Germline cyst formation in Drosophila.

INTRODUCTION

Gametes develop in association with specialized cells that assist their growth and differentiation. Despite plentiful variation, two types of germ cell interactions can be recognized. First, in many species, germ cells associate with somatic cells of distinct embryological origin, for example with testicular Sertoli cells, Drosophila spermatogenic cyst cells, or ovarian follicle cells. Beginning in the early embryo when migrating germ cells approach the presumptive gonad, interactions between germline and somatic cells mediate gonad formation, sex determination, growth and gametogenesis. In some organisms somatic and germ cell cytoplasms are brought into direct contact via intercellular bridges (as in lizards; 9), and sometimes they even fuse completely (as in gall midges; 6).

Second, and equally important, are interactions among germline-derived cells that are joined together in small clusters or cysts. Germ cell clusters arise by a process of incomplete cytokinesis, which links the daughter cells via distinctive, stable intercellular bridges known as ring canals. Spermatogenesis in all animals except nematodes is thought to take place in cysts, and a wide variety of vertebrate and invertebrate oocytes develop at least for a time within similar clusters. In ovarian cysts, generally all cells but one differentiate as nurse cells and transport materials through the ring canals to accelerate the growth of the remaining cell, which becomes the oocyte. Developing cysts often contain a large and unusual cytoplasmic organelle, the fusome, that is thought to play a central role in cyst formation and oocyte determination. Understanding germline cysts and the role played by the fusome in their construction and function remains a fundamental issue in the study of gamete development.

This review examines the molecular and cellular processes underlying germline cyst formation and function. Several previous summaries (6, 7, 40, 89) continue to provide invaluable information on the phylogenetic distribution of cyst types and on the cellular mechanisms of their formation. Because molecular genetic studies of germline cysts have been carried out primarily in Drosophila melanogaster, we focus on this species, whose gametogenesis has been broadly reviewed (24, 84). Particular aspects of Drosophila cyst development have also been summarized elsewhere, including ring canal structure (16, 77, 78), fusome biogenesis (65), germline stem cell function (53), and regulation of cyst production (84a).

FUNCTION

The occurrence of germline cysts among phylogenetically diverse organisms suggests that they serve an important function. Male gametes in particular almost universally develop in tightly synchronized clusters. Developing sperm are thought to benefit from interconnection because they differentiate after completing meiosis and consequently contain different genomes (4). Because gene products are shared through persistent ring canals, selection for genotypes that favor particular sperm over others (meiotic drive) and the production of nonfunctional gametes may be reduced. Efficient sharing probably requires that sperm develop synchronously, since some gene products might be needed at particular times and be deleterious at other times. Moreover, a synchronization mechanism might mitigate meiotic drive by linking the fates of advantaged and disadvantaged gametes.

Cysts play a very different role in egg development. Developing oocytes remain genetically homogeneous since they complete the bulk of their development prior to meiosis, but they must grow to enormous size and accumulate large amounts of stored materials. Consequently, in many female cysts, all but one of the cystocytes differentiate into nurse cells that support the growth of a single oocyte. Organisms such as higher insects that use this strategy are able to produce relatively large eggs much more quickly than species that lack nurse cells. Moreover, relieving the oocyte nucleus of synthetic duties allows it to become inactive, which may reduce its susceptibility to mutagenesis and parasitism. Surprisingly, cystocyte synchrony is lost after female cysts form, despite the fact that ring canals persist and continue to enlarge. Transport through ring canals is a highly regulated and directional process (16, 77, 78); either female cysts have evolved mechanisms to prevent the sharing of cell cycle regulators in mature cysts, or male cysts have developed an active system to use the ring canals to effect synchrony.

MORPHOGENETIC PRINCIPLES

Phylogenetic comparisons of cyst structure among insects (6) provide several clues to the principles guiding their construction. Nearly all male cysts, and female cysts from most insect species, have characteristic properties. First, the number of cells per cyst is usually a power of two, i.e. [2.sup.n], though n can range from 1 (2-cell cysts) in some caddis flies (Trichoptera) to at least 8 (256-cell cysts) in the Strepsipteran Elenchinus japonicus (6, 28). Male and female cysts in the same species often differ in size; for example, in many Lepidopterans, females form 8-cell cysts whereas male cysts contain 64 cells prior to meiosis (89). In cases where developing cysts have been studied, the [2.sup.n] rule can be seen to result from the fact that the cystocytes' mitotic cycles are synchronized from the very beginning [ILLUSTRATION FOR FIGURE 1A OMITTED]. For example, a 16-cell cyst (24) is produced from a single cell, a cystoblast, that undergoes four synchronous rounds of mitosis, giving rise to 2, 4, 8, and finally 16 cells. In Drosophila melanogaster, both male and female clusters contain 16 cells at the end of mitosis; male cystocytes will eventually undergo meiosis, producing a mature cyst of 64 cells. There are exceptions to the [2.sup.n] rule, however. Ovarian cysts in lacewings (Neuroptera) contain 12-14 cells and in honeybees (Apis) approximately 48, apparently due to the failure of some cells to participate in all rounds of division (6). Despite these exceptions, the general validity of the [2.sup.n] rule emphasizes the importance of understanding the mechanisms that synchronize cystocyte mitoses.

A second general principle of cyst organization is their"maximally branched" pattern of interconnections. Cysts with 2 or 4 cells can only be interconnected in a linear fashion, but larger cysts can be linear or branched, depending on how new cells arise during subsequent divisions [ILLUSTRATION FOR FIGURE 1B OMITTED]. If new cells intercalcate between older cells, then the cyst will remain linear; but if new cells branch off of the chain of older cells, then the cyst will be branched. How the cells behave depends upon how preexisting ring canals segregate between daughter cells during subsequent divisions. For example, as 4 cells divide to form 8, if each daughter cell retains one preexisting ring canal, then new cells will intercalcate between older cells; but if 4 of the daughter cells retain all preexisting ring canals, and the other 4 none, then new cells will branch off of older cells. Cysts in the great majority of species always contain the maximum possible number of branches, however, indicating that all preexisting ring canals must segregate to only one of the daughter cells in each pair, at every division (89). In such cysts, the number of ring canals in the two cells arising at the first division, is always equal to n. For example, in the 16 - cell cysts [(2.sup.4)] of Drosophila, the two oldest cells always contain 4 ring canals each.…