Inside-out flowers characteristic of Lacandonia schismatica evolved at least before its divergence from a closely related taxon, Triuris brevistylis

Lacandonia schismatica, a mycoheterotrophic, hermaphroditic monocotyledon endemic to the Lacandon rain forest of southeast Mexico, is the only flowering plant for which a spatial inversion (heterotopy, complete homeosis) of the reproductive floral whorls (stamens and carpels) is known to occur in natural populations. In order to investigate if this autapomorphic inside-out arrangement of the reproductive organs is fixed in natural populations, we have undertaken extensive and intensive fieldwork spanning several years to locate new populations in addition to the type locality. In parallel, we have also searched for natural variation in floral organ arrangement in Triuris brevistylis, a closely related dioecious triurid that is found in nearby areas of the Lacandon forest. We have found that a small proportion of L. schismatica inflorescences bear unisexual flowers of both sexes, as well as bisexual flowers with differences in the number of reproductive organs. However, in all bisexual flowers, the stamens were always central and the carpels peripheral to them. More important, we have also found that a few T. brevistylis individuals have bisexual flowers with altered positions of stamens and carpels. Among these, flowers with an inside-out L. schismatica-like floral organ arrangement were observed. We document our findings with scanning electron micrographs, histological sections, and dissection microscope views. The information presented implies that the developmental-genetic mechanism putatively responsible for homeotic/heterotopic transformations involving floral reproductive organs in the two triurid species originated at least before these taxa diverged from each other. The Mexican triurids may be an example in which the molecular evolutionary events causally related to a major morphological change in plants can best be understood at the microevolutionary scale.