The LAMB1 gene from the clubmoss, Lycopodium annotinum, is a divergent MADS-box gene, expressed specifically in sporogenic structures.

Transcription factors encoded by the large MADS-box gene family have important developmental functions in angiosperms, the flowering plants. Mutations in certain MADS-box genes are known to cause homeotic alterations in floral organ identity, and the establishment of floral organ identity is the most well-studied developmental process in which MADS-box genes are known to function. Our interest is in the potential connection between the duplication history of this gene family and the evolutionary origin of the structures that the different MADS-box genes developmentally regulate in plants. Previous studies have demonstrated that the origin of the MADS-box genes that control floral organ identity predate the evolutionary origin of the flower itself, since gymnosperms have genes that are orthologous to angiosperm floral homeotic MADS-box genes, whereas ferns appear to lack such genes. Here we report on the isolation of a MADS-box gene from Lycopodium annotinum, which belongs to the clubmosses, the phylogenetic sister group to other vascular plants. The gene, LAMB1, in the sporophyte is expressed exclusively in the reproductive structure, the strobilus, during sporogenesis. LAMB1 is similar to other plant MADS-box genes in that it contains a MADS-box as well as a second conserved element, a K-box. However, it differs in length and in exon/intron structure in the region between the MADS- and K-box, and also in the length and structure of the C-terminal region. A phylogenetic analysis indicates that LAMB1 is not closely related to other plant-type MADS-box genes, and may represent one of the basal branches in the phylogenetic tree of plant MADS-box genes.