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

doi:10.1073/pnas.0606434104

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

*Division of Biological Sciences, Section of Cell and Developmental Biology, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0116; and
^†Department of Biology, University of Missouri, One University Boulevard, St. Louis, MO 63121

Edited by John F. Doebley, University of Wisconsin, Madison, WI, and approved November 25, 2006 (received for review July 28, 2006)

Abstract

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.

Footnotes

^§To whom correspondence should be addressed. E-mail: rschmidt{at}ucsd.edu
Author contributions: C.J.W. and M.J.Z. designed research; C.J.W. and M.J.Z. performed research; E.A.K. contributed new reagents/analytic tools; C.J.W., M.J.Z., E.A.K., and R.J.S. analyzed data; and C.J.W. and R.J.S. wrote the paper.
↵ ^‡Present address: Purdue University, Botany and Plant Pathology, Lilly Hall, 915 West State Street, West Lafayette, IN 47907-2054.
The authors declare no conflict of interest.
This article is a PNAS direct submission.
Data deposition: The sequences reported in this paper have been deposited in the GenBank database (accession nos. DQ662237–DQ662246).