New Theory On The Evolution of Eukaryotic Cell

 

The emergence of eukaryotic cell was the precursor that heralded the process of evolution.  A eukaryotic cell with its functional organelle, organised nucleus enveloped in a nuclear membrane with a well connected endoplasmic reticulum was a quantum leap from the primitive prokaryotic cells. It laid the basic foundations for multicellularity and development of independent organisms capable of adapting to new ecological niches. Prokaryotes are believed to be the predecessors for the development of a eukaryotic cell. Existing theories believe that protomitochondria entered the cell via phagocytosis. According to those theories, the invaginations of the plasma membrane and internalisation of the membranes led to formation of compartments, endoplasmic reticulum that is continuous and organised around the chromatin to form inner and outer nuclear membrane. Assumption has been that nucleus was formed within the boundaries of existing cell with unaltered plasma membrane and these theories collectively were referred to as outside-in models.  Other theories are the endo-symbiotic theory which hypothesised that origin of nucleus and mitochondria has been consequence of repeated phagocytosis and endo symbiosis. This necessitates the presence of three partners- host, nucleus and mitochondria. Another was Syntropic consortium model wherein simultaneous fusion of cytoplasm, nucleus and mitochondria was envisaged. Endospore model suggested that nucleus evolved when cell enclosed its sister after cell division. In all of these models nucleus was derived from the internalised plasma membrane.

As against the reigning theories, Baum and Baum proposed a new model wherein Archae generated extracellular protrusions which led to development of eukaryotic cell and that eukaryotic architecture was formed by the extrusions. As expounded by the outside-in model, Archae are not known to undergo the process of endocytosis or phagocytosis. The new theory of inside-out has convincingly enunciated step by step process in the development of eukaryotic cell wherein the nuclear membrane, the oldest part of the cell is equivalent to the ancestral prokaryotic cell. Under this outer nuclear membrane, plasma membrane and cytoplasm are derived from the external protrusions or blebs and endoplasmic reticulum represents the gaps between the blebs. The process usually occurs during an intimate mutualistic association between an Archael host cell and a α-proteobacteria (containing a proto-mitochondrion). Initially mitochondria are trapped by the ER and it slowly penetrates and makes way into the cytoplasm. Eukaryogenesis (the process of development of eukaryotic cell) is sealed off by the formation of continuous plasma membrane and closing off the ER to the exterior. In this model prokaryote is termed as eocyte having a single lipid bilayer membrane and a simple cell wall rich (S-layer) in N-glycosylated proteins, cytoskeleton homologs of actin and tubulin and a membrane manipulating protein ESCRTIII.

The new theory is empirical and experimental evidence nearly validates the theory, firstly Archae are known to form protrusions to increase their surface to volume ratio. Formation of protrusions is facilitated the S-layer proteins and the cytoskeleton while positive curvature of the protrusions are stabilised by proteins containing seven blade beta propeller domains homologous to the Coat  Protein (COP II) that form outer ring of nuclear pore. Accordingly, the outer membrane of the nuclear pore complex (NPC) will be a highly conserved portion and it is found that most eukaryotes have most stable proteins in NPC. Recent data suggest that mitochondria are closely related to proteo-bacteria precisely to the Rickettsiales group of intracellular parasites of eukaryotes. These are known to enter the food-vacoule and then enter cytoplasm proper by lysing the vacuole. Since the external protrusions swelled beyond S-layer, osmoregulation is another feature which has to be accounted. It is known that Archael cells thrive in an environment containing high external osmolytes wherein the chances of bursting of cell due to thinning or loss of S-layer are negligible. Moreover, the expansion of the blebs pressed against each other generate a continuous network against each other is akin to the lumen of the nuclear envelope and ER of highly developed eukaryotes.

Three different kinds of evidence support the inside-out theory elucidated by authors. The first set of evidence support the characteristic features of the eukaryotes like existence of nuclear membrane without any internal membrane bound organelles, presence of double nuclear membrane with periplasmic space continuous with ER and why N-glycosylation is initiated in the nuclear envelope is explained parsimoniously by this model. The second set of evidence explains the unusual or quirky features of eukaryotes like the close connections between the ER and mitochondria and their prominent roles in lipid synthesis and provides logical explanation for the presence of phosphoinositides in the nucleus and their role in mRNA processing are convincingly detailed by the inside-out model. The third set of evidence draws parallels between the phylogenetic analyses of eukaryotic gene families. Phyogenetic analyses of the Ras GTPases superfamily propounds that secretion and exocytosis evolved before endocytosis. The inside-out clearly predicts similar phenomenon. Evidence identify that α-proteo-bacteria, (protomitochondrion) is a source of eukaryotic lipid biosynthesis and transport genes. This compelling evidence offers a plausible historical narrative for the origin of the Eukaryotes.

Evolutionary theories envisage the process of development that had occurred billions of years ago. Though the events are not testable, these hypotheses help us to understand and analyse the slow and steady process of development and the underlying biology.