The nuclear-encoded granule-bound starch synthase gene (GBSSI or waxy) has recently been characterized for its potential phylogenetic utility in a number of plant groups. In most plant families studied to date GBSSI appears to be a single-copy gene (e.g., Poaceae, Solanaceae), although analysis of GBSSI in Rosaceae revealed two copies. To explore its phylogenetic utility in Malvaceae, GBSSI genes have been isolated and characterized from several Malvaceae species representing four of the five tribes (all except the monogeneric Decaschistieae). In addition, two groups have been more intensively sampled for GBSSI: the cotton genus (Gossypium), and the North American Rose Mallows (Hibiscus sect. Muenchhusia). This combination of data allows an evaluation of the phylogenetic utility of GBSSI at both lower (intrageneric) and higher (intrafamilial) taxonomic levels. Two GBSSI genes are present in most diploid Malvaceae species and have been designated GBSSI-1 and GBSSI-2, although copy number does vary among species. Phylogenetic utility of the GBSSI genes is similar to other low-copy nuclear genes studied to date in Gossypium, and greater phylogenetic resolution is obtained with GBSSI genes relative to chloroplast or nuclear ribosomal ITS sequences.

Key words: GBSSI, Gossypium, Hibiscus, Malvaceae, molecular phylogenetics