Response to Moreira et al.

related explanation for the origin of the nucleus. A third argument deals with structural characteristics of the nuclear membrane. As Staub et al. posit ‘‘no free-living prokaryote is separated from the environment in the same manner inwhich thenucleus is separated from thecytoplasm’’. This is true, but the question is also whether we should expect, after many hundred millions years of evolution, to find a nuclear membrane remaining identical to that of a free-living prokaryote.We think this is highly improbable.On the contrary, during the huge time span of eukaryotic evolution important structural changes and innovations have occurred (a clear example are the nuclear pores). Innovations exist for the mitochondria and chloroplasts (for example, specific membrane transport systems) but they do not refute their endosymbiotic origin. No free-living algae are separated from the environment as cryptophyte and chlorarachniophyte fourmembrane chloroplasts do, but their endosymbiotic origin is beyond any doubt. Endosymbiotic hypotheses need to explain the structural peculiarities of the nuclear membrane, but the origin of the nuclear membrane and its selective advantages are also puzzling for non-endosymbiotic supporters. Finally, Staub et al. argue that nucleus replication involves membrane disintegration, something that obviously does not occur in prokaryotes. However, whilst several eukaryotic groups break down their nuclear membrane during mitosis (the so-called ‘‘open mitosis’’), many others do not (‘‘closed mitosis’’,moresimilar to the replicationofaprokaryoticcell). Insomegroups(e.g.greenalgae),bothmitosis formsexist.The key question is to know which mitosis type, open or closed, is ancestral. Since both types are widespread in the eukaryotic tree, a simple inspection of their distribution across the eukaryoticphyladoesnotprovideaclearanswer. Interestingly, most hypotheses to explain the evolution of mitosis based on the characteristics of the closed and open mechanisms state that the closed mitosis predated the open one, an idea consistent with an endosymbiotic origin of the nucleus. In summary, Staub et al. have carried out a very interesting analysis that sheds light on the origin and evolution of the nucleolus, but their results are compatible with very different hypotheses for the origin of the nucleus, including the endosymbiotic theories. Their arguments against a possible endosymbiotic origin can be debated and are disconnected from their observations concerning the nucleolus. The origin of the nucleus remains an open question. References 1. Andersen JS, Lyon CE, Fox AH, Leung AK, Lam YW, et al. 2002. Directed proteomic analysis of the human nucleolus. 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