The role of Numb in honeybee ovary activation

Abstract

Environmental signals in utero are throught to induce plastic changes to metabolism and gene regulation, causing diseases such as Type 2 Diabetes. For example, fetal undernutrition is associated with obesity, hyperglycemia, and high circulating lipids. These multifactorial responses are challenging to study in humans, so we need a better model of phenotypic plasticity than humans to understand the fundamental mechanisms underpinning these diseases. Honeybees are an ideal model for phenotypic plasticity, as they have a manipulable response to a defined stimulus, queen mandibular pheromone (QMP): when QMP exposure is lost, worker honeybees become fertile. One molecular mechanism underpinning this response is Notch signalling: when Notch cell signalling is active, the worker ovary is repressed, while a loss of Notch signalling causes ovary activation. Notch signalling is active in the honeybee germarium, the region of the ovary where oocytes are specified and produced, but its function there is unknown. Changes in expression of Numb, a known inhibitor of Notch, coincides spatially and temporally with Notch activation. Therefore Numb is hypothesised to regulate Notch signalling.

To investigate the regulation of Numb in honeybee ovary activation, I predicted a cis-regulatory element within the Numb gene. This drove expression in the gut of Drosophila, and may be QMP-responsive, suggesting Notch signalling may mediate the Drosophila response to QMP in the gut. In addition, to investigate the role of Notch signalling in the honeybee ovary, I performed hybridization chain reaction (HCR) to visualize expression of Numb and the germ-cell marker vasa. HCR in this context was very successful, revealing a new expression domain of vasa, and when quantified, gave results similar to other studies. HCR is, therefore, a powerful method to investigate and quantify gene expression in an unbiased manner. I found that Numb is expressed in a broad domain in the anterior germarium, instead of localizing to a specific cell type. Notch signalling may, therefore, prevent the cell clusters in the ovary from dividing and differentiating into more mature oocytes. This hypothesis is supported by data in the wasp Nasonia vitripennis, where more information is available about the structure of the germarium: Notch protein expression correlates spatially with the presence of undifferentiated, self-renewing cell clusters, though the Notch modifier fringe is expressed throughout the whole germarium. Therefore, the role of Notch signalling in the honeybee (and Nasonia) germarium may be to hold cell clusters in an undifferentiated state, preventing them from becoming mature oocytes, thus repressing worker bee reproduction.