Recent Proliferation and Translocation of Pollen Group 1 Allergen Genes in the Maize Genome

Elene R. Valdivia , Javier Sampedro ^*, Jonathan C. Lamb , Surinder Chopra , and Daniel J. Cosgrove

Department of Biology, 208 Mueller Lab, Penn State University, University Park, PA 16802, USA; Division of Biological Sciences, 117 Tucker Hall, University of Missouri, Columbia, MO 65211, USA; and Department of Crop and Soil Sciences, 116 Ag. Sci. & Ind. Bldg, Penn State University, University Park, PA 16802, USA

^* Corresponding author; email: jus16{at}psu.edu.

The dominant allergenic components of grass pollen are knownby immunologists as group 1 allergens. These constitute a setof closely-related proteins from the ${beta}$ -expansin family and havebeen shown to have cell-wall loosening activity. Group 1 allergensmay facilitate the penetration of pollen tubes through the grassstigma and style. In maize the group 1 allergens are dividedinto two classes, A and B. We have identified 15 genes encodinggroup 1 allergens in maize, 11 genes in class A and 4 genesin class B, as well as 7 pseudogenes. The genes in class A canbe divided by sequence relatedness into two complexes, whereasthe genes in class B constitute a single complex. Most of thegenes identified are represented in pollen-specific EST librariesand are under purifying selection, despite the presence of multiplecopies that are nearly identical. The group 1 allergen genesare clustered in at least six different genomic locations. Thesingle class B location and one of the class A locations showsynteny with the rice regions where orthologous genes are found.Both classes are expressed at high levels in mature pollen butat low levels in immature flowers. The set of genes encodingthe maize group 1 allergens is more complex than originallyanticipated. If this situation is common in grasses, it mayaccount for the large number of protein variants, or group 1isoallergens, identified previously in turf grass pollen byimmunologists.