These two readings emphasize on how it is interesting to know that cell evolution research is advancing in new and exciting ways at the intersection of genomics, cell biology, molecular biology, biochemistry, and microbiology. The aim of the papers is to reexamine one of the more revolutionary conjectures made to account for the origin of the complicated kind of cell that is the structural unit of the higher animals and plants.
The first reading is entitled “Origin of eukaryotic cells: 40 years on”, the paper is authored by John M. Archibald. The book is aimed at discussing the 40 years that have elapsed since the landmark publication of the book Origin of eukaryotic Cells by Lynn Margulis with a focus on the “molecular era”. This paper gives a good review of Margulis’ contributions to the field of cellular Evolution. And goes further to establish why she is considered, in the eyes of many, as the champion of endosymbiotic theory. The second reading, entitled The Large, Free-living Amoebae: Wonderful Cells for Biological Studies, was authored by Kwang W. Jeon in 1995, it aims at showing that amoebae are uniquely suited as model cells with which to study a variety of biological phenomena including; cell motility, nucleocytoplasmic interactions, membrane function, and symbiosis.
How the eukaryotic cell originated still remains a puzzle that has fascinated and perplexed biologists for over a century. The extent of the genetic diversity of prokaryotes and eukaryotes revealed by metagenomics has been breathtaking. Combined with ‘next-generation ’DNA sequencing, flow cytometry, and single-cell genome amplification techniques, it is impossible to predict how culture-independent molecular investigations of the natural world will impact current views on the evolution of eukaryotic cells and their organelles. Lynn Margulis published the book Origin of Eukaryotic Cells in 1970. Forty years after the publication of the book, it is still recognized as one of the most influential bodies of work that contributes to the, now widespread, acceptance that endosymbiosis has been—and still is—a creative force in cellular evolution. Some of the other reasons why Margulis book is influential are because it brought the exciting and weighty problems of cellular evolution to the scientific mainstream, while simultaneously, breaking new grounds and re-discovering the decades-old ideas of German and Russian biologist. The book also discusses how DNA sequencing and comparative genomics have proven beyond all doubt the central tenets of the endosymbiont hypothesis for the origin of mitochondria and plastids and, at the same time, revealed a genomic and genetic complexity in recent eukaryotes that could not have been imagined in decades ago. This book is devoted to a reexamination of one of the more revolutionary conjectures made to account for the origin of the complicated kind of cell that is the structural unit of the higher animals and plants. Dr. Margulis has brought together with infectious enthusiasm a vast amount of very disparate material, largely derived from the most recent biochemical studies, to bear on this subject. The final chapter of Margulis’ book is focused on the problem of speciation in multicellular eukaryotes. Dr. Margulis emphasized the importance of symbiotic theory as a framework for comprehending all aspects of advanced cellular evolution, from polyploidy in plants to tissue development in animals.
The first reading focuses on molecular phylogenetic and genomics-enabled investigations of the evolution of mitochondria and plastids. It gave examples of endosymbiotically derived organelles—and on recent advances in their understanding of less firmly entrenched endosymbiosis between prokaryotes and eukaryotes. Pertaining to the Problems of cellular evolution, it showed that the endosymbiotic origin of some specific eukaryotic organelles is not consistent with many facts, and it went further to proffer a research-generative alternative. Researchers such as Margulis focused less on the diversity of prokaryotes and the distinction between eubacteria and archaebacteria than on what this diversity could reveal about the prokaryote-to-eukaryote transition. Two competing hypotheses were debated in the primary literature, the so-called ‘direct-filiation’ model and the symbiosis model, the latter also referred to as the ‘serial endosymbiosis theory’. The ‘blue-green algae’, known today as cyanobacteria, featured prominently in both hypotheses but in very different ways.
The direct-filiation (i.e., autogenous or non-endosymbiotic) model for the evolution of eukaryotes held that a specific relative of present-day blue-green algae gave rise to, by vertical evolution, an ancestral ‘phytoflagellate’ from which all nucleus-bearing cells ultimately evolved. In contrast, the symbiosis model posited that key eukaryotic sub-cellular organelles—mitochondria, plastids and the 9+2 flagellar apparatus—were of xenogenous origin. A critical feature uniting the various incarnations of the direct-filiation hypothesis is the existence of a specific evolutionary link between photosynthetic prokaryotes and photosynthetic eukaryotes: the common ancestor of all eukaryotes had a plastid (or something like it) and non-photosynthetic, plastid-lacking groups such as animals and fungi had lost the organelle secondarily and became heterotrophic. Early proponents of direct filiation included Allan Allsopp (1969), Rudolf Raff and Henry Mahler (1972), Tom Cavalier-Smith (1975) and Max Taylor, with the latter author playing a particularly prominent role in the marshaling of evidence for and against. According to Margulis, “eukaryotic plants did not evolve oxygen-eliminating photosynthesis which later ‘packaged’ into membrane-bounded plastids; they acquired it by symbiosis”. The question of what “photosynthetic ancestral form”, or “uralga”, linked the cyanobacteria with plastid-bearing eukaryotes was to Margulis a “non-question”; she later referred to direct filiation as the “botanical myth”.
According to the second reading, Amoeba was described by Von Resenhof as “little Proteus” in 1970, ever since then," the large, free-living amoeba have been favorable organisms for both cellular research and teaching. Many prominent researchers have attested to the fact that “Life among amoeba” has created a lot of excitement, this is because amoeba offers unique advantages as a material for performing experiments.
It’s good to know that the large, free-living amoebae offer many advantages as a model system for the study of cell structure and functions, including motility, nuclear-cytoplasmic interactions, membrane functions, and symbiosis. The article establish particular, the amoeba/X-bacteria symbiosis system has a unique and novel system, in which the process of symbiont integration causing cellular character changes and the origin of new cell components can be studied.
In recent years, however, the number of workers using amoeba as research organisms has dwindled because free-living amoebae are perceived to be unsuitable for modern cellular and molecular-biological studies because their genetics are difficult to study and they cannot be grown axenically, and hence research funds are not easily obtainable. In the article, experimental results presented showed that amoebae still provide a useful model system for studies in cell and molecular biology in spite of the problems mentioned earlier. Symbiosis, membrane functions, cell motility, nucleocytoplasmic interactions, and other varieties of biological phenomena have been studied using the large, free-living amoebae. The author hoped that, with the result that this survey has demonstrated, biologists would be able to see the usefulness of amoebae as experimental cells and that more cell biologists will be encouraged to use amoebae as research organism.
During a study of the amoeboid movement, it was discovered that amoebae exhibited a chcmotactic behavior toward whole and pieces of hydra as demonstrated in a film. In the late 1940s, Danielli and his co-workers initiated microsurgical studies involving nuclear transplantation and injection of cytoplasm to elucidate the mechanism for possible cytoplasmic inheritance, using the newly developed dc micromanipulator. It had been observed that some of the cellular phenotypic characters seemed to be controlled by the cytoplasm and not by the nucleus
However, microsurgical studies in amoebae yielded useful results related to nuclear-cytoplasmic hybrids, their viability, strain-specific organelle compatibility, cellular character changes and instability, and the role of nucleus on cell attachment and motility. It came to be recognized that free-living amoebae harbored other microorganisms within their cytoplasm either as transient inclusions or as permanent residents. The large, free-living amoebae offer many advantages as a model system for the study of cell structure and functions, including motility, nuclear-cytoplasmic interactions, membrane functions, and symbiosis. In particular, the amoeba/X-bacteria symbiosis system is a unique and novel system, in which the process of symbiont integration causing cellular character changes and the origin of new cell components can be studied.
These studies carried out, has demonstrated the usefulness of amoebae as experimental cells, it has shown that amoebae are uniquely suited as model cells with which to study these biological phenomena, it established that free-living amoebae offer many advantages as a model system for the study of cell structure and functions, including motility, nuclear-cytoplasmic interactions, membrane functions, and symbiosis. it is hoped that more cell biologists will be encouraged to use amoebae as research organisms.
Reference
Kwang W. Jeon.1995. The Large, Free-living Amoebae: Wonderful Cells for Biological Studies. J. Euk. nflcrobiol..
42(1). 199$. pp. 1-7. Society of Protozoologists.
John M. Archibald. 2011. Origin of eukaryotic cells: 40 years on. Symbiosis (2011) 54:69–86 DOI 10.1007/s13199-
011-0129-z
