A Chloroplast DNA Study of the Agavaceae

Abstract

A chloroplast DNA restriction site analysis of 40 taxa in the Agavaceae s.l. was undertaken to test the accuracy of several current systems of classification. In Cronquist's system, the Agavaceae contain 18 genera of fibrous-leaved, more or less woody plants such as Yucca, Hes- peraloe, Agave, Manfreda, Polianthes, Dracaena, Sansevieria, Nolina, Beaucarnea, Dasylirion, and Cordyline. In Dahlgren's system, the Agavaceae are restricted to Yucca, Hesperaloe Agave, Manfreda, Polianthes, Beschorneria, and Furcraea, while the other genera are placed in different families. These contrasting phylogenetic schemes were investigated by a chloroplast DNA restriction site analysis of these genera plus Xanthorrhoea, Hosta, Hypoxis, Hemerocallis, Asparagus, Liriope, Aspidistra, Maianthemum, and Convallaria. Phylogenetic analysis of 110 restriction site mutations, 90 of which were phylo- genetically informative, strongly supports the conclusion that there are two major lineages in this group of taxa. One well supported clade contains, sensu Dahlgren, the Nolinaceae, Dracaenaceae, and Convallariaceae. It appears that Nolina and Dasylirion are more closely related to genera such as Liriope, Maianthemum, and Dracaena than to Yucca and Agave. The other major lineage contains Yucca, Hesperaloe, Agave, Manfreda, Polianthes, Beschorneria, and Furcraea, with Hosta and perhaps Xanthorrhoea at the base. Asparagus appears to be intermediate between the two lineages. The cpDNA suggests that Cordyline, Hypoxis, and Hemerocallis are not closely related to the Agavaceae. A major problem in monocot taxonomy con- cerns the classification and phylogeny of the genera in the Liliaceae s.l. and smaller families that have been split off from it, such as the Agavaceae. Two distinct systems of classifica- tion have been proposed and both are in use at the present time (Cronquist 1981; Dahlgren et al. 1985). The apparent frequency of convergent and parallel evolution of characters in these taxa makes it difficult to recover the phylogeny us- ing morphological characters alone. In this pa- per these problems are examined using evi- dence from chloroplast DNA restriction site analysis. In the traditional taxonomic systems of Ben- tham and Hooker (1883) and Engler and Prantl (1888), most liliaceous plants with a superior ovary, including Yucca L. and Dasylirion Zucc., were placed in a broadly conceived Liliaceae with many tribes, while those with an inferior ovary, such as Agave L., were placed in the Amaryllidaceae. Hutchinson (1934) is generally