All of the cells in your body derive from a single fertilized egg, and most of those cells contain the same genes. Yet your body contains about 200 distinct cell types precisely arranged in a variety of tissues and organs. How can two cells with the same genes become different cell types? The answer lies in development, the system of gene regulation that guides growth from zygote to adult (Chapter 20).

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Development is the result of molecular communication between cells. Cells have different fates depending on which genes are switched off or on, and genes are switched off and on by the molecular signals that cells receive. A signal commonly alters the production of proteins—inducing the reorganization of the cytoskeleton, for example. As a result, a stem cell may become an epithelial cell, or a muscle cell, or a neuron. This observation leads to another question: What causes the same gene to be turned on in one cell and off in another? The ultimate answer is that two cells in the same developing organism can be exposed to very different environments.

When we think about development as a process of programmed cell division and differentiation, the link to unicellular ancestors becomes clearer. Many biological innovations accompanied the evolution of complex multicellularity, but, as noted in Chapter 27, the differentiation of distinct cell types is not one of them. Many unicellular organisms have life cycles in which different cell types alternate in time, depending on environmental conditions. For example, if we experimentally starve dinoflagellate cells, two cells undergo sexual fusion to form morphologically and physiologically distinct resting cells protected by thick walls. That is, a nutrient shortage induces a change in gene expression that leads to the formation of resting cells. When food becomes available again, the cells undergo meiotic cell division to form new feeding cells. Many other single-celled eukaryotes form resting cells in response to environmental cues, especially deprivation of nutrients or oxygen.

The innovation of complex multicellularity was to differentiate cells in space instead of time. In a three-dimensional multicellular organism, only surface cells are in direct contact with the outside environment. Interior cells are exposed to a different physical and chemical environment because nutrients, oxygen, and light become less abundant with increasing depth within tissues. In effect, there is a gradient of environmental signals within multicellular organisms. We might, therefore, hypothesize that in the earliest organisms with three-dimensional multicellularity, a nutrient or oxygen gradient triggered oxygen- or nutrient-starved interior cells to differentiate, much as happens to trigger the formation of resting cells in their single-celled relatives. With increasing genetic control of cellular responses to signaling gradients, the seeds of complex development were sown.

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Green algae provide a fascinating example that links cell differentiation in unicellular and multicellular organisms. The simple multicellular organism Volvox (see Fig. 27.15) has two types of cells, vegetative cells that photosynthesize and control movement of the organism, and reproductive cells. Cell differentiation in Volvox is regulated by a gene also involved in the formation of distinct cell types in the life cycle of Volvox’s single-celled relative Chlamydomonas, supporting the hypothesis that the spatial differentiation of cells in multicellular organisms began with the redeployment of genes that regulate cell differentiation in single-celled ancestors.

Bulk flow, which transports nutrients, oxygen, and water within complex multicellular organisms, also carries developmental signals. Signals carried by bulk flow can travel far greater distances through the body than signals transmitted by diffusion alone. For example, in animals the endocrine system releases hormones directly into the bloodstream, enabling them to affect cells far from those within which they formed (Chapter 38). Thus, the sex hormones estrogen and testosterone are synthesized in reproductive organs but regulate development throughout the body, contributing to the differences between males and females. In this way, signals carried by bulk flow can induce the formation of distinct cell types and tissues along the path of signal transport.

The genome of the choanoflagellate Monosiga brevicollis, discussed earlier, has been a treasure trove of information on the antiquity of signaling molecules deployed in animal development. In addition to expressing proteins that govern cell adhesion and epithelial cohesion in animals, M. brevicollis expresses a number of proteins active in animal cell differentiation. For example, signaling based on specific receptor kinases was long thought to be restricted to animals, but it also occurs in M. brevicollis. In other cases, individual components of signaling proteins are present in the M. brevicollis genome, but not the complex multidomain proteins formed by animals. This is true, for example, of several protein complexes that are important in development. Similarly, molecules that play an important role in plant development are also being identified in the genomes of morphologically simple green algae. The key point is that genome sequences interpreted in light of eukaryotic phylogeny are now enabling biologists to piece together the patterns of gene evolution that accompanied the evolution of morphological complexity and diversity in plants and animals.