BIOLOGY:ORIGIN OF LIFE:PART 3-EVOLUTIONARY DEVELOPMENTAL BIOLOGY

The embryo contains the unique information that defines a person-Todd akin

Origin of the eukaryotic cell

The embryonic development of metazoans consists of repeated division of cells from a zygote. While undergoing divisions the cells differentiate into various specialised cells. Moreover the cells organise themselves into structures such as plane sheets(epithelium) tubes( blood vessels, bronchi). The ability to organise cells in above manner is not present in prokaryotes. It arose when the first eukaryotic cell arose with capability to form Endo membranes ,phagocytosis ,secretion .

          The prokaryotic cell was surrounded by a rigid cell wall .Over a period of time the cell wall was lost and the outer wall remained as a flexible plasma membrane. The cell wall prevented any movement or modification of the PM. With its loss there was scope for the PM to be modified. The PM gained three types of capability. Firstly it became capable of secretion through a process of translocation of proteins across the PM. This helped in digesting surrounding food materials but the enzymes got diluted by the environment. Thereafter it developed the capability of folding . This was achieved by placing specific proteins in the PM. With folding the PM could form infolding and then a vesicle. The food could be now ingested and digested within the body. Third was the development of cytoskeleton of actin proteins and molecular motors such as myosin. This allowed vesicles to be transported and used to construct nuclear membranes,ER ,Golgi complex. It also enabled chromosome separation and cytokinesis as in mitosis.

     The capability to carry out above depends upon certain proteins such as SAR which can fold membranes in eukaryotes. Moreover the cell developed a signalling system based on proteins of GTPase system which enabled the control and coordination of membrane construction inside the cell

Embryonic development of metazoans

Embryo develops by construction of spheres, tubes, sheets of cells. The morphology of these structures are possible because eukaryotic cells have a cytoskeleton which can change the shape of cells. A tube forms when a sheet of cells undergo in folding. At the site of in folding the apex cell changes it’s shape to a conical form. The surrounding cells undergo similar change in shape till a tube is formed. The entire change has to be coordinated.This coordination is achieved by a gradient of morphogens secreted by one of the cells nearby. Again the ability to secrete proteins is present only in eukaryotes. The diffusion of the morphogens across the embryo occurs in a gradient with highest concentration near the origin. The cells in the embryo respond to the gradient only when the concentration near them is optimum. Thus the apex cell in a invaginating sheet of cells responds to a certain morphogenetic concentration by changing to a conical shape. The surrounding cells have a different concentration of morhogens and do not change their shape as much.

   Genetic and epi genetic coordination of development

Development from a single cell occurs by a sequence of structure formation. Thus the single zygote cell divides to form a ball of cells ,the blastula. Thereafter the blastula forms a cavity and then forms a three layered structure the gastrula. The gastrula forms infolding dorsally to form a solid tube the notochord. 

        Each sequence in development is under the control of a gene and development can thus be considered as sequential gene activation. Genes have an upstream regulatory segment called cis region or promoter. The region is responsible for regulating the transcription of the gene . The promoter region binds to various trans regulatory proteins called transcription factors. These trans factors are secreted by other genes and include morphogens. 

     Though the above forms the basic mechanism to control developmental sequences there is another layer of control. In protocells and RNA world the RNA is the principal genetic material. Moreover it has the capability to cut and ligate   segments of RNA to duplicate itself. In eukaryotic cells this function of RNA has been modified to produce epi genetic control during development. After transcription has occurred the mRNA  can be further modified by sRNA and long RNA in cells. These cut and ligate  the mRNA to produce different nucleotide sequences from the same RNA . This is usefull as the same  RNA can be translated differently into different proteins. This occurs widely in different organ in the body and is responsible for varieties of neuronal cells in the brain for example. The activation of organ specific transcription is under control of cis region of Regulatory DNA . However it may be activated by envirnmental factors such as cold or chemicals in the environment. This is called epigenetic control .

Evolution of development

Evolution takes place by changes in sequences of cell growth and differentiation  in the embryo. Genes which control these sequences are duplicated to form a duplicated sequence. Thereafter the gene undergoes mutations and creates a new form . The form change occurs due to new trans regulatory factors altering the activation of the cis region of the gene. There may in addition be a mutation and change in the structural gene as well  but most evolutionary changes in embryo occurs due to change in regulatory regions.

  As a result of change in cis regulatory region a gene may be activated at a site not usual for it to occur. Thus genes controlling curvature can change site of activation from neural tube to bronchial tube in lungs. Apart from site change a regulatory gene can get activated to add a new sequence to existing structure. Thus nephron formation sequences has been gradually added to a primitive pouch sequence from neprhogenic mesoderm to form the adult kidney.

     Evolution of sequences requires a stable base sequence to build upon. This is so because regulatory genes are being constantly reshuffled from their sites during recombination and crossing over in meiosis. A fundamental function controlled by a regulatory gene cannot undergo reshuffling as the entire embryo will become dysfunctional. Genes controlling later sequences can be reshuffled without lethal effects on survival of the embryo.Thus a gene controlling pigment formation in eyes serves as an anchor. This is a basic function. All eyes have added sequences to this function in the form of lens,eyelids,cornea . The basic genes in eye is thus common to most animals with eyes such as vertebrates ,insects and molluscs. 

 Another example is the box genes. These genes are activated from anterior to posterior in the genome during development. They serve as scaffolds for formation of body segments and their attachments such as wings and legs. They have been preserved as fundamental regulatory genes as they serve to give a framework to build the body plan. Numerous regulatory sequences have thus been added to hoxgenes and they have been preserved in evolution.

Ack: Life unfolding;sequence evolution function;Genomic regulatory systems development and evolution;Eukaryotic membranes and cytoskeleton;Mechanisms of morphogenesis