Factors controlling development

Factors Controlling Development
While the genetic constitution of the zygote largely controls the course of development, there are several other factors, which have been shown to play an important role in controlling the developmental patterns. These include the following : (i) Cytoplasmic factors in some cases play a very important role in early embryonic development. For instance, yolk and mitochondria are not evenly distributed in an amphibian egg, and get unevenly partitioned into different daughter cells. Experimental demonstration of the extra-nuclear control of development has been possible by grafting a part of cell cortex (without nuclei) from an embryo into a fertilized egg, which led to the development of an extra spinal cord, (ii) Environmental factors also play an important role in development as shown in phenocopies in an organism, where different genetic constitutions lead to same phenotype if grown in different environments. However, treatment to a developing embryo with a definite chemical or temperature, etc., should be given at a specific critical period during development in order to obtain each particular phenocopy. (iii) Maternal effects on development have been witnessed in many cases, where the cytoplasm of the egg was found to regulate the early events in embryonic development. For instance, even an enucleated egg of a frog will still undergo the normal early cleavage, although one may argue that the architecture and composition of the cytoplasm are themselves under the control of the nuclear genome of the mother. Some other examples of maternal and cytoplasmic effects are given in Maternal Effects and Cytoplasmic Inheritance.