Characterization and Phylogenetic Analysis of MADS-Box Gene Family in Magnoliids: Insights into the Evolution of Floral Morphogenesis in Angiosperms

Magnoliids represent one of the most basal lineages within angiosperms, and their ancestral floral morphology provides crucial insights into the evolution of flowers in angiosperms. MCM1-AGAMOUS-DEFICIENS-SRF (MADS)-box transcription factors play crucial roles in specifying floral organs. To understand their evolutionary history and functional divergence in magnoliids, we identified MADS-box genes, and conducted phylogenetic and expression analysis in 33 magnoliids and 8 other angiosperm plants. A total of 1310 MADS-box genes were identified and classified into Type I and Type II. The expansion of MADS-box genes in magnoliids mainly arose from whole-genome duplication events. In <i>Liriodendron chinensis</i> and <i>Chimonanthus praecox</i>, we identified floral homeotic MADS-box genes that are orthologous to the ABCDE model genes of floral organ identity determination. The broad expression pattern of A and B genes in floral organs and overlapping activity of ABCDE-model genes are consistent with the "shifting-fading borders" scheme proposed in basally diverging angiosperm lineages. Our results not only elucidate the driving forces underlying the diversification of MADS-box genes in magnoliids, but also shed light on the evolutionary models of floral development in angiosperms.