Bouteloua Curtipendula (Poaceae): Reproductive Biology, Phenotypic Plasticity, and the Origins of an Apomictic Species Complex

Abstract

The Bouteloua curtipendula (Michx.) Torr. complex (BCC) is composed of 11 species and five varieties distributed in North and South America with its center of diversity in Mexico. It is characterized by tremendous morphological variation and taxonomic complexity that is most likely due to reticulate evolution, phenotypic plasticity, and the development of asexual reproduction (apomixis). Climate-induced range changes over evolutionary time may explain the origin of the morphologically diverse BCC as suites of species came into contact over time, hybridized, and created new species, cytotypes, and reproductive modes. I investigated the origins of the BCC by creating habitat suitability models based on present-day occurrence records for eight BCC taxa and hindcast these models to paleoclimate reconstructions for the Last Glacial Maximum and Last Interglacial. By estimating range dynamics over time, coupled with phylogenetic data, I inferred the locations of contact zones among taxa and identified likely progenitor taxa for various cytotypes found in the BCC. Species with large and environmentally heterogeneous distributions may have large ranges due to plastic responses to environmental variation for adaptive traits or they may maintain differently adapted ecotypes to specific habitats. I evaluated phenotypic plasticity for stolon and rhizome production in the three taxonomic varieties of B. curtipendula. My results indicate that expression of these traits is correlated with specific, local environmental conditions and not to broadly defined environments in geographic space and that phenotypic plasticity accounts for a greater proportion of trait expression than does total genetic variance. Apomixis, asexual reproduction via seed, most likely results from interploidy hybridization. There are many possible pathways that lead to asexual seed formation and understanding these pathways is important to understanding genetic diversity, demography, and evolutionary potential in apomictic and mixed apomictic-sexual populations. I discovered that B. curtipendula var. caespitosa, the only recognized apomictic taxon in the BCC, is pseudogamous, indicating that although fertilization is unnecessary to produce the embryo, it is necessary to produce the endosperm. These data also indicate that meiosis is highly irregular and results in sperm nuclei with variable chromosome numbers, which affects endosperm production, fertility and demographics in apomictic B. curtipendula.