Worm Breeder's Gazette 5(1): 27
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Two descendants of the gonadal somatic precursor cells, Z1 and Z4, appear to regulate germ cell development in both sexes. These two cells - the distal tip cells - arise in homologous positions in the Z1- Z4 lineage during L1. In hermaphrodites, one distal tip cell resides at the growing tip of each gonadal arm. In males, both distal tip cells reside at the stationary end, and the linker cell precedes the elongating tip. Ablation of both distal tip cells has two effects on germ cell development in both sexes : divisions cease among all descendants of the germ cell precursors, Z2 and Z3, and all Z2 and Z3 descendants enter meiosis. In hermaphrodites, ablation of the distal tip cell(s) stops elongation of the relevant gonadal arm. In males, the elongation or leader function is allocated to a different somatic cell, the linker cell. When the linker cell is destroyed, elongation is blocked, but proliferation of the germ cells continues to generate a large oval mass instead of a reflexed tube. The laser ablation experiments reported here address three questions concerning the control of germ cell development: 1) Is the primary function of the distal tip cells stimulation of mitosis or suppression of meiosis? 2) How is the spatial organization of the germ cell component of the gonad established and maintained? 3) Is the sperm/oocyte decision in hermaphrodites made according to a lineage mechanism? In the intact animal, the distal tip cells might stimulate mitosis which secondarily suppresses entry into meiosis, or they might suppress entry into meiosis which secondarily permits proliferation of the germ cells. Superficially, these two possibilities seem difficult to distinguish. However, because germ cells become meiotic at a characteristic point in development, one can ask how that time of entry is affected by various experimental manipulations. Such knowledge can be used to infer what role the distal tip cells play in the decision of Z2-Z3 descendants to enter meiosis. The following experiments suggest that the primary function of the distal tip cells is to suppress meiosis. After ablation of the distal tip cells as early as possible in L1 the germ cells undergo 2-3 rounds of division and enter meiosis at approximately the same time as in unoperated animals. After ablation of the distal tip cells during L2, the germ cells undergo no more than one round of division and, again, enter meiosis at the appropriate time. If the distal tip cells serve a purely mitogenic function, one would expect to detect meiotic nuclei earlier than normal in these experiments, and one would expect the germ cells to stop dividing after an equivalent number of divisions in all experiments. However, if the distal tip cells primarily suppress meiosis, one would expect the germ cells to continue to divide until signalled to enter meiosis if the distal tip cells had been ablated. A second experiment is consistent with this hypothesis. It is possible to extend the time during which the distal tip cells influence the germ cells by reducing the number of germ cells early during development. For example, if Z2 or Z3 is ablated just after hatching, the number of germ cells is effectively reduced by half at any time during gonadogenesis. Such experiments show that entry into meiosis can be delayed significantly by reducing the size of the gonad and thereby bringing the distal tip closer to the most proximal point in the germ cell tube. The effect of the distal tip cells on germ cells also sheds light on how the polarity and spatial organization of a gonadal arm is established and maintained. In the adult, the germ cells are arranged to display a progression of maturation along the distal-proximal axis, with mitotic cells located most distally and the most mature meiotic cells found most proximally. This organization is established and maintained when all the gonadal somatic cells except the distal tip cells are ablated during L1. In contrast, ablation of the distal tip cells severely disrupts that organization. It therefore seems likely that the distal tip cells are responsible for establishing the polarity of the germ cell component of the gonad. The most appealing mechanism for this is that, as distal tip cells become positioned progressively further away from the proximal end, germ cells escape their influence and enter meiosis. Such a scheme would produce the gradient of maturation observed in the gonad. The ability to manipulate the number of germ cells made (by ablation of the distal tip cells) has been used to study the sperm oocyte decision in hermaphrodites. If ablation of both distal tip cells is performed during L1 or L2, all the germ cells made differentiate as sperm. This means that no oocyte precursor is set aside irreversibly early during development. Since the decision whether or not to produce oocytes seems to be made during L1, based on shift experiments of the ts transformer mutant B202, this argues that a lineage mechanism is not relevant to this decision. When the distal tip cells are ablated later and later, the number of sperm precursor cells increases until the normal complement of sperm is made per arm. Once this number is reached one begins to see oocyte differentiation. Experiments are in progress to try to understand how the number of sperm precursor cells is determined.