Date of Award
Doctor of Philosophy (PhD)
Lytton J. Musselman
Elizabeth A. Zimmer
Lisa E. Wallace
Isoëtes (Isoëtaceae, Isoetales, Lycopodiophyta) is a cosmopolitan genus of aquatic lycophytes, occurring on every continent except Antarctica. Of approximately 200 total taxa, about half are in a clade of species mostly occurring in North and South America. Eastern North America accounts for 22% of global taxonomic diversity, containing 32 fertile taxa and 16 named hybrids. This taxonomic diversity is built upon relatively little morphological difference, and even combined with phylogenetic analysis using several nuclear and chloroplast DNA markers, no well-resolved systematic treatment within this clade exists.
This study aims to clarify the relationships between all species and subspecies of Isoëtes in eastern North America using a phylogenomic approach. Subsets of taxa were analyzed separately depending on their presumed mode of evolution: fertile diploid taxa thought to have originated through allopatric speciation, and fertile allopolyploids derived from whole genome duplication following primary hybridization. Phylogenies inferred from whole chloroplast genome DNA sequences using maximum-likelihood and Bayesian inference were fully resolved with high support. Ancestral state reconstruction of megaspore and microspore ornamentation, megaspore color, and seasonality of spore maturation found that more than 80% of these character state transitions occurred on terminal tips of the tree, and that some shared morphological characters are the result of homoplasy. Only I. ‘graniticola-NC’, I. ‘laurentiana’, I. septentrionalis, and I.tuckermanii showed very strong relationships indicating a clear maternal ancestor, with other polyploids suggesting ancestral or unknown diploid progenitors often in conflict with nuclear phylogenetic data.
Parentage of polyploid taxa was inferred by comparing DNA sequences of a low-copy nuclear marker (LEAFY intron 2) to all diploid taxa present in the eastern US under phylogenetic and similarity criteria. Some hypotheses based on previous work, such as I. engelmannii and I. valida as parents of I. appalachiana and I. engelmannii and I. echinospora as parents of I. septentrionalis, were validated, but most polyploid taxa were found to be derived from different sets of parental species. Using a lineage-based species concept may require the recognition of ca. 50 new species of auto- and allopolyploid Isoëtes in eastern North America.
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Schafran, Peter W..
"Molecular Systematics of Isoëtes (Isoëtaceae) in Eastern North America"
(2019). Doctor of Philosophy (PhD), Dissertation, Biological Sciences, Old Dominion University, DOI: 10.25777/37tg-7846