Figure 1.
Model for evolution of MADS box genes in non-seed plants. MIKC-type MADS proteins seem to have evolved in streptophytes (700 MYA) by addition of K domain from a MEF2-like ancestor in chlorophytes. A gene duplication event in the common ancestor of bryophytes and tracheophytes (450 MYA) led to MIKCC and MIKC* proteins where MIKC*-type proteins have an elongated K domain. The origin of Type I is still not clear which lacks the I region and K domain but in the MRCA of bryophytes, lycophytes and tracheophytes there are two subtypes M alpha and M beta-gamma. The information on Type I genes in the MRCA of monilophytes and spermatophytes is not available. The MRCA of monilophytes/ferns and seed plants (380 MYA) had at least 2 MIKCC type and 2 MIKC* type (S and P clades) genes. Further diversification and expansion took place independently in MIKCC type genes in the lineages leading to extant ferns and spermatophytes. This gave rise to the large number of MIKCC genes in present day ferns and seed plants. In leptosporangiate ferns, there are genes belonging to three clades CRM1, CRM3, and CRM6 whereas in Ophioglossum (a eusporangiate fern) in addition to the three CRM6-like genes there are two unique genes OPM3 and OPM4 which may be specific to eusporangiate ferns. It should also be noted that in each taxa from bryophytes onwards there has been lineage specific diversification and expansion of MIKC genes. It is a possibility that with increase in number of MIKC genes there has been increase in complexity (from single celled algae to multicellular algae to multicellular non-vascular plants to early vascular plants and finally seed plants) which needs further evidence. MYA, million years ago.