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Modern Genetic Analysis Chapter 16. The Genetic Basis of Development
Binary Fate Decisions: the Germ Line Versus the Soma

In animal development, the earliest developmental decision is that of separating the germ line from the soma. Once this separation occurs, it is irreversible. Germ cells do not contribute to somatic structures. Somatic cells cannot form gametes, and thus their descendants never contribute genetic material to the next generation. This early separation means that genetic or regulatory modifications of somatic cells that occur in the course of development have no consequences on gamete formation.

In making this decision of germ line versus soma, the embryo exploits its machinery for creating asymmetriesthe cytoskeleton, the girders that support and shape the cellto localize a germ-line determinant to a subset of early embryonic cells. Before directly addressing the question of how the germ-line versus soma decision is made, we need to consider the nature of cytoskeletal and cellular asymmetries.

The Cytoskeleton of the Cell

The cytoskeleton consists of several networks of highly organized structural rods that run within each cell: microfilaments, intermediate filaments, and microtubules (Figure 16-7). Each has its own architecture formed of unique sets of protein subunits and proteins that promote production or disassembly of the rods. Furthermore, each type of rod forms higher-order networks through different sets of proteins that reversibly cross-link the individual rods to one another.

Several roles of the cytoskeleton are relevant to our discussion of pattern formation: control of the location of the mitotic cleavage plane within the cell, control of cell shape, and directed transport of molecules and organelles within the cell. All of these roles depend on the fact that the cytoskeletal rods are polar structures. The contributions to pattern formation of microfilaments and microtubulespolymers of actin and tubulin subunits, respectivelyare better documented, and so we will focus on these two classes of cytoskeletal elements.

The Intrinsic Asymmetry of Cytoskeletal Filaments

The polarity of microfilaments and microtubules is crucial to their roles as intracellular highways. The ability of other molecules to move up and down these highways is an important aspect of all of their cellular roles. Microfilaments and microtubules are linear polymers with polarityconceptually like the 5-to-3 polarity of DNA and RNA strands, even though the mo-lecular basis of polarity is quite different (Figure 16-8). Furthermore, the polarity of the cytoskeletal elements can be organized within a cell. Consider microtubules. Near the center of most cell types, all the "" (minus) ends of the microtubules are found (Figure 16-9). This location is called the microtubule organizing center (MTOC). The "+" (plus) ends of microtubules are located at the periphery of the cell. Very much as an automobile uses the combustion of gasoline to create energy that is then transduced into motion, special "motor" proteins hydrolyze ATP for energy that is utilized to propel movement along a microtubule. For example, a protein called kinesin is able to move in a to + direction along microtubules, carrying cargoes such as vesicles from the center of the cell to its periphery (Figure 16-10a and b on the following page). The motorthe part of the kinesin protein that directly interacts with the microtubule rodis contained in the globular head of the protein (Figure 16-10c). The tail of kinesin is thought to be where the cargo is attached. These cargoes might be individual molecules, organelles, or other subcellular particles to be towed from one part of the cell to another. (Comparable motors exist for actin microfilaments.)

What is the value of having multiple independent cytoskeletal systems? A part of the answer is probably division of labor. Just like cities have complex grids of intersecting streets to permit travel from a starting point to any other location, cells use multiple cytoskeletal systems to move cargo from one part of the cell to any other.

MESSAGE
The cytoskeleton serves as a highway system for the directed movement of subcellular particles and organelles.

Localizing Determinants through Cytoskeletal Asymmetries: The Germ Line

In many organisms, a visible particle is asymmetrically distributed to the cells that will form the germ line. These particlescalled P granules in Caenorhabditis elegans, polar granules in Drosophila, and nuage in frogsare thought to be transport vehicles that ride on specific cytoskeletal highways to deliver the attached germ-cell determinants (regulatory molecules) to the appropriate cell. In C. elegans and Drosophila, the evidence relating germ-line determination to the cytoskeleton is particularly strong. We will consider both of these cases.

The early cell divisions of the C. elegans zygote provide an example of how cytoskeletal asymmetries help form the germ line. One of the favorable properties of C. elegans as an experimental system is that the same pattern of cell divisions occurs from one animal to anothera pattern that can be readily followed under the microscope. A lineage tree can then be constructed that traces the descent of each of the thousand or so somatic cells of the worm (refer to Genetics in Process 15-2).

The one-cell zygote of C. elegans that is produced on fertilization is called the P0 cell. It divides asymmetrically across the long axis of the ellipsoidal P0 cell to produce a larger, anterior AB cell and a smaller, posterior P1 cell (Figure 16-11). This is a very important division in that it already sets up specialized roles for the descendants of these first two cells. The AB cell descendants will produce most of the skin cells of the worm (the hypoderm) and most of the neurons of the nervous system, whereas most of the muscles and all the digestive system and the germ-line cells will come from the P1 cell.

The germ-cell fate in the earliest divisions of the P0 cell and its posterior descendants (P1, P2, and so forth) correlate with the distribution of certain fluorescent cytoplasmic particles called P granules. These granules are incorporated exclusively into the P1 cell at the first division. When the P1 cell divides, also asymmetrically, the P granules are incorporated into the progeny P2 cell and, similarly, at the next division into the P3 cell and so forth. Only the Px cell that has these P granules becomes the germ line of the wormall other cells are somatic. The asymmetric distribution of P granules is microfilament dependent. When applied at the right time to the P0 cell, drugs such as cytochalasin disrupt actin subunit polymerization into microfilaments. After disruption of microfilament polymerization, P granules are distributed symmetrically between the two progeny cells. (Presumably because other fate determinants are abnormally distributed owing to the actin disruption, the resulting embryos are quite "confused" and die as masses of cells that look nothing like a normal worm.)

In early Drosophila development, the cytoskeleton also is exploited to localize the structure containing the germ-line determinants: the polar granules. During oogenesis in the ovary of the mother, the polar granules are constructed and become tethered to the posterior pole of the oocyte by virtue of their attachment to one end of the microtubules. They remain in this location throughout early embryogenesis until nuclear division 9, when a few nuclei migrate to the posterior pole. (Note that an unusual feature of early Drosophila development is that the first 13 mitoses are nuclear divisions without concomitant cytoplasmic division, making the early embryo a syncitiuma multinucleate cell.) After nuclear division 9, the plasma membrane of the oocyte evaginates at the posterior pole to surround each nucleus and pinches off some of the polar granule-containing cytoplasm. This creates the pole cells, the first mononucleate cells of the embryo and the cells that will uniquely form the fly's germ line (Figure 16-12).

How do the polar granules get tethered to the posterior pole of the oocyte? Again, the subcellular localization is accomplished by one of the cytoskeletal networks. In contrast with C. elegans, in which the actin-based microfilaments seem to form the polar structure to which the P granules attach, here the tubulin-based microtubules provide the essential asymmetry. Probably this is just an accident of the evolutionary history of these organisms. In each case, the germ-line determinant evolved to co-opt any cytoskeletal system that had the appropriately oriented asymmetry.

© 1999 by W. H. Freeman and Company. All rights reserved.