The researchers fed larvae divided into 72 different groups by combining 9 qualitatively different diets, from the poorest to the richest, with 8 different quantitative rations (160μl->370μl), plus 1 group in which the medium-quality diet was offered to the larvae “ad libitum”.

The results seem to show that it is not the quality of the larval diet (exclusive royal jelly or not) that determines its caste, but rather the amount of food available to it. Nurse bees would offer more food to future queens than to future workers. The elongation of a queen cell would allow nurse bees to provide more food to the larva inside.

Abstract

In species that care for their young, provisioning has profound effects on offspring fitness. Provisioning is important in honey bees because nutritional cues determine whether a female becomes a queen or a sterile worker. A qualitative difference between the diet of queen and worker larvae is thought to drive this divergence; however, no single compound appears to be solely responsible. Diet quantity may play a role in honey bee caste determination, but it has never been formally studied. Our goal was to determine the relative contributions of diet quantity and quality to queen development. Larvae were reared in vitro with nine diets varying in the amounts of royal jelly and sugars, and these were given to larvae in eight different quantities. For the intermediate diet, a quantitative ad libitum treatment was included. Once adults had eclosed, queen status was determined using a principal components analysis on seven morphological measurements. We found that larvae fed ad libitum could not be distinguished from commercially reared queens, and that queen status was independent of the proportion of proteins and carbohydrates in the diet.

 

Neither protein nor carbohydrate content had a significant influence on principal component 1 (PC1), which explains 64.4% of the difference between queens and workers. In contrast, the total quantity of diet explained a significant portion of the variation in principal component 1 (PC1).

Influence of diet quantity on principal component 1 (p < 0.0001, R2 = 0.3814)

Large quantities of diet in the final instar were able to induce queen traits, contrary to the prevailing notion that queen determination can occur only in the third instar. These results indicate that the total quantity of diet given to larvae can regulate the difference between queen and worker castes in honey bees.

Authors’ conclusion

Nutritional status regulates many insect polyphenisms, such as the horns of dung beetles and the increased production of non-worker castes in termites. A biologically active substance in royal jelly has been thought to determine honey bee caste since the late 1890s. Many studies have evaluated caste determination in vivo and in vitro, and while these studies offer important insights, the influence of food quantity has not been explicitly tested.

 

Our results indicate that diet quantity is an important factor in caste determination. Nutritional stress is a simple and elegant mechanism for controlling reproductive potential. In social vertebrates, nutritional stress regulates reproductive state. Over the course of the evolution of eusociality in wasps, the lower nutritional status of workers likely regulated their non-reproductive status. In solitary bee species, which represent the ancestral state relative to sociality, all female larvae become reproductive adults. Therefore, caste determination is a loss of reproductive potential in worker larvae, and not necessarily a gain of reproductive potential caused by ingesting a special type of food. Over the course of the evolution of eusociality, the loss of reproductive potential in workers coincides with the development of multiple morphological and behavioral traits that promote hive fitness. The miRNAs and p-coumaric acid present in pollen inhibit ovary development and are present only in the diet of larvae destined to become workers. In our study, low quantities of food inhibited queen development. The third day has long been considered the critical period for queen determination, but our results suggest that reproductive state can be maintained until the sixth day if food is sufficient. Thus, our study suggests that bee larvae become workers through the suppression of feeding, such as food restriction and p-coumaric acid. In this framework, the mechanism underlying eusociality in bees is food restriction, which suppresses reproduction in the worker caste.

 

Source: Bowsher JH. 2020 Diet quantity influences caste determination in honeybees (Apis mellifera). Proc. R. Soc. B 287: 20200614. http://dx.doi.org/10.1098/rspb.2020.0614

 

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See also:

• Royal Jelly: One of Several Factors in Queen Development
• Epigenetics Leads Genetics
• Queen Cells
• From Egg to Imago
• Principles and Methods of Queen Rearing
• DNA Adaptation According to Role