The Roots of Land Plants: Recent Research on Early Diverging Lineages in the Evolution of Higher Drier Algae

Abstract

AbstractRecent multigene phylogenetic analyses of charophycean algae sensu Mattox and Stewart have advanced our understanding of the relationships of the green algae that are most closely related to land plants. Several clades of green algae are monophyletic with land plants (i.e. liverworts, mosses, nonvascular and vascular plants that arose from an aquatic ancestor) to the exclusion of other green algae: Mesostigma (a flagellate unicell); the Chlorokybales (small packets of cells); Klebsormidiales (unbranched filaments); Zygnematales (conjugating unicells, filaments, and colonies); Coleochaetales (filamentous and parenchymatous thalli); and Charales (branched filamentous thalli with complex nodal structure). The morphological and molecular diversity of the extant members of this nearly exclusively freshwater clade is remarkable. Moreover, the taxon diversity within clades is highly asymmetric, having yielded several distinctive albeit small groups of green algae, as well as one of the major evolutionary success stories on earth, viz. land plants. The most recent analysis (Karol et al. (2001) Science 294 : 2351–2353) of four genes from plastid, mitochondrial, and nuclear compartments strongly support the sister relationships of Charales (stoneworts) to the most primitive land plants. The analysis also shows that within this clade there has been a trend toward increasing complexity of thallus form (from unicells, to filaments, to branching and parenchymatous thalli) and reproduction (absence of sexual reproduction to iso‐ or anisogamy, to oogamy). Although the major clades mentioned above are well supported by molecular analyses, relationships among these groups are less well‐resolved, especially for the Klebsormidiales, Chlorokybales, and Mesostigma. This paper reviews the morphological and molecular evidence for the relationships among and within these groups. More sequence data from a broader sampling of taxa at the base of the charophyceans (including perhaps some algae not now known to be charophytes) should provide a clearer picture of the evolution of this group and the reasons for its success.