Evolutionary aspects of MADS-box genes in the eusporangiate fern Ophioglossum

The MIKC-type MADS-box genes of seed plants encode transcription factors which are typically expressed in a tissue- or organ-specific way. In contrast, the MIKC genes of the fern Ceratopteris isolated so far, show fairly ubiquitous expression in both reproductive and non- re productive organs. in order to determine whether this is a feature which is unique to a lineage of highly derived, leptosporangiate ferns, we initiated a characterization of the family of MIKC-type MADS-box genes of the eusporangiate fern Ophioglossum. Within the large clade of ferns and their allies, Ophioglossum is only very distantly related to Ceratopteris. cDNAs were isolated which represent at least four different Ophioglossum MIKC-type genes, termed OPM1, OPM3 - OPM5. Hybridization of genomic DNA gel blots, however, revealed that appreciably more MADS-box genes are present in the Ophioglossum genome. As for the MIKC-type genes from Ceratopteris, phylogeny reconstructions did not reveal orthology to seed plant genes for any of these. For OPM1 and OPM5, however, a relatively close relationship to some Ceratopteris genes was moderately supported. Semi-quantitative RT-PCR revealed that OPM1, OPM3 and OPM5 are expressed in both the vegetative (trophophore) and the reproductive (sporophore) part of the Ophioglossum leaf. In contrast, expression of OPM4 is restricted to the sporophore, suggesting a specific role in generative development for that gene. These findings suggest that relatively ubiquitous expression is a typical, but not absolute, feature of MIKC-type MADS-box genes from ferns and their allies. Taken together, the data outlined here corroborate the view that the phylogeny of MIKC-type MADS-box genes took different pathways in ferns and their allies on the one hand, and seed plants on the other hand, involving numerous independent gene duplications in both lineages, and distinct differences in the ubiquity of the expression patterns and, by inference, function. The implications of these findings for a better understanding of the evolution of vascular plants are discussed.