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The morphological diversity of land plants is astounding. However, what we see mostly is the sporophytic phase that is dominant in the majority of land plants. In contrast, the diversity of the secret gametophytic phase is unseen and rather uninvestigated. Recently, evidence has accumulated that the so-called MIKC* group of MADS-domain transcription factors is important for the proper functioning of the Arabidopsis male gametophyte (pollen). Already earlier, MIKC* genes were identified in the moss Physcomitrella patens, which has a dominant gametophytic phase. MADS-domain proteins are well known for the roles they have in flower development and thus for the establishment of the sporophytic body plan. That MIKC* genes have a similar role in the gametophytic phase is not granted, but a tempting hypothesis. To study the function of MIKC* genes and their possible role in land plant gametophyte development and its evolution, they were isolated from a broad variety of vascular land plants, namely, the lycophyte Selaginella moellendorffii, the fern Ceratopteris richardii, the basal eudicot Eschscholzia californica, the monocot Oryza sativa and the basal angiosperm Aristolochia fimbriata. Sequence comparison showed that MIKC* MADS-box genes probably evolved from classical MIKCc genes by a duplication event in the Keratin-like domain. Further phylogenetic analysis revealed that 2 phylogenetic subclades emerged early in the evolution of vascular plants and indications were found for a recent subfunctionalization of one of the subclades in angiosperms. MIKC* genes from different, remote, plant lineages were heterologously expressed in an Arabidopsis MIKC* mutant and it could be shown that they were able to perform the same function as Arabidopsis MIKC* genes. This information plus the results that were gathered by performing expression and yeast-2-hybrid interaction studies, were unified in a hypothesis concerning the function of MIKC* genes during land plant evolution. |
