Epiphytism in the Cactaceae:
I am particularly interested in the evolution of epiphytism in the Cactaceae, an American plant family primarily distributed in the arid, semi-arid, subtropical and tropical regions of the New World. Epiphytism occurs in 10% of vascular plants, is in fact, one of the least studies lifestyles in plants and little is known about its evolution, morpho-physiological specialization, phorophyte specificity, and the ecological and evolutionary significance of plant-plant interaction. Few vascular plant groups/families, e.g, Pteridophytes, Bromeliaceae, Ericaceae, Gesneriaceae, Melastomataceae and Orchidaceae, possess the complex suite of morphological and physiological adaptations needed for the epiphytic habitat to be successful. Thus, understanding the epiphytic lifestyle in the Cactaceae will provide insight into the evolution of morphological novelties in this characteristic growth habit of vascular plants.
At present the phylogeny and evolution of epiphytic cacti remains poorly understood. I am interested in testing two hypotheses:
1) that epiphytism has independently evolved several times in the family in the tribes
Hylocereeae and Rhipsalideae, and
2) that epiphytic cacti originated from terrestrial ancestors in semi-desert habitats or
perhaps from columnar cacti.
Morphological data suggest that the epiphytic habit has evolved independently at least three times in the family, in particular in the Cactoideae with both holoepiphytes (strict epiphytes) and hemiepiphytes (facultative and accidental) having been derived separately. Testing these hypotheses and hence the monophyly of epiphytic cacti is central to my research program.
Another long-term goal involves the biogeography and biodiversity of epiphytic cacti. This area will investigate the highly contrasting disjunct distribution of epiphytic cacti in North versus South America to propose hypotheses about their putative origin, migratory routes and patterns of radiation in order to better understand their distribution and centers of diversity and endemism as an approach to propose areas for preserving epiphytic diversity.
At present, my research team and I are generating a molecular phylogeny of the major terrestrial, facultative epiphytic and strict epiphytic lineages of the Cactaceae to better understand the multiple origin of the epiphytic lifestyle in the family. Epiphytic cacti occur mainly in the subfamily Cactoideae, in particular the tribes Hylocereeae and Rhipsalideae; however, facultative species also occur in tribes of the Pachycereeae and Trichocereeae (sensu Anderson, 2001). Thus far, our data set includes molecular data from the chloroplast (rbcL and trnL-F) and nuclear (ITS) genomes. In addition, we are investigating the morphological changes that have played major roles in the evolution of epiphytism, in particular during the transition from ground to canopy. Our long-term goal is to reconstruct a familial molecular phylogeny, including morphological characters that have been important in habitat change. It is clear that more laboratory and field research is required to better understand the evolutionary and morphological changes involved in the transition from ground to canopy.
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Vivipary in the Cactaceae:
Vivipary, the germination of seeds within the fruit prior to abscission from the maternal plant, is a rare event in angiosperms involving complex ecophysiological processes (Cota-Sánchez, 2004). Two main types of vivipary are known in vascular plants: true vivipary and pseudovivipary. In crop grasses, pseudovivipary is an undesirable character as it results in lower yields. For instance, in some annual crops, such as rice, the lack of seed dormancy is undesirable since the premature sprouting of grains creates a major challenge in maintaining food supplies, resulting in lower yields. In addition, some aspects of vivipary and the ensuing desiccation intolerance are relevant to plant conservation. Since recalcitrant seeds generally lose viability upon drying, desiccation intolerance and vivipary can be detrimental because it affects species diversity in seed bank repositories.
The fundamental goal in the investigation of vivipary is to determine the morphophysiological basis, ecological requirements and evolutionary significance of this character. In fact, the investigation of the ecophysiological basis governing this process and the central role of phytohormone signaling in developing seeds and its putative compartmentalized synthesis in embryonic and maternal tissues should be instrumental in understanding vivipary in plants. In addition, we will investigate whether vivipary is associated with thermoregulation, parental care and conspecific nursing as recently suggested based on our studies in viviparous individuals of Epiphyllum phyllanthus and Ferocactus herrerae.
To date, reports of vivipary in the Cactaceae exist in less than 20 species, most of which are cultivated. More recently, several cases of pseudo- and cryptovivipary (a subcategory of true vivipary) have been reported in nature in members of the tribes Cacteae and Pachycereeae (Cota-Sánchez et al., 2007) and Hylocereeae (Cota-Sánchez & Abreu, 2007) of the subfamily Cactoideae. Vivipary has also been reported in the tribes Rhipsalideae and T richocereeae. Pseudovivipary and cryptovivipary occur in approximately 30 (2%) species of the cactus family. In the Cactaceae viviparous individuals are occur in coastal flooded and non-flooded halophytic environments and there is a potential trend indicating that the overall proportion of viviparous plants is higher in coastal flooding areas relative to halophytic, non-flooding environments. While germination is not a limiting factor in the perpetuation of this viviparous species, seedling establishment is. In our view, vivipary is a reproductive strategy that has evolved to provide a more efficient mechanism favoring germination and new avenues for survival by contributing to population maintenance and short-distance dispersal in halophytic substrates. Further, viviparity is an intrinsic, genetic event involving high metabolic costs favoring germination and dispersal of the fittest offspring regardless of substrate and environmental conditions.
The incidence of viviparity in phylogenetically distinct lineages of the cactus family is evidence of a parallel directional change towards a more practical reproductive mechanism. In other words, it is a reproductive advantage that in addition to allowing propagules to root and grow almost immediately, it favors quickly establishment whenever seedlings land on suitable substrates. Vivipary is an unconventional reproductive means deserving more attention in the laboratory and the field. These and other recent findings indicate that ample opportunity to investigate this trait from the ecophysiological and eSvolutionary perspectives exists. Future studies involving native cacti distributed in coastal regions, especially areas subject to estuarine influence would likely be instructive in improving our understanding of the relationship of cactus vivipary with saline/marine environments.
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