Sequences for the Rubisco large subunit (rbcL) gene were used to test hypotheses about the evolution of chloroplast shape and thallus type in genera of two families of conjugating green algae (Zygnematales): the Mesotaeniaceae (saccoderm desmids, mostly unicellular) and the Zygnemataceae (strictly filamentous). Unicellular (u) and filamentous (f) genera exhibit a series of three similar chloroplast shapes: ribbonlike (e.g. Spirotaenia [u], Spirogyra [f], and Sirogonium [f], laminate (e.g. Mesotaenium [u] and Mougeotia [f]), and twin-stellate (e.g. Cylindrocystis [u] and Zygnema [f]. Two conflicting phylogenetic hypotheses have been proposed: 1) families are polyphyletic constructs drawn from three lineages, each with unicellular and filamentous taxa characterized by a specific chloroplast shape; or 2) unicells form one monophyletic lineage (Mesotaeniaceae) and filaments form another (Zygnemataceae), with some chloroplast shapes independently derived. The rbcL data strongly refute hypothesis 2 (monophyly of the two traditional families) and support hypothesis 1 in part. Parsimony, maximum likelihood, and neighbor-joining analyses of the rbcL data strongly support monophyly of a clade containing taxa with ribbonlike chloroplasts and, to a lesser extent, monophyly of a second clade of the four genera with the other two chloroplast shapes. Two saccoderm genera (Roya, curved laminate chloroplasts; Netrium, ''cucumber''-shaped chloroplasts) are not members of either of these clades, but they are included in a monophyletic Zygnematales.
