ARS | Publication request: Genes Related to Immunity as Expressed in the Alfalfa Leafcutting Bee, Megachile rotundata During Pathogen Challenge

Submitted to: Insect Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 6, 2009
Publication Date: November 6, 2009
Citation: Xu, J., James, R.R. 2009 Genes Related to Immunity, as Expressed in the Alfalfa Leafcutting Bee, Megachile rotundata During Pathogen Challenge. Insect Molecular Biology. 18(6):785-795.

Interpretive Summary: Bees are very important as pollinators. Honey bees are easily the most well known bee, but there are actually many different kinds of bees, at least 3000 species in the U.S. alone. Most of these other bees do not produce honey, nor do they form large colonies. They are sometimes called the pollen bees. Honey bees and bumble bees form colonies with a queen and worker bees, and so are called “social bees” because they form societies. Other bees are called “solitary bees” because each female bee is her own queen, and she lays eggs in her own nest without workers to help her. Many scientists think that social insects are more vulnerable to disease outbreaks because they live in crowded conditions, feed each other, and share the same space, increasing the chances for the spread of diseases. Thus, these bee may have evolved specialized ways to combat disease, including special immune systems. However, such hypotheses cannot be tested when we know virtually nothing about the immune response of solitary bees. We surveyed for all the genes that are active in both healthy and diseased larvae of a solitary bee, the alfalfa leafcutting bee, identifying 116 immunity genes, then compared these immunity genes to those found in the honey bee and other insects that have been studied. The genes in the alfalfa leafcutting bee were more like honey bee genes than other insects. Although the genetic composition was similar to the honeybee, it was not exactly the same. The data presented provides the first analysis of immune function in a solitary bee, and provides a foundation to better understand how bees defend themselves against pathogens. This is the first step towards understanding how environmental stress, such as pesticide exposure and climate change, can affect the susceptibility of bees to disease.

Technical Abstract: Bees are a large, diverse group of insects that collect pollen and nectar from plants to feed to their young, and as a result, play an important ecological role as pollinators. Honey bees, which form large colonies with a complex social structure, are probably the most well known bee, but the majority of species are actually solitary, with little or no social structure. It has been proposed that social insects are more vulnerable to disease outbreaks due to the density and frequency of interactions among individuals in a nest, and as a result, may have evolved special mechanisms for evading disease, including specialized immune responses, but such hypotheses cannot be tested when we know virtually nothing about the immune response of solitary bees. Using an EST database for both healthy and pathogen challenged larvae of a solitary bee, the alfalfa leafcutting bee, we identified 104 genes known to have immunity-related functions in other insects. An additional 12 putative immune response genes were identified in this solitary bee using PCR amplification with primer design based on the sequence of honey bee immune response genes. Identified genes coded for proteins with a wide variety of innate immune response functions, including pathogen recognition, phagocytosis, the prophenoloxidase cascade, melanisation, coagulation and several signaling pathways. The data presented provides the first analysis of immune function in a solitary bee, and provides a foundation for the further analysis of gene expression patterns in bees.