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Abstract For over a century, the origin of eukaryotes has been a topic of intense debate among scientists. Although it has become widely accepted that organelles such as the mitochondria and chloroplasts arose via endosymbiosis, the origin of the eukaryotic nucleus remains enigmatic. Numerous models for the origin of the nucleus have been proposed over the years, many of which use endosymbiosis to explain its existence. Proposals of microbes whose ancestors may have served as either a host or a guest in various endosymbiotic scenarios abound, none of which have been able to sufficiently incorporate the cell biological as well as phylogenetic data which links these organisms to the nucleus. While it is generally agreed that eukaryotic nuclei share more features in common with archaea rather than with bacteria, different studies have identified either one or the other of the two major groups of archaea as potential ancestors, leading to somewhat of a stalemate. This paper seeks to resolve this impasse by presenting evidence that not just one, but a pair of archaea might have served as host to the bacterial ancestor of the mitochondria. This pair may have consisted of ancestors of both Ignicoccus hospitalis as well as its ectosymbiont/ectoparasite ‘Nanoarchaeum equitans’.

The process of development can be viewed as a series of linker histone replacements which take place throughout spermatogenesis and oogenesis, as well as following fertilization or somatic nuclear transfer (SNT). Although few of the histone H1 variants in question have been shown to be essential for viability, the timing of their appearance as well as the affinity with which they are able to bind to chromatin seem to be important factors in their developmental role. A looser binding of linker histones to chromatin seems to correlate with the meiotic phases of gametogenesis and the establishment of a totipotent, as well as the maintenance of a pluripotent, state in early embryos, while tighter binding of linker histones to chromatin appears to be associated with the mitotic phases, as well as the increased levels of condensation that are required for the packaging of DNA into sperm. This latter process also involves the binding of certain basic non-histone proteins to DNA. While all proteins involved in chromatin compaction during development are highly basic in nature, in general they can be seen to change from lysinerich variants to arginine-rich ones, and back again. The fact that linker histone transitions are conserved across diverse metazoan species speaks of their importance in packaging DNA in a variety of ways during this crucial period. [ABSTRACT FROM AUTHOR]