When ABC becomes ACB.

Understanding how the information contained in genes is mapped onto the phenotypes, and deriving formal frameworks to search for generic aspects of developmental constraints and evolution remains one of the main challenges of contemporary biological research. The Mexican endemic triurid Lacandonia schismatica (Lacandoniaceae), a mycoheterotrophic monocotyledonous plant with hermaphroditic reproductive axes is alone among 250,000 species of angiosperms, as it has central stamens surrounded by a peripheral gynoecium, representing a natural instance of a homeotic mutant. Based on the classical ABC model of flower development, it has recently been shown that the B-function gene APETALA3 (AP3), essential for stamen identity, was displaced toward the flower centre in L. schismatica (ABC to ACB) from the early stages of flower development. A functional conservation of B-function genes from L. schismatica through the rescue of B-gene mutants in Arabidopsis thaliana, as well as conserved protein interactions, has also been demonstrated. Thus, it has been shown that relatively simple genetic alterations may underlie large morphological shifts fixed in extant natural populations. Nevertheless, critical questions remain in order to have a full and sufficient explanation of the molecular genetic mechanisms underlying L. schismatica's unique floral arrangement. Evolutionary approaches to developmental mechanisms and systems biology, including high-throughput functional genomic studies and models of complex developmental gene regulatory networks, constitute two main approaches to meet such a challenge. In this review, the aim is to address some of the pending questions with the ultimate goal of investigating further the mechanisms of L. schismatica's unique homeotic flower arrangement and its evolution.