Lacandonia

Description

Lacandonia is a small parasitic plant that lacks chlorophyll and has a rhizomatous, mycotrophic habit. This genus exhibits racemous inflorescences and bract-like leaves. The flowers are actinomorphic and are considered "inverted" from the typical flower arrangement–usually 3 (but sometimes two to four) stamens are in the center of the flower surrounded by 60 to 80 pistils. This characteristic where the position of the androecium and the gynoecium are inverted is unique in the known and described taxa of flowering plants.

Flowers of Lacandonia are bisexual and self-pollinate and fertilize before the flower opens (preanthesis cleistogamy). They are true flowers as opposed to pseudanthia as had been suggested earlier in the literature. The three-celled pollen grains germinate within the anthers and the pollen tube grows through the receptacle to reach the ovaries. L. schismatica can be found flowering year-round when its environment is moist enough, with a particularly active flowering period in November and December. Owing to the prenathesis cleistogamy, a form of autogamy (self-pollination), the known population of L. schismatica lacks genetic variability and has a high instance of homozygosity. The haploid chromosome number of this species is n = 9.

Taxonomy and botanical history

Lacandonia is a genus of mycoheterotrophic plants in the Triuridaceae, with very unusual inverted floral morphology. It has two known species:

1. Lacandonia schismatica E.Martínez & Ramos 1989 - Chiapas in Mexico
2. Lacandonia brasiliana A.Melo & M.Alves 2012 - Paraíba in Brazil

Lacandonia schismatica was first described by Martínez and Clara Hilda Ramos in 1989, who placed the species in its own family, Lacandoniaceae, which itself was placed in the Triuridales. In 1991, Traudel Rübsamen-Weustenfeld suggested that L. schismatica be included in the family Triuridaceae within the genus Sciaphila, Peltophyllum, or its own genus. Another study in 1998 presented data that supports the separation of L. schismatica into its own family. The APG II system transferred the genus to the Triuridaceae and placed that family in the Pandanales. The group was long considered monotypic until the discovery of a second species, Lacandonia brasiliana, in Guaribas Biological Reserve in Brazil in 2012.

Evolution

The difficulty expressed in placing the species in the proper family is due to the unique floral morphology. How this inverted position of the androecium and gynoecium evolved is unknown, but some studies have posed hypotheses. Davidse and Martínez suggested that L. schismatica could be one of Richard Goldschmidt's "hopeful monsters", meaning that the inverted floral morphology could have arisen from a macromutation in the genes that control floral development. It is also possible that chromosomal repatterning, also known as chromosomal rearrangement, was the origin of this species.

Since the original description and early work on this species in the 1990s, other field work has revealed some instances of L. schismatica flowers that were unisexual. The closely related species Triuris brevistylis was discovered to be mostly dioecious but a few individuals were located that had bisexual flowers with the flower arrangement inverted, just like that of the normal L. schismatica flowers. This discovery led the authors of the study to conclude that the inverted floral morphology evolved before L. schismatica and T. brevistylis diverged. Isolated populations during the Quaternary Period (around five million years ago) when temperatures in the Lacandon lowland rainforest were six to eight °C (10.8 to 14.4 °F) cooler than today. This hypothesis is supported by the geographic distribution where L. schismatica is restricted to the warmer lowlands and T. brevistylis has a distribution in the cooler highlands.