Date of Award
Master of Science (MS)
Houstonia (Rubiaceae) is a strictly North American genus of 24 species distributed from Mexico, throughout the United States, up to Canada. Houstonia has proven to be a taxonomically difficult genus since the Linnaean description of Houstonia and the related genera: Hedyotis and Oldenlandia in 1753. For over 250 years botanists have lumped and separated Houstonia from Hedyotis and Oldenlandia based on various morphological characters. The most recent circumscription of Houstonia (Terrell 1996) separated the genus into two subgenera with each subgenus containing two sections. Nuclear (ITS) and plastid (trnL-F, rps16) DNA sequences were used to build a molecular phylogeny depicting relationships within Houstonia and among the closely related genera Stenaria and Stenotis, all of which used to be considered Hedyotis. Separate and combined datasets show Stenaria is nested within the Houstonia lineage and therefore Houstonia, as currently circumscribed, is not a monophyletic lineage. These results disagree with the use of crateriform seeds to distinguish Houstonia (crateriform seeds) from Stenaria (non-crateriform seeds). It appears the most useful characters to define this group are the loss of chromosomes through the major clades as the Houstonia-Stenaria lineage radiated north and east in North America. Descending aneuploidy has been accompanied by slight modifications of the pollen aperture types from a simple endoaperture in Stenotis referred to as colporate type A with modifications in Houstonia-Stenaria resulting in compound aperture types referred to as colporate type B and colpororate.
Shanks, Hunter L.. "Molecular Phylogeny of the Genus Houstonia and Allies in Rubiaceae" (2015). Master of Science (MS), thesis, Biological Sciences, Old Dominion University, https://digitalcommons.odu.edu/biology_etds/3