The enigmatic monocotyledon family Triuridaceae is composed of inconspicuous mycoheterotrophs, that have been variously interpreted. We present here the first report of a thorough floral developmental series for any member of the Triuridaceae. The two known Mexican Triuridaceae species were studied with anatomical sections and scanning electron microscopy. While both species have ephemeral and reduced radially symmetric flowers arising in a counterclockwise spiral on racemose inflorescences, Lacandonia schismatica is hermaphroditic with a central androecium and Triuris brevistylis is dioecious. Tepals are connately fused at their bases, and during development the subapical caudal tips continue to elongate, while at maturity the tepals are reflexed. Carpel primordia develop centrifugally from compound primordia in both species, with contrasting androecium development. In Lacandonia schismatica stamen and carpel primordia arise from a common precursor. The two species differ in tepal and carpel number and timing of organ development. This paper provides a developmental framework to understand floral characters in the Triuridaceae. Notably, we addressed if L. schismatica and T. brevistylis bear true flowers or pseudanthia, and our data support the former. The role of particular genes in determining the floral developmental patterns studied and the evolutionary significance of these patterns are discussed.

No abstract

Spontaneous homeotic transformations have been described in natural populations of both plants and animals, but little is known about the molecular-genetic mechanisms underlying these processes in plants. In the ABC model of floral organ identity in Arabidopsis thaliana, the B-and C-functions are necessary for stamen morphogenesis, and C alone is required for carpel identity. We provide ABC model-based molecular-genetic evidence that explains the unique inside-out homeotic floral organ arrangement of the monocotyledonous mycoheterotroph species Lacandonia schismatica (Triuridaceae) from Mexico. Whereas a quarter million flowering plant species bear central carpels surrounded by stamens, L. schismatica stamens occur in the center of the flower and are surrounded by carpels. The simplest explanation for this is that the B-function is displaced toward the flower center. Our analyses of the spatio-temporal pattern of B-and C-function gene expression are consistent with this hypothesis. The hypothesis is further supported by conservation between the B-function genes of L. schismatica and Arabidopsis, as the former are able to rescue stamens in Arabidopsis transgenic complementation lines, and Ls-AP3 and Ls-PI are able to interact with each other and with the corresponding Arabidopsis B-function proteins in yeast. Thus, relatively simple molecular modifications may underlie important morphological shifts in natural populations of extant plant taxa. INTRODUCTION An ABC Model-Based Hypothesis of the Developmental Genetic Factors Underlying the Unusual Reproductive Morphology of Lacandonia schismaticaThe ABC model for the specification of floral organ identity Coen and Meyerowitz, 1991;Meyerowitz et al., 1991) has played a critical role in the modern explanation of the molecular-genetic determinants of the ontogenetic development of reproductive structures in angiosperms. This combinatorial genetic model has guided diverse plant evolutionary developmental biology studies, especially during the formative years of this young field (Cronk, 2001;Cronk et al., 2002;Pruitt et al., 2003). As part of the larger discipline of evo-devo, the articulation of explanatory tools like the ABC model is considered essential for understanding the developmental mechanisms that underlie morphological innovation throughout evolutionary time (Cronk et al., 2002;Carroll et al., 2004;Gilbert, 2006;Wolpert et al., 2006).The ABC model of flower development was based on the interpretation of floral homeotic phenotypes in Arabidopsis thaliana and Antirrhinum majus (Coen and Meyerowitz, 1991). However, the participation of homeotic transformations and other forms of heterotopy in the appearance of new organ arrangements during the evolution of angiosperms had already been considered in the botanical literature, long before the age of plant developmental genetics (e.g., Meyer, 1966;Sattler, 1984;Bowman et al., 1989;Weston, 2000). Outstanding instances of spontaneous floral homeotic phenotypes have continued to be recorded in well-characterized taxa (s...