A CINDERELLA STORY

Abstract

In human societies, not everyone is born equal. The phrase `born with a silver spoon in the mouth' refers to the fact that, at birth, some people find themselves in positions of privilege and wealth whereas others do not. Similar inequalities exist within animal societies and, in the case of some insects such as ants and bees, are taken to great extremes. Ants and bees have evolved eusociality – several generations live together in colonies, only one or a few individuals (queens) have offspring, and non-reproductive colony members, the workers, care for these offspring. Becoming a queen is not something that workers can aspire to. Although some sneaky workers do try to have offspring themselves, individuals born as workers cannot become queens. Nutrition is an important determinant of social class in many eusocial insect species. In some species, larvae destined to become queens are fed a particular substance (royal jelly in honey bees), whereas in other species queen larvae get more food than worker larvae. This latter system is found in the stingless bee Schwarziana quadripunctata. Now, an international team of researchers from Belgium, Britain and Brazil led by Tom Wenseleers has discovered that some stingless bees manage to beat the system and change their fate.The team investigated previous reports that dwarf queens occur in stingless bee colonies in addition to the normal, large queens. Wenseleers and his co-workers weighed the dwarf queens and showed that they are indistinguishable in weight from regular worker bees. In addition, dwarf queen larvae are raised in cells that are identical to the cells in which worker larvae are raised,which are smaller than the large specialized cells in which the queen larvae develop. Dwarf queens aren't a rare occurrence; when the team assessed the social class of 11 574 individual stingless bees from 19 colonies, they found six times more dwarf queens than normal queens. The team used a series of morphological measurements to identify adult dwarf queens and distinguish them from normal queens. They showed that the dwarf queens headed 12 out of a further 54 colonies, suggesting that the dwarf queens are able to reproduce. Although this may seem like a large proportion of colonies, it actually suggests that dwarf queens aren't as successful as normal queens – 86%of queens reared were dwarf queens, but they only headed 22% of the colonies that the team studied. Wenseleers and his co-workers suggest that this discrepancy may be due to workers actively discriminating against the dwarf queens.Many questions remain about this intriguing system in which individuals seem able to choose whether to become queens. In particular, it would be interesting to know exactly which mechanisms determine the fate of stingless bee larvae. What role does nutrition play in determining their fate? If nutrition determines whether an individual becomes a queen or a worker, then some larvae destined to become workers are apparently overcoming nutritional limits and becoming queens despite their limited food supply. What physiological mechanisms control nutritional responses in this system? Recent advances in Drosophila have given us some places to start looking for answers, such as insulin receptor pathways. Perhaps these or other, as yet unknown, pathways play a role in the production of dwarf queens in stingless bee colonies. Whatever the case, this system has the potential to keep ecologists and physiologists busy for many years to come.