Conservation of B class gene expression in the second whorl of a basal grass and outgroups links the origin of lodicules and petals

Clinton J Whipple, Michael J Zanis, Elizabeth A Kellogg, Robert J Schmidt
Proceedings of the National Academy of Sciences of the United States of America 2007 January 16, 104 (3): 1081-6
Studies of flower development in core eudicot species have established a central role for B class MADS-box genes in specifying petal and stamen identities. Similarly in maize and rice, B class genes are essential for lodicule and stamen specification, suggesting homology of petals and lodicules and conservation of B class gene activity across angiosperms. However, lodicules are grass-specific organs with a morphology distinct from petals, thus their true homology to eudicot and nongrass monocot floral organs has been a topic of debate. To understand the relationship of lodicules to the sterile floral organs of nongrass monocots we have isolated and observed the expression of B class genes from a basal grass Streptochaeta that diverged before the evolution of lodicules, as well as the outgroups Joinvillea and Elegia, which have a typical monocot floral plan. Our results support a conserved role for B function genes across the angiosperms and provide additional evidence linking the evolution of lodicules and second whorl tepal/petals of monocots. The expression data and morphological analysis suggest that the function of B class genes should be broadly interpreted as required for differentiation of a distinct second floral whorl as opposed to specifying petal identity per se.

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