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4 The Epigenetic Inheritance Systems A person’s liver cells, skin cells, and kidney cells, look different, behave differently, and function differently, yet they all contain the same genetic information. With very few exceptions, the differences between specialized cells are epigenetic, not genetic. They are the consequences of events that occurred during the developmental history of each type of cell and determined which genes are turned on, and how their products act and interact. The remarkable thing about many specialized cells is that not only can they maintain their own particular phenotype for long periods, they can also transmit it to daughter cells. When liver cells divide their daughters are liver cells, and the daughters of kidney cells are kidney cells. Although their DNA sequences remain unchanged during development, cells nevertheless acquire information that they can pass to their progeny. This information is transmitted through what are known as epigenetic inheritance systems (or EISs for short). It is these systems that provide the second dimension of heredity and evolution. Until the mid-1970s, the existence of epigenetic inheritance was barely recognized. Developmental biologists devoted most of their efforts to trying to find out how cells became differentiated. They were concerned with the signals that switched genes on and off, and with the cascade of events that led to cells in one place becoming specialized for one particular function , while those somewhere else were induced to have a different function. The emphasis was on how cells acquired their specialized roles rather than on the complementary problem of how, once the appropriate genes had been turned on and off, cells remembered their new epigenetic state and transmitted it to their progeny. In 1975, two rather speculative articles drew attention to the problem by suggesting a possible solution to it. Robin Holliday and John Pugh, two British biologists, and Arthur Riggs in America independently suggested a mechanism that would enable states of gene 112 Chapter 4 activity and inactivity to be maintained and transmitted to future cell generations .1 Their ideas generated a lot of interest, and after a rather slow start the study of cell memory and epigenetic inheritance began to take off. It was given even more impetus when it was realized that understanding epigenetic inheritance was going to be crucial to the success of cloning and genetic engineering projects. Today, epigenetics is quite a buzzword, and biologists are well aware of the existence of EISs and their importance in development and medicine. However, there is still a reluctance to recognize that they may also have a significant role in evolution. So, to illustrate how EISs can affect evolution, we are going to resort to another thought experiment. The scenario that we are going to describe will show that evolution is possible on the basis of heritable epigenetic variation even when there is no genetic variation at all. To avoid any misunderstanding, we need to stress at the outset that we do not underestimate the importance of genetic variation in evolution. We are using the thought experiment merely to show that it is possible to think about evolutionary change based solely on variations transmitted by nongenetic cellular inheritance systems. Evolution on Jaynus Imagine that on Jaynus, a planet not too far or too different from our own, there is life. The organisms found there are very diverse, having all sorts of amazing shapes and behaviors, although their complexity does not exceed that of a jellyfish (figure 4.1). All Jaynus creatures multiply solely by asexual processes: there is nothing like the meiotic cell division that leads to the production of gametes in Earth’s animals and plants, and there is no sexual reproduction of any other type. But, just as here on Earth, there are several types of asexual reproduction. Some creatures multiply by shedding buds from the adult body; in most others multiplication is through single cells that become detached, start dividing, and develop into adults; and in a few it occurs through the assembly of cells from several different individuals to form a kind of “embryo,” which then begins the developmental process.2 Jaynus organisms have a genetic system that is based on DNA, and replication , transcription, and translation are much the same as on Earth. However, there is one very extraordinary thing about the DNA of Jaynus creatures—every organism has exactly the same DNA sequences. From the simplest organism, a tiny unicellular creature, to the enormous fanlike colonial worms, the DNA is identical. Their genomes are large...


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