Extensive developmental studies have been shown that the pre-existing cellular structures play a key role in the formation of new cellular structures. Various processes such as homonucleation, heteronucleation and allosteric transitions are observed to involve in the developmental phenomenon. The studies on the development of cellular structures such as ciliary basal bodies and their associated structures, the repeating unit territory of the cortical structure, the patterns of the rows of basal bodies and unit territory, the ingestatory apparatus (comprised of gullet and vestibule) in the unicellular organism such as paramecium aurelia (ciliated protozoan), and the studies on the organization of bacterial flagella, assembly of the viral particles (bacterial viruses) and the parts of ciliary microtubules and their appendages have shown that the pre-existing structures determine the organization of new structures.
Bacterial flagellum is a polymer of flagellin protein. Under non-physiological conditions, the flagellin proteins can be polymerized to form flagella like precipitates. In an experiment (Abraham and Koffler 1964) to assemble the flagellin into flagella like precipitates under physiological conditions, it was observed that only on the addition of small fragments of organized flagella triggers the formation of precipitates of flagella and the number of flagella like structures formed was equal to the number of flagellar fragments added. This indicates that the organized flagella acts as seeds or nucleation centres for the polymerization of the flagellin molecules into flagella. In another experiment (Oosawa et al. 1966) that involved the work with flagellar mutants (wild type- wavy flagella, curly type- curly flagella) it was observed that the wavy flagellar fragments when added to the wavy flagellin solution gave wavy flagellar type structures. On the other hand, the curly flagellar fragments when added to the curly flagellin solution gave curly flagellar type structures. Interestingly, when the curly flagellar fragments were added to the wavy flagellin solution gave rise to curly flagellar type structures. These observations proved that the existing flagellar type determine the form of flagellin to be assembled to form the flagellar structures. Similar experiments (Stephens 1968) with cilia (made of a ring of 9 pairs of microtubules) have shown that the fragments of tubules were essential in the solution of microtubular proteins for the formation of cilia like structures.
The studies on the bacteriophage viral particle assembly (Levine 1069) have shown that even though the tails form earlier to the head structure (capsid), the assembly of these tail fibers does not take place until the capsids are completely formed. This can be linked to the sites on the capsids where the tail can attach, that these sites need to be processed by the reactions that occur during the head assembly. This shows that during the development of structures, an ordered succession of molecular events take place that are governed by the allostreric transitions.
Several features have been studied in Paramecium aurelia (Sonneborne 1970) from the development perspective. It was observed that the spatial and temporal developmental events in the cortex of the paramecium are determined by its microgeography. Also, the site of initiation of the basal bodies assembly, its migration to the cell surface and the orientation of the associated structures around it are not determined by the outside or nuclear or cellular influences, but they all have been shown to depend on the molecular geography within the cortical unit territory. Similarly, the study on the ingestatory apparatus of the paramecium has shown that the site of the origin of the new gullet (component of ingestatory apparatus) is determined by the local microgeography. The studies on the row patterns of cortical unit territories have shown that the highly coordinated and locally differential reproduction of the unit territories may affect the development of row patterns during cell division. In the same way, it was shown that the distribution of the two kinds of cortical unit territories is determined by the gullet-vestibule region. The gullet-vestibule region thus acts as a triple determinant from the developmental aspect; first, its own reproduction, second, paths of the rows around it and third, distribution of two types of cortical unit territories. Therefore, all these above examples clearly show that the pre-existing structure plays a key role in the development of new structures.
Influence of cortex of Paramecium in the inheritance
As far as the development of the cortical structures at cell division is concerned, the cortex of the paramecium acts as a controller of genic action, this can be supported by many examples. In the areas of the cortex that lack pre-existing basal bodies, the assembly of the newly formed genic products of the basal bodies does not take place. Similarly, genic products do not assemble into the gullet of the ingestatory apparatus unless certain cortical geographical features (at the juncture of the vestibule and gullet) already exist. The determinative cortical differentiations that control the hereditary development of all the highly differentiated features of the cell cortex at the time of each developmental event cannot be traced back solely to prior actions of DNA. This raises a thought from evolutionary perspective that how this non-DNA but hereditary determination of developmental process accounts for.
