Two ancient classes of MIKC-type MADS-box genes are present in the moss Physcomitrella patens

Characterization of seven MADS-box genes, termed PPM1-PPM4 and PpMADS1-PIWADS3, 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 MIKCc-type genes-(c)stands for classic-(PPM1, PPM2, PpMADS/) 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, MIKCc-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, L4MB1 from the clubmoss Lycopodium annotintan was identified as the only other MIKC*-Lype gene published so far. Our findings strongly suggest that the most recent common ancestor of mosses and vascular plants contained at least one MIKCc-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 MIKCc-type and MIKCc-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.