Theories Of Endosymbiosis And Autogenesis

School of Bioscience

913369293

Alternative Assessment: BI1051 Genetics and Evolution

Question 1

I. Introduction

The two most advanced and scientifically supported hypotheses of evolution from a prokaryote precursor to a eukaryote are The Theories of Endosymbiosis and Autogenesis. The hypotheses both base their claims on the fact that eukaryotic genomes are chimeric, they don’t have a vertical lineage from one common ancestor, but rather a varying ancestry with diverse lineages of archaea and bacteria. Endosymbiosis is the idea that one prokaryotic organism engulfed another which formed a symbiotic relationship between the two, leading to the creation of the eukaryote and its hallmark semi-autonomous organelles (Sagan 1967). Autogenesis is

Mitochondria and chloroplasts are descendants from two separate free-living prokaryotic cells that joined together endosymbiotically (Margulis 1991).There has been some debate on the order of these events and the amount of times an organism undergoes an endosymbiotic event to become a functioning eukaryote (Yonas 2009). According to The Shopping Bag hypothesis, there can be multiple endosymbiotic events occurring until the endosymbiont can successfully survive and thrive in the other cells environment (Howe, 2008). Meaning that the larger heterotrophic prokaryote may have to engulf the endosymbiont multiple times. Each time it adds more and more genetic information from the endosymbiont until the environment is stable enough to support it. There is molecular evidence for this in the form of eukaryotes with more than two subcellular membranes and the retention of bacterial DNA (Allen 2003). The Theory of Autogenesis for eukaryotic cell components, or self-birth, consists of the idea that the cells organelles arose through differentiation and natural evolutionary changes in a single prokaryotic lineage

Mitochondria and Chloroplasts do not replicate by mitosis like their eukaryotic counterparts, they replicate using a process very similar to binary fission seen in more recent prokaryotes (Zimorski 2014). All eukaryotes have mitochondria and only plants and algae have a chloroplast, this suggests that The Shopping Bag Theory is correct and that the mitochondria arose from the first endosymbiotic event. Chloroplasts then developed as eukaryotic organisms continued to undergo endosymbiotic events until the environment was stable enough to support life almost six billion years