EVOLUTION OF AGL6-LIKE GENES IN GRASSES (POACEAE): OVULE EXPRESSION IS ANCIENT, PALEA EXPRESSION IS NEW
REINHEIMER RENATA; ELIZABETH A. KELLOGG
AMER SOC PLANT BIOLOGISTS
AGAMOUS-like 6 (AGL6) genes encode MIKC-type MADS box transcription factors, and are closely related to SEPALLATA and AP1/FUL-like genes. Unlike the latter two groups, the AGL6-like genes have not been well characterized in any group of plants and their function is largely unknown. Here we focus on the molecular evolution and expression of the AGL6-like genes in grasses. We have found that AGL6-like genes are expressed in ovules, lodicules (second whorl floral organs), paleas (putative first whorl floral organs), and floral meristems. Each of these expression domains was acquired a different time in evolution, indicating that each represents a distinct function of the gene product and that the AGL6-like genes are pleiotropic. Expression in the inner integument of the ovule appears to be an ancient expression pattern corresponding to the expression of the gene in the megasporangium and integument in gymnosperms. Expression in floral meristems appears to have been acquired in the angiosperms, and expression in second whorl organs in monocots. Early in grass evolution, AGL6-like orthologues acquired a new expression domain, in the palea, and thus are the first of the MADS box genes to be implicated in specifying identity of that floral organ. Stamen expression is variable but appears to correlate with stamen number. Most grasses have a single AGL6-like gene (orthologous to the rice gene OsMADS6). However, rice and its relatives in the genus Oryza have a second copy (orthologous to OsMADS17) that appears to be the result of an ancient duplication.