AGAMOUS subfamily MADS-box genes and the evolution of seed cone morphology in Cupressaceae and Taxodiaceae.

In this comparative developmental genetics study, we test hypotheses based on fossil and morphological data on reproductive organ morphology and evolution in conifers--specifically, the ovule-bearing organ in Cupressaceae and Taxodiaceae. Genes homologous to the Arabidopsis gene AGAMOUS are expressed in ovuliferous scales of spruces (Picea) throughout development. Previous studies have shown that the AGAMOUS subfamily of MADS-box genes predates the split between angiosperms and gymnosperms, and that these genes have in part conserved functions in reproductive development among seed plants, especially in the specification of identity of the ovule-bearing organs. These data indicate that their expression in conifer families other than Pinaceae might be used as markers for organs homologous to the Pinaceae ovuliferous scale. Here we have isolated putative AGAMOUS orthologs from Cupressaceae and Taxodiaceae and analyzed their expression pattern in seed cones to test for the presence of morphological homologs of ovuliferous scales. Our results were not congruent with the hypothesis that the tooth of the Cryptomeria seed cone is homologous to the Picea ovuliferous scale. Likewise, the hypothesis that the bracts of Thujopsis and Juniperus contain fused ovuliferous scales was not supported. However, we found expression of AGAMOUS homologs in the sterile bracts of Cupressaceae seed cones at late developmental stages. This expression probably represents a novel gene function in these conifer families, since no corresponding expression has been identified in Pinaceae. Our study suggests that the evolutionary history of modern conifer cones is more diverse than previously thought.