Asexual Reproduction in Pediastrum

     The two principle functions of all beings from the biological point of view are survival and reproduction. All creatures living on this earth have to carry out both these processes over their lifetime. Many types of survival and reproduction that are most suited for each organism have been chosen and inherited. Over hundreds of millions of years, various forms of evolutionary process have selected the most suitable organisms for residing in this biosphere.

     Several kinds of survival can be expressed in terms of an organism's behaviour. Many behaviors, such as warning behavior in groups of animals for escaping from their predators, foraging, feeding, mating etc. are exhibited. In addition to various behaviors, adaptations, homeostases, and other biological mechanisms have many effects on survival. Why do all living things want to survive? Some people believe that the organisms maintain their life for reproductive purpose.

     Since reproduction is the only way that genetic information from ancestors to their descendants can be inherited, all organisms have to try to make this process for keeping their genetic material. A good example that can illustrate the importance of this essential procedure is the sexual reproduction which occurs in mayflies, a group of insects belonging to the Order Ephemeroptera. This group of insects spends its egg and larval life in freshwater aquatic habitats and a larva feeds on microscopic algae, organic debris, etc. Until a mayfly grows up to be an adult, its mouth becomes reduced and cannot function as that of a larval stage. An adult mayfly has only a short time to live and has a crucial duty, which it must do - that is to reproduce. Mating takes place between mature male and female insects. After reproduction and laying a number of eggs, mayflies will soon die. As a result of this type of life cycle, it was designated that the order name "EPHEMEROPTERA" which means a wing or insect lasting for a day.

     In nature, a great multitude of organisms have evolved and every type has their own way of living and reproducing. It is really difficult to mention and explain every type of reproduction in each different organism but this vital process can be simply divided into two types, asexual reproduction and a sexual one. The former is reproduction having no exchange or fusion of genetic material. On the other hand, the latter is a process having exchange or fusion of genetic material.

Figure 1. Autocolony formation in Pediastrum duplex showing aggregated round-shaped cells (middle top) before transforming into a fully formed daughter colony as the one on bottom. (Taken by D.I.C.; objective 20X.)

       In fact, this article will concentrate on a form of asexual reproduction called "Autocolony Formation" in a genus of green algae, Pediastrum. The process is started by production of a number of biflagellate zoospores, i.e. motile spores having two flagella, from a parent cell. These zoospores are generated by repeated mitotic nuclear division without cytokinesis, (i.e. cytoplasmic division), therefore, at this stage, the parent cell is multinucleate. After that, cytokinetic processes for each nucleus take place concomitantly and several mononuclear daughter cells transform into biflagellate zoospores kept within a vesicle made from innermost layer of a parent cell wall. Subsequently, zoospores will aggregate and arrange themselves into a flat circular colonial form. During this time, motile spores will lose their flagella and adopt a round shape. (Figure 1.) The transformation process from round cells to flat butterfly-shaped cells like those of Pediastrum duplex will take some time. In other species of this green alga, cell shape may differ depending on the species. The word "butterfly-shaped cell" referred to here is unofficially used by myself. An emergence of a new daughter colony will make a parent cell expire. This can clearly be seen by the appearance of an empty cell and its cleft. (Figure 2 and 3.)

Figure 2. A diagram which illustrates the process of asexual reproduction in Pediastrum duplex; autocolony formation. From left to right, a parent colony produces a number of biflagellate zoospores kept within a vesicle. An emergent vesicle which leaves a break in the mother cell. An aggregation and arrangement of round shaped motile spores take place and spherical spores transform into "butterfly-shaped cells". These cells organise and finalise into a new complete daughter colony drifting and floating along water current.

       As mentioned above, every organism has their own methods of living and reproducing. As long as those means have been successful, the genetic information of those organisms will be inherited from ancestor to descendant by means of reproduction.

Figure 3. A phase contrast photomicrograph shows four empty cells and their clefts, the rupture indicating an emergence of a new daughter colony, (objective 40X).

       All comments to the author Chitchai Chantangsi are welcomed.
 
 

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Published in the February 2003 edition of Micscape Magazine.

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