Stranger than Fiction: Loss of MADS-Box Genes During Evolutionary Miniaturization of the Duckweed Body Plan

Duckweeds (Lemnoideae or Lemnaceae) are unusual flowering plants. They all lack a stem, many species even do not have roots, and most of them flower only rarely, if at all. Evolution of duckweeds obviously involved extreme miniaturization and simplification of their structure and life cycle. This raises the question as to whether the evolutionary changes of the duckweed body plan were accompanied, and thus potentially causally linked, to a loss of function of genes that control the development of affected structures. MADS-box genes are involved in the control of many developmental processes in flowering plants, including root, flower, and fruit development. Their phylogeny is quite well known and reveals a fairly strong correlation between some gene clades and function. Therefore, we used the available genome sequences of several duckweed species to test as to whether the loss of specific MADS-box genes can be linked to the reduction of morphological structures. In all duckweed genomes analysed, 5 of the 17 clades of MIKCC-group MADS-box genes that probably existed in the most recent common ancestor of extant flowering plants, appear to be absent. Our analyses thus demonstrate that duckweeds are the plant taxon with the lowest number of MIKC-type MADS-box gene clades of all flowering plants that were investigated so far. While AGL15-like genes have probably been lost in the stem group of extant monocots already, and FLC-like genes have been lost in the stem group of the order Alismatales (to which duckweeds belong), AGL9- (SEP3-), AGL12- and OsMADS32-like genes were lost very likely in the stem group of extant duckweeds. The potential functional relevance of our findings is discussed. For example, the loss of AGL12-like genes might be linked to the vestigial or absent root formation in duckweeds.