Several systems of classification have been proposed for the family Agavaceae. A distinctive bimodal karyotype and similarities of fruits and seeds strongly support close relationships among Yucca, Hesperaloe, Beschorneria, Furcraea, Agave, Manfreda, Polianthes, Prochnyanthes, and perhaps Hosta. However, Dasylirion, Beaucarnea, Nolina, Calibanus, Dracaena, and Sansevieria differ in so many cytological and morphological features that many have concluded they should be excluded from Agavaceae and separated into two families, Nolinaceae and Dracaenaceae. Chloroplast DNA restriction site data support these separations and indicate that Nolinaceae and Dracaenaceae are very close to Convallariaceae (Malanthemum, Convallaria, Aspidistra, Liriope, etc.). In this paper we report the results of an ITS rDNA sequencing study of 40 taxa in Agavaceae sensu lato and related groups in the order Asparagales. Sequence alignments were optimized using the Consistency Index, Retention Index, and Rescaled Consistency Index to find the alignment that exhibited the least amount of homoplasy. The results of our study are congruent with the conclusions drawn from cytological, immunological, cpDNA, and rbcL studies, which support a narrow interpretation of Agavaceae and a close relationship among Convallariaceae, Dracaenaceae, and Nolinaceae. In addition, the ITS sequence data provide evidence for some interesting relationships within these families.

Aim The aims of this study were (1) to investigate whether the two growth forms of Darwiniothamnus Harling (Asteraceae) originated from the colonization of a single ancestor, (2) to identify the closest relative(s) of Darwiniothamnus, and (3) to review molecular phylogenies from other plant groups to infer the origin of Galápagos endemics.Location Darwiniothamnus is endemic to the Galápagos Islands.Methods All putative relatives of Darwiniothamnus plus 38 additional species were included. Nucleotide sequences of the internal transcribed spacers of the nuclear ribosomal DNA were used for Bayesian and parsimony analyses.Results Darwiniothamnus is polyphyletic. Two species (D. lancifolius (Hook. f.) Harling and D. tenuifolius (Hook. f.) Harling) are woody shrubs that usually grow to 1-2 m in height; they belong to a clade composed of species otherwise restricted to the Caribbean. These two species are sister to Erigeron bellidiastroides Griseb., a herbaceous species endemic to Cuba. The third species (D. alternifolius Lawesson & Adsersen) is a perennial herbaceous plant, woody at the base and reaching only up to 50 cm in height. It is sister to two Chilean (CoquimboValparaiso region) species that also have a perennial herbaceous habit: E. fasciculatus Colla and E. luxurians (Skottsb.) Solbrig. They are placed in an assemblage restricted to South America. The review of previous molecular phylogenetic studies revealed that two of the endemic genera and endemic species of three non-endemic genera have their closest relatives in South America. Endemic species belonging to three non-endemic genera have sister species in North America or the West Indies. One endemic genus and endemic species in three non-endemic genera have sister taxa with a widespread continental distribution, or their molecular phylogenies yielded equivocal results.Main conclusions The flora of Galápagos has affinities with both North America (including the Antilles) and South America. Darwiniothamnus exhibits both patterns: two species of this genus are sister to a taxon endemic to Cuba, supporting a connection between the Cocos plate and the West Indies; the third species, D. alternifolius, provides a link with the Coquimbo-Valparaiso region, suggesting a biogeographical connection between the Nazca plate and southern South America.