Characterization of homeodomain-leucine zipper genes in the fern Ceratopteris richardii and the evolution of the homeodomain-leucine zipper gene family in vascular plants

The homeodomain-leucine zipper (HD-Zip) genes encode transcription factors that are characterized by the presence of both a homeodomain and a leucine zipper motif. They belong to the homeobox gene superfamily and have been reported only from flowering plants. This article is the first report on the fern HD-Zip genes (named Crhb1-Crhb11) isolated from the homosporous fern Ceratopteris richardii. Phylogenetic analyses of the ii Ci Crhb genes with previously reported angiosperm HD-Zip genes show that the Ci hb genes belong to three of the four different angiosperm HD-Zip subfamilies (HD-Zip I, II, and IV), indicating that these subfamilies of I-ID-Zip genes originated before the diversification of the fern and seed plant lineages. The Crhb4-Crhb8 and Crhb11 genes belongs to the HD-Zip I subfamily but differ from angiosperm KD-Zip I genes by the presence of a seven-amino-acid indel in the leucine zipper motif. By the northern analyses, Crhb1 and Crhb3 were expressed only in gametophyte tissue. Expression of Crhb2 and Crhb11 genes could not be detected in any tissue examined, while all other CI hb genes were expressed in most sporophytic and gametophytic tissues. Although the functions of the Crhb genes in Ceratopteris are unknown, their patterns of expression suggest that they regulate developmental or physiological processes common to both the gametophyte and the sporophyte generations of the fern. Differences in the expression of Crhb1 between male gametophytes and male-hermaphrodite mixed populations of gametophytes suggests that the Crhb1 gene is involved in gametophytic sex determination.