Characterization of seven MADS-box genes, termed PPM1-PPM4 and PpMADS1-PpMADS3, from the moss model species Physcomitrella patens is reported. Phylogeny reconstructions and comparison of exon-intron structures revealed that the genes described here represent two different classes of homologous, yet distinct, MIKC-type MADS-box genes, termed MIKC(c)-type genes-"(c)" stands for "classic"-(PPM1, PPM2, PpMADS1) and MIKC(*)-type genes (PPM3, PPM4, PpMADS2, PpMADS3). The two gene classes deviate from each other in a characteristic way, especially in a sequence stretch termed intervening region. MIKC(c)-type genes are abundantly present in all land plants which have been investigated in this respect, and give rise to well-known gene types such as floral meristem and organ identity genes. In contrast, LAMB1 from the clubmoss Lycopodium annotinum was identified as the only other MIKC(*)-type gene published so far. Our findings strongly suggest that the most recent common ancestor of mosses and vascular plants contained at least one MIKC(c)-type and one MIKC(*)-type gene. Our studies thus reveal an ancient duplication of an MIKC-type gene that occurred before the separation of the lineages that led to extant mosses and vascular plants more than about 450 MYA. The identification of bona fide K-domains in both MIKC(*)-type and MIKC(c)-type proteins suggests that the K-domain is more ancient than is suggested by a recent alternative hypothesis. MIKC(*)-type genes may have escaped identification in ferns and seed plants so far. It seems more likely, however, that they represent a class of genes which has been lost in the lineage which led to extant ferns and seed plants. The high number of P. patens MADS-box genes and the presence of a K-box in the coding region and of some potential binding sites for MADS-domain proteins and other transcription factors in the putative promoter regions of these genes suggest that MADS-box genes in mosses are involved in complex gene regulatory networks similar to those in flowering plants.

The MADS-box genes of land plants are extensively diverged to form a superfamily and are important in various aspects of development including the specification of floral organs as homeotic selector genes. The closest relatives of land plants are the freshwater green algae charophyceans. To study the origin and evolution of land plant MADS-box genes, we characterized these genes in three charophycean green algae: the stonewort Chara globularis, the coleochaete Coleochaete scutata, and the desmid Closterium peracerosum-strigosum-littorale complex. Phylogenetic analyses suggested that MADS-box genes diverged extensively in the land plant lineage after the separation of charophyceans from land plants. The stonewort C. globularis mRNA was specifically detected in the oogonium and antheridium together with the egg and spermatozoid during their differentiation. The expression of the C. peracerosum-strigosum-littorale-complex gene increased when vegetative cells began to differentiate into gametangial cells and decreased after fertilization. These expression patterns suggest that the precursors of land plant MADS-box genes originally functioned in haploid reproductive cell differentiation and that the haploid MADS-box genes were recruited into a diploid generation during the evolution of land plants.charophytes ͉ land plants ͉ Chara ͉ Coleochaete ͉ Closterium D iversity in form, a hallmark of extant species, is probably caused by modifications of ancestral gene networks regulating development and by the generation of novel developmental processes (1). The evolution of transcription factors, which have critical functions in development, through gene duplication and subsequent functional divergence has been hypothesized to be a major force in developmental evolution (reviewed in ref.2). The adaptation of green plants to a terrestrial environment and their subsequent diversification are tightly linked to the evolution of the body plan of land plants (3, 4). Members of the MADS-box gene family regulate various aspects of development in flowering plants and therefore were probably involved in the evolution of the morphology of land plants.MADS-box genes are characterized by the conserved MADS domain and are found in a wide range of eukaryotes including metazoans, fungi, slime mold, and green plants (5). These genes have been classified into several groups (6). MIKC C -and MIKC*-type MADS-box genes contain intervening (I), keratinlike (K), and C-terminal (C) domains (7,8). They are present in all major land plant taxa including seed plants, pteridophytes, and bryophytes but have not been found in other organisms such as green algae (5, 6, 9). The Arabidopsis genome (10) contains Ϸ38 MIKC C -type and 5 MIKC*-type genes (6, 9). The MIKC*-type genes tend to form a monophyletic group, including several subgroups of genes with unknown functions (9), but the MIKC Ctype MADS-box genes have been classified into about a dozen subfamilies with diverse functions and expression patterns (reviewed in ref. 6). The floral homeotic genes ar...

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