A major hallmark of multicellular life is differentiation. Cells reproduce, and a single cell becomes a complete organism. Much of the work done on this subject before and even since our recently increased ability to document genetic mechanisms experimentally has concerned the determination of overall body plans. Multicellu-lar organisms are quite diverse, but in fact there are only about 35 body plans associated with the major animal phyla (e.g., Raff 1996) and an additional set of basic plant forms. There is a similarly limited repertoire of basic patterns of structural organization of systems within an organism. We will see in the next chapter that a limited repertoire of developmental processes brings this about, helping to explain how the great diversity of life has been achieved.
Once a basic system has become highly integrated, early in development, it can be canalized or its general features retained, a term introduced by CH Waddington (Waddington 1942; 1957; Wilkins 2002) that we have several times mentioned. A view fully compatible with classical darwinian theory is that, once entrenched, so much of later development depends on the early establishment of a body plan that the latter is difficult to change. This leads to evolutionary and developmental stability, although at the expense of flexibility (e.g., Siegal and Bergman 2002). Other ways of buffering include heat shock (or chaperone) proteins that can serve as a "capacitor" against harmful environmental or mutational effects, epistatic feedback (Rutherford and Henikoff 2003; Wagner 2000), and redundancy. In Chapter 7, we discussed the way that tolerant enhancer elements can contribute to these kinds of stability over individual as well as evolutionary time.
Balancing selection, that is, selection against all extreme phenotypes (e.g., too big, too small) can facilitate the evolution of such stability. Of course, no foresight is involved in any case. Transmitted chromatin modification that affects gene expres sion is a way in which genetic variation can exist in a population but not be expressed, perhaps easily "reawakened" under stress (if the chromatin modification is itself variable or released by fortunate mutation). Selection need merely maintain the suitable forms in their respective environments. Later, crosses between what appear to be invariant strains, or other kinds of selection, can release the constraints on this variation so that it becomes expressed. Various effects of this kind have been observed in animals and plants (e.g., Dun and Fraser 1959; Lauter and Doebley 2002; Sollars et al. 2003).
Here, we will attempt a cursory survey of the basic aspects of multicellular organization that these processes have produced. Examples, illustrations, and even animations are plentifully available on the internet, and there are several excellent texts concerning morphological development and its genetics (Gilbert 2003; Raff 1996; Wilkins 2002; Wolpert et al. 1998). Tudge (Tudge 2000) has provided an excellent compendium of animal form, and there is an excellent resource on known evolutionary relationships among the creatures on Earth (Maddison 2003).
Animal body plans include various types of symmetry from essentially none (as in hydra and sponges) to radial (coelenterates), to pentameral (starfish and other echinoderms), to bilateral (ourselves). Plants establish radial symmetries in their roots, shoots and flowers, and branching symmetries in the arrangement of leaves on stems or veins within leaves. These symmetries define or reflect anatomic axes, most notably the anterior-posterior (AP) axis from head to tail and the dorsalventral (or dorsoventral, DV) axis from back to front and lateral or left-right or proximal-distal (or proximodistal, PD) when viewed relative to the midline; in plants, correspond to the main vertical and radial axes and the longitudinal axes of shoots and roots.
Evolutionary explanations of body plans usually relate to their current function. For example, the segmentation of the vertebral column enables animals to become long and flexible, and yet rigid from head to tail, and to have separate structures in different places along the way. The bilateral symmetry of our eyes and ears, the forked tongue in snakes, and the antennae in insects allow perception of the world in stereo. Of course, keeping in mind the step-by-step nature of complex trait evolution, these may or may not be the original selective advantages.
Not all body plans are as organized or simply explained. Bacterial aggregates form from free-floating single-celled organisms that, under environmental conditions that favor their congregation, can emit chemical signals that induce others nearby to change behavior and join together with no fixed kind of plan (Bonner 2000; Shapiro JA 1998). By aggregating on a surface, to which they adhere through the secretion of various polysaccharide or other substances in which they also become covered, the bacteria act in a sense as a single entity. The secreted adherent film can serve to concentrate trace nutrients, attract commensal organisms to process waste, or provide or recycle nutrients. The adherence may cause the walls to thicken, protecting the individual cells.
Different microstructures in the aggregates affect microenvironments, the flow of and access to water, response to antibacterial agents, and the like. In a sense, by organizing fluid flow in their environment, bacterial colonies become vascularized. More remarkably, it has been discovered that bacteria use different sets of genes when living in biofilm aggregates than when planktonic (free-floating), with major changes in the protein composition of their cell walls. More complex and diverse are ways that bacteria aggregate and accomplish things they could not do alone
(Armitage 1999; Hoyle and Costerton 1991; Miller and Bassler 2001; Parsek and Greenberg 2000; Pratt and Kolter 1999; Prigent-Combaret et al. 1999); sometimes, different species cooperate in this regard (we are anthropomorphizing in discussing these things as if they were done on purpose).
Here we consider general characteristics of multicellular organization, and in Chapter 9 we will consider the genetic mechanisms that bring this structure about. First, we should repeat what has been noted by countless other biologists: the range of multicellular organization cannot be arrayed in a hierarchical Great Chain of Being graded from worst to best. The simplest organisms are still with us and, if anything, seem far less likely to become extinct than most complex organisms (who may, in fact, be at an overall disadvantage and more vulnerable to extinction in the long run). If there is one multicellular structure that is "better" in this sense, it might be the human brain because it is the only structure that seems to make it possible to exterminate so many others, around the globe, at least in the short run.
Very simple organisms that can live as isolated single cells but can also form associations in various ways appear to have multiple evolutionary origins (Bonner 1988; 1998; 2000; Buss 1987). The simplest organisms that we think of as truly multicellu-lar, such as sponges, have little organized structure or cellular differentiation. Most are referred to as diploblast, because they have an organized outer and inner cell layer but little cellular material or organization in between. Choanoflagellates are single-celled organisms consisting of a cell body and a collar of microvilli through which water is filtered, surrounding a single flagellum used for propulsion. Some choanoflagellates aggregate to form what may resemble an early form of metazoan life.
Sponges have only a few distinct cell types: a protective external layer; a ciliated internal layer that moves water through the organism; and a middle layer composed of freely migrating archeocytes that scavenge food particles and can differentiate into all the other cell types, as well as cells that lay down an extracellular matrix of spicules (Sponges 2002).A sponge is a hollow structure pierced by numerous incur-rent and excurrent pores through which water flows. There is species-specific variation in shape, which implies that there is also underlying developmental programming, and specialized functions such as contractile ability and color variation. However, sponges do not develop from a single cell in the highly orchestrated hierarchical way we will see is typical of more complex animals. Their lesser level of tissue commitment and organization can be seen by the ability of disaggregated cells to reaggregate into a normal whole, something "higher". Cells in vertebrate embryos have similar reaggregation properties but only early in embryogenesis (in a way that may be informative about evolution and mechanisms of early development) (Steinberg 1998). Environmental conditions can affect sponge morphology, reflecting their rather loose level of genetic programming.
Sponges can reproduce asexually by shedding buds or "gemmules," small groups of cells that float away to anchor at a new location (Darwin used the term to hypothesize ubiquitous rudimentary and rather lamarckian units of inheritance). Even the simple sponge can also differentiate into haploid sperm and eggs and undergo fertilization and the shedding of ciliated larvae. Reproduction can even be seasonal. Thus, without much real histology or organized form, some developmental complexity occurs.
Coelenterates (or Cnideria) include corals, jellyfish, and hydra. These species are diploblasts but make only a few specialized cell types, including the stinging nematocysts. They are typically bag-shaped with a single oral opening but can have internal septa, and they have more species-specific body plans than sponges. Co-elenterates reproduce asexually as sedentary polyps or via sexual, free-swimming forms known as medusae; their life cycle often includes both forms. Corals are aggregates of polyps, whose association is so highly constrained that it is somewhat moot whether they should be considered separate individuals. They exemplify organisms so integrated with each other and with the environment that they themselves construct that it is difficult to consider the organism as only what is within each unit (e.g., Turner 2000). Some forms branch in a simple treelike way as they grow, with branches occurring in a more or less random pattern but affected by external conditions. Coral polyps grow on acellular (corallite) stalks that occasionally divide into two similar units, each forming a new stalked polyp, overall resulting in a treelike structure that approximates a fractal pattern.
More Complex Animals: Basic Characteristics
Much of our knowledge about the remaining forms of animal life is generalized from the intense study of only a few model organisms (e.g., mice, fruit flies, nematodes, sea urchins) and a few special traits (e.g., eyes, early embryonic stages, limbs), usually studied for historical reasons (because so much is already known) or convenience (transparent embryos, short lifespan, easy to manipulate or raise in the laboratory). But the broad picture is likely to be representative of the general array of "strategies" used.
These "higher" forms of animal life essentially are committed to multicellular life derived from a single starting cell. Subsequently, a highly orchestrated four-dimensional dance of development in space and time takes place. Differentiated organ development includes a few simple processes: make a ball; change its shape by differential, asymmetric growth; fill space by local division of similar cells; pinch it; make a local bulge outward or indentation inward; close or open a hole in the ball forming tubes; and split growing areas into two or more branches. Cells then differentiate to produce specific substances such as extracellular materials that provide structural rigidity or other properties, perhaps comprising dead cells or protein debris (e.g., hair, lenses), or may become mineralized and quite hard, as in shell, bone, or cartilage. Some cells secrete particular functionally important products such as hormones or digestive enzymes.
Despite sharing these basic characteristics, sometimes in a highly programmed way within a given species, there is great variation in most aspects of development among species, sometimes even among closely related species. A few rounds of rather unspecialized cell divisions generically referred to as cleavage turn the single fertilized egg into a simple primary shape, generally a hollow ball (blastula). But there are various basic patterns, including very regular and symmetric division in echinoderms, many asymmetric patterns, and the initial formation of a syncytium, a large single cell that contains many nuclei not surrounded by nuclear membranes or cell walls (in many insects) (Gilbert 2003).
Commitment of cells quickly occurs in two basic ways that have been defined by manipulation of embryos, or more recently, of gene expression. Polarity is also established early and involves basic axes of symmetry mentioned above. In insects like Drosophila, polarity is established through gradients of maternal gene products in the syncytium. Vertebrates first separate a yolk or nutrient section from a section that will generate the embryo itself. Then, the AP and DV axes of the embryo form, instructed by signals produced by centers known as organizers that affect nearby cells that proliferate as growth extends away from the organizer region. For example, Spemann's organizer is a region of cells capable of establishing the early AP and DV axes in a vertebrate (the homologous structure has different names in different species), as is shown by its removal or surgical relocation in an embryo and by mutations that alter or prevent it from developing.
piwynx oviduct oocytes bulbs oocytes spermatheca
(for sperm storage)
rectum anus uterus piwynx oviduct oocytes bulbs oocytes spermatheca
(for sperm storage)
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