Early plant history: something borrowed, something new?

In response to the article “Deep Green rewrites evolutionary history of plants” by Kathryn S. Brown (News of the Week, 13 Aug., p. [990][1]), the statement that “plants should be divided into three kingdoms rather than one” is neither new nor particularly insightful. Botanists have long recognized that the organisms called “plants” represent a grade-level of cytological and physiological organization (“plants” = photosynthetic eukaryotes); that the various lineages that make up the “algae” are polyphyletic, each tracing its evolutionary debut back in deep time, presumably as a consequence of either a primary endosymbiotic event (leading to the red and green algae, as attested to by, among other things, their double-membrane-bound chloroplasts) or secondary endosymbiotic events (which gave rise to other lineages, such as the cryptomonads, whose chloroplasts have a nucleomorph, a nucleus-like organelle, sandwiched between their “supernumerary” second and third chloroplast membranes); and that the five- (or seven)-Kingdom system still illustrated in most introductory textbooks conflates grade (for example, “protista”) with clade (for example, embryophytes). Likewise, the report that the last common ancestor of the land plants was related to modern-day Chara or Coleochaete is hardly new information, although its reconfirmation based on new molecular data is newsworthy indeed. ![Figure][2] New view of plant kingdoms. Amborella, possibly the world's most primitive flowering plant(bottom).CREDIT: (PHOTO) SANDRA FLOYD, UNIVERSITY OF COLORADO; (DIAGRAM) B. MISHLER ![Figure][2] The new cladistic analyses of plant evolutionary relationships deserve to be reported, but it is vital that all realize that every cladogram is a hypothesis and, perhaps more important, that such hypotheses depend on both the algorithm used to generate the hypotheses (for example, the algorithim generating the reported trees has now been superseded by a much faster algorithim capable of generating shorter and more accurate hypotheses of relationship) and on the character matrix used as the basis of the analysis (such as, cladistic hypotheses will change as new data or taxa are added to the matrix). The character matrices are themselves evolving rapidly and are affected by additions of new characters, such as new gene sequences, and by the selection and definition of morphological-structural characters. Thus, the phylogeny presented as the basis for a radical shift in our understanding of green plant relationships is a transitory hypothesis that likely will be replaced by other different hypotheses. It would be far better to present that understanding as being in a dynamic state evolving in response to newly available techniques and data. Finally, while on the subject of classification, contrary to the This Week in Science item in the same issue (p. 981), the cyanobacteria are not eukaryotes, but in fact are photosynthetic and oxygen-evolving prokaryotes similar in many respects to the ancient bacteria some believe evolved by means of endosymbiosis into red and green algal chloroplasts. # Response {#article-title-2} Niklas et al. contend that some of the Deep Green findings described in the News article are “neither new nor particularly insightful.” That point is debatable. The findings presented at the meeting offer a more comprehensive picture of green plant phylogeny, and confirm that the common classroom view of plant classification—such that plants belong to one kingdom—is inaccurate. As the article clearly noted, the emerging phylogenetic framework is preliminary and will surely change with time. But researchers interviewed for the article suggest that current systematic methods are sufficient to ensure that cladistic analyses of plant evolutionary relationships are as reliable and valid as other types of scientific analysis, and that many branches of the tree are well supported and unlikely to change radically. [1]: /lookup/doi/10.1126/science.285.5430.990 [2]: pending:yes