In Arabidopsis, the shoot apical meristem (SAM) homeostasis is finely regulated by the WUSCHEL-CLAVATA antagonism. WUSCHEL (WUS) encodes for a homeodomain protein essential for the SAM maintenance and its expression marks the organizing center (OC). On the other hand, the interaction between the three CLAVATA (CLV) proteins, which all together code for a heterodimeric transmembrane leucin-rich repeat (LRR) receptor like kinase (CLV1-2) and its specific ligand (CLV3), correctly restricts the WUS expression to the OC. In contrast to Arabidopsis, in maize two different WUS orthologs and a single CLV1 ortholog, Thick tassel Dwarf1 (TD1), have been so far characterized. Like in Arabidopsis, the TD1 and ZmWUS2 expression domains overlap but, unlike Arabidopsis, their expression is detected in cells recruited for leaf primordia. Conversely, ZmWUS1 is expressed within the SAM dome, not in a OC-type manner but rather in a dynamic fashion that always correlates to phytomer establishment. The expression of the single CLV1 ortholog TD1 does not overlap with ZmWUS1 expression domain, leaving an open question over the putative regulator of ZmWUS1 function. To answer this question, the closest TD1 paralogs were identified and their expression pattern elucidated. Unfortunately, none of the three maize candidate genes identified has shown the potential to regulate ZmWUS1 activity, indicating that none of the closest CLV1 relatives in maize are able to regulate ZmWUS1 activity. WUSCHEL is the founding member of a large gene family, the WUSCHEL-related homeobox (WOX) genes, which appear to be involved in several aspects of plant development, from defining the organizers of the shoot and root apical meristems, to conferring distinct cell fates as early as the 2-cell stage during Arabidopsis embryogenesis. The WOX gene family is present throughout the plant kingdom, from the most basal algae and land plants to the most evolved angiosperms. As the members of this gene family take part in key plant developmental aspects, it is intriguing to study the evolution of the WOX gene family. In this respect, the lycophyte scenario is described in this work, in which both Selaginella kraussiana and S.moellendorffii has been the object of study. As for moss Physcomitrella patens, also the Selaginella WOX genes belong to the WOX13-like clade. S.moellendorffii genome has nine putative WOX homeodomains, six of them grouping together in a S.moellendorffii specific WOX13 sister group, whereas only three WOX-like gene were identified by degenerate primer PCR in S.kraussiana, all belonging to the WOX13-like clade. Despite the expression analysis of the three S.kraussiana WOX13-like genes and their S.moellendorffii closer orthologs demonstrate their subfunctionalization and their high conservation through the Selaginellaceae evolution, the phylogenetic reconstruction is in favor of the presence of only a single ancestor WOX13-like gene before the separation of the lycophyte and euphyllophyte lineage, which was probably present from the dawn of the plant kingdom
