Evolutionary affinities of the Southern Appalachian clonal hornwort, Nothoceros aenigmaticus and the repercussions of clonality on genetic diversity

Date of Completion

January 2011


Biology, Genetics|Biology, Systematic




The first chapter deals with the taxonomic circumscription of the genus Nothoceros and transferring the widely recognized species of American Megaceros to Nothoceros. In this chapter I used the genes rbcL and nad5 and sampling from most hornwort genera. The second and third chapters focus on the genus Nothoceros using Bayesian dating analyses and maximum likelihood approaches using 5 chloroplast markers (rbcL, matK, trnL-F intron, trnL-F spacer, rps4-trnS spacer), the mitochondrial nad4-nad5 spacer and the nuclear ITS2. I explored the evolution of sexual condition; biogeography and phylogenetic affinites of Nothoceros aenigmaticus. This species survives as gametophytes only and displays a unique pattern of sex allopatry in the Southern Appalachians: male plants are ca. 30 miles apart from the closest female populations. This unisexual clonal species originated from sexually reproducing dioicous high elevation tropical populations. We provide the first phylogenetic evidence of a reversal from monoeicy to dioecy (to asexuality in N. aenigmaticus), in marked contrast to the assumed plesiomorphic condition of dioecy in bryophytes. The most conservative mean estimates for asexuality in N. aenigmaticus are from the Pleistocene (less than 1.5 mya), older than most calculated estimates for clonal—only plants published to date. In the third chapter, I explored the biogeography of Nothoceros. We sampled multiple accessions per species to test the potential concordance between three main geological events: West-Gondwana break up in the Late Cretaceous, Atacama Desert formation in the Miocene and Páramo formation in the Pliocene with cladogenic events within the Nothoceros. Our dated phylogeny suggests that those geological events were not likely shaping the current distribution patterns of the genus Nothoceros. In contrast, short-range dispersal may account for the distribution of the genus. The last part of the dissertation deals with the population genetics of N. aenigmaticus. Here we present a unique case of sex allopatry in a non-flowering plant that seems to reproduce clonally and to be genetically isolated from their closest relative in Mexico. The genetic structure of the Southern Appalachian plant appears to be partly influenced by the geological consequences of the Last Glaciation and the impact of stream capture is discussed. ^