feeding and food collection (although cell construction was stopped). However, the queens showed conspicuous behaviors. They walked through the entire colony, including in the food pots. The queens’ enlarged abdomen (a typical morphological aspect of post-mating stingless bee queens), did not disappear during reproductive diapause, but we observed that the posterior portion of abdomen decreased, suggesting oocytes were resorbed.
So, faced with the behavioral and morphological changes, why were queens not replaced by gynes when they stopped oviposition? The answer can be related to the chemical communication between the castes, which allows cohesion in the social insect colonies. The chemical analysis of Melipona marginata queens showed that the cuticular hydrocarbons profile does not change qualitatively during the diapause phase. Probably, this may explain why the workers have not killed the queens in this period, and why the workers did not lay eggs, a common occurrence in Meliponini colonies. Chemical and behavioral evidence suggest that two specific groups of hydrocarbons, the methyl-branched alkanes and alkenes, may act as fertility signals. The cuticular profiles of Melipona marginata before and during reproductive diapause had a greater and similar amount of hentriacontene isomers (alkenes). These results reinforce the idea that the chemical signals are crucial to maintaining the organization in insect societies, even in periods of adversity
References
Borges FVB, Blochtein B (2006) Variação sazonal das condições internas de colônias de Melipona marginata obscurior Moure, no Rio Grande do Sul, Brasil. Rev Bras Zool 23:711-715
Denlinger DL (2002) Regulation of diapause. Annu Rev Entomol 47:93-122
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