The Genus Isoëtes L., evolution, diversification and population structure in a free-sporing heterosporous lycophyte.

Among land plants, one of the most cryptic lineages is the genus Isoëtes L., both in terms of public awareness and appearance. Taxonomically, \textit{Isoëtes} are members of the vascular plant Division Lycopodiophyta, the sister lineage to the more well-known and lineage rich Euphyllophyta, which includes both the Monilophytes (ferns) and Spermatophytes (seed plants). Lycophytes are comprised of three extant lineages, the homosporous order Lycopodiales, which have the greatest extant generic diversity within the lineage, and the heterosporous Selaginellales and Isoetales, both of which are contemporarily monogeneric. Lycophytes are united by a few key features, including the microphyll, a leaf type unique to the lineage, and adaxial placement of the sporangia on the sporophylls. Like the Monilophytes, Lycophytes are free-sporing plants. As such, they have a pronounced alternation of generations, with a large, dominant diploid sporophyte producing haploid spores via meiosis, which go on to germinate into haploid gametophytes. These gametophytes produce either or both haploid eggs and sperm, which unite within the gametophyte’s archegonia to produce a new diploid sporophyte embryo./nThe Isoëtalean lycopods, both extinct and extant, are particularly unique among their relatives due to both a suite of unusual characters. One of the first and most obvious of them is their growth form. Unlike their fellow lycophytes, which have unipolar growth, meaning they grow and elongate via a terminal apical meristem on the shoot system, and produce adventitious roots from this stem axis, Isoëtaleans have bipolar growth like most Spermatophytes. And, like the non-monocot Spermatophytes, the Isoëtaleans undergo secondary growth, adding both cortical tissue and secondary vascular tissue via a meristematic region known as the prismatic layer. In the extinct Isoëtalians, these two traits allowed them to become arborescent, with some of the largest members, such as Lepidodendron Sternberg growing to over ten meters tall./nToday, Isoëtes is the only remaining member of Isoëtales Prantl. The genus appears to have arisen in the Triassic and can be found throughout the non-polar regions of the world in seasonally to permanently hydric to aquatic habitats. Morphologically, Isoëtes have a highly conserved base bodyplan; almost all of them are small, perennial, semi-herbaceous geophytes whose body is comprised of a highly reduced 1-3 lobed, corm-like trunk, an apical rosette of long, simple, linear leaves, and numerous roots that emerge from their basal furrow. Species are identified through a combination of habitat, spore color, ornamentation and size, leaf morphology, corm lobe numbers, and size of the plants themselves. Because their morphology is so conserved, species identification can be difficult in areas where multiple taxa overlap, especially when the taxa in question are close relatives, as the plants are known to readily hybridize with one another, or form allopolyploids./nModern Isoëtes can be broken up into five distinct sub-clades, which predominantly correspond to their geographic range. There is the Gondwanan clade, which is found in Southern Africa, South America, India and Australia. The Laurasian clade, which occur in the Mediterranean region of Europe and North Africa, North America, and India. The Italian clade, which occurs in and around the Italian peninsula. The Austro-Asian clade, found in Eastern and Southern Asia, India and Australia. And the American clade, which is found in North and South America, as well as a few circumboreal species, and the only known species in Oceania. Because Isoëtes is so morphologically conserved, prior to molecular phylogenetics it was assumed that spore morphology or habitat types defined the taxonomic groups, which we now know not to be the case. In fact, these traits are quite labile, particularly when polyploids are involved./nIn this dissertation, I explore the evolution of one of the traits in extant Isoëtes through ancestral state reconstruction, as well as conduct multi-locus population genetic and phylogenetic analyses to determine if the species composition in one of the sub-clades found on the West Coast of North America.

Advisor: Rothfels, Carl J.