Systematics of the xeric-adapted genus Argyrochosma

 

Three varieties of Argyrochosma nivea. A. A. nivea var. nivea with white farina. B. A. nivea var. flava with yellow farina. C. A. nivea var. tenera without farina. (Image from Sigel et al. 2011, Systematic Botany) 

Three varieties of Argyrochosma nivea. A. A. nivea var. nivea with white farina. B. A. nivea var. flava with yellow farina. C. A. nivea var. tenera without farina. (Image from Sigel et al. 2011, Systematic Botany

The cheilanthoid ferns are xeric-adapted members of the family Pteridaceae and provide a striking example of how convergent evolution of morphological traits has obscured the delineation of monophyletic lineages. In collaboration with Dr. Michael Windham (Duke Herbarium) and working entirely from herbarium vouchers, I completed a systematic survey of the New World cheilanthoid genus Argyrochosma and reconstructed the character evolution of farina, a powdery substance hypothesized to reduce water loss and reflect light. By combining phylogenetic analyses of plastid sequence data with morphological data, we were able to determine that the farinose species of Argyrochosma form a monophyletic group and that major chemical variants of farina characterize specific clades. In addition, by incorporating spore size measurements, data on the number of spores per sporangium, and chromosome counts, I was able to evaluate the distribution of polyploid and apomictic species across the genus. This work identified several putative, cryptic polyploid species in need of further study (Sigel et al. 2011, Systematic Botany). At present, I am collaborating with Dr. Eric Schuettpelz and Spencer Goyette (both of the Smithsonian Institution) to expand sampling of the polyploid species, use low-copy nuclear sequence markers to resolve their parentage, and provide taxonomic treatments of previously unrecognized taxa.

The number of spores per sporangium and spore size vary with mode of reproduction and ploidy, respectively. Sexual diploid plants produce 64 small spores per sporangium (left image). Apomictic plants produce 32 relatively larger (unreduced) spores per sporangium (right image).  

The number of spores per sporangium and spore size vary with mode of reproduction and ploidy, respectively. Sexual diploid plants produce 64 small spores per sporangium (left image). Apomictic plants produce 32 relatively larger (unreduced) spores per sporangium (right image).  

Average spore lengths for specimens of Argyrochosma; error bars indicate one standard deviation. Symbol shading depicts the number of spores per sporangium (see inset legend). Lines beginning and ending in arrows indicate specimens observed to have two sporangia types, each with a different number of spores per sporangium.  The letter below each data point corresponds to a major monophyletic clade within Argyrochosma. Sporophyte ploidy is indicated for documented chromosome count vouchers. Image from Sigel et al. 2011, Systematic Botany. 

Average spore lengths for specimens of Argyrochosma; error bars indicate one standard deviation. Symbol shading depicts the number of spores per sporangium (see inset legend). Lines beginning and ending in arrows indicate specimens observed to have two sporangia types, each with a different number of spores per sporangium.  The letter below each data point corresponds to a major monophyletic clade within Argyrochosma. Sporophyte ploidy is indicated for documented chromosome count vouchers. Image from Sigel et al. 2011, Systematic Botany.