Diversification of Galium within Tribe Rubieae (Rubiaceae): Evolution of Breeding Systems, Species Complexes, and Gene Duplication

Abstract

Tribe Rubieae is unique within Rubiaceae with its herbaceous habit, temperate distribution, and whorls of leaf-like structures. This dissertation examines the evolution of the tribe, evolution a clade within the tribe (Cruciata-Galium-Valantia [CGV] clade), and finally a section of the genus Galium (sect. Baccogalium). A molecular phylogeny of the tribe, based on three chloroplast (cp) regions, strongly supports seven major clades within the tribe. The resulting phylogeny is used to examine geographic distribution patterns and evolution of leaf-like whorls in the tribe. An Old World origin of the tribe is inferred, followed by at least eight dispersal events into North America. The ancestral whorl morphology of the tribe is inferred as composed of six organs, from which whorls of four organs are derived. Polygamy, dioecy, and hermaphroditism all occur within the CGV clade, in which dioecy is hypothesized to have evolved from hermaphroditism via polygamy. A molecular phylogeny of the CGV clade, based on cp and nuclear ribosomal data, strongly supports nine lineages of New World Galium taxa. The resulting phylogeny is used to examine evolution of breeding systems, fruit types, and fruit hairs. Dioecy is inferred to have arisen at least three times from hermaphroditism; polygamy is inferred to have arisen at least twice from dioecy and at least six times from hermaphroditism. Polygamy appears to be a terminal condition in the CGV clade and not a pathway to dioecy. Fruit characters traditionally used in the taxonomy of this group have arisen multiple times within this clade and are not reliable indicators of shared evolutionary history. Approximately 30 Galium taxa are designated rare by the California Native Plant Society, ten of which occur within G. sect. Baccogalium. Within G. sect. Baccogalium, relationships among taxa are not well resolved with either cp or nuclear data. A molecular phylogeny of the section, based on cp data, indicates that subspecies from three species complexes do not form respective monophyletic groups, which will have implications for management of rare infraspecific taxa. A molecular phylogeny based on nuclear RPB2 indicates that Galium taxa examined lack the I copy and contain a duplicated D copy.