Worm Breeder's Gazette 9(2): 59
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.
From a screen (after EMS mutagenesis) for animals with morphologically abnormal gonads, we have isolated the mutation n1321. Mutants homozygous for n1321 have a fertile gonad of apparently normal morphology that is usually displaced to a position considerably anterior of normal. We have nicknamed this phenotype 'egghead' (Egh), since the eggs in the adult are shifted toward the head. In most n1321 L1 animals, the gonad primordium is located somewhat anterior of normal (with respect to the V and P cells and other body markers) and is in a ventral position (as in N2). In about 10% of the animals (at 20 C), the gonad primordium is located dorsally, separated from the ventral cord and hypodermis by the intestine. During postembryonic development the center of the gonad remains in whatever position it occupied in the L1. The gene defined by n1321 lies in the cluster on chromosome III in the deficiency interval covered by nDf16 but not nDf20. The Egh and dorsal gonad phenotypes of n1321 are recessive, but the mutation appears not to be null, because the position of the gonad primordium is shifted even further anterior in animals of genotype n1321/nDf16. We have identified three other developmental abnormalities in n1321 animals. All of these defects are apparently caused by the displacement of the gonad. First, the cells of the vulval equivalence group P(3-8).p can be altered in fate in a manner dependent on the position of the gonad (the gonad is required for the induction of vulval cell lineages in N2). Second, the sex myoblasts, which generate the vulval and uterine muscles that control egg laying, migrate further anterior than in the wild type, to a position determined by the displaced gonad. Third, the HSN neurons, which innervate the vulval muscles and drive egg-laying, have normally positioned cell bodies but appear to innervate the displaced vulval muscles, suggesting that their processes extend further anterior than normal before making synapses with the muscle cells. It is interesting to note that these three developmental alterations are precisely those necessary to ensure that an animal with a displaced gonad will be egg-laying proficient. The observations that have led to these conclusions are discussed below. 1. In many n1321 animals in which the gonad was mispositioned, the vulval cell lineages were generated by the three P(3-8).p cells located nearest the anchor cell (which is generated postembryonically within the somatic gonad and induces the vulval cell lineages in N2). For example, in several n1321 animals the vulva was formed from P(3-5). p, whereas in N2 it is formed from P(5-7).p. Induction of vulval lineages in n1321 animals is anchor cell dependent in cases of both ventrally and dorsally located gonads, since no vulval lineages were induced in 4/4 animals in which the gonad was ablated at the L1 stage ( two ventral and two dorsal gonads), and no vulval lineages are induced in animals of genotype n1321 lin-12(n302) (the gonads of which lack an anchor cell as a result of the lin-12 mutation). We conclude that the location of the anchor cell determines which cells of the vulval equivalence group will undergo vulval cell lineages. These observations support earlier conclusions reached by Paul Sternberg. The anchor cell can induce normal vulval cell lineages by P(3-8).p even when located dorsally, separated from them by about half the body width. In about 2/3 of the animals with a dorsally located gonad between one and four P(3-8).p cells are induced to undergo vulval lineages. Since the anchor cell is clearly not in contact with its target Pn.p cells at any time in these animals, this result means that the signal for induction of the vulval lineages is diffusible. A diffusible anchor cell signal is an important aspect of the model of Sternberg and Horvitz (Cell 44: 761-772, 1986) that determination of the vulval cell lineages occurs via a graded anchor cell signal. Since the gradation of the anchor cell signal should be altered in animals with a dorsally located anchor cell, we may be able to use n1321 to further test this model. 2. The migration of the hermaphrodite sex myoblasts (SMs) is altered in n1321 animals. Normally the two bilateral SMs migrate during the L2 stage from a posterior position to a position directly lateral to the middle of the developing gonad, and there undergo three rounds of division to generate the sex muscles. Several observations support a role for the gonad in determining the SM migration. First, the SMs in n1321 animals extend their migration further anterior than normal and stop when located directly lateral to the middle of the displaced gonad. Second, when the gonad is located dorsally the SMs migrate anteriorly along their normal ventral route but then turn and move dorsally when they have reached the midpoint of the gonad. Third, when the gonad primordium is ablated in the early L1 in either N2 or n1321 animals, the SMs migrate anteriorly as usual but stop in a variable position usually near, but rarely exactly at, the normal N2 position. The simplest explanation of these results is that the SMs themselves determine the direction and approximate extent of their migration, but a signal generated in the gonad determines their final position more precisely. Ablation of the germ line precursors Z2 and Z3 has no effect on the migration of the SMs in either N2 or n1321. However ablation of the somatic gonadal precursor cells Z1 and Z4 results in the characteristic migration defect described above. We conclude that the somatic but not the germ line gonad is required for the generation of the signal that directs the SM migration. 3. The embryonic migration of the HSN cell body is unaffected in n1321 animals (the HSN is located in the wild-type position with respect to the body). This observation means that the sex muscles that it normally innervates are abnormally far away anteriorly. Nevertheless, of eight young adult n1321 animals with vulvae that were displaced well anterior of normal, only one later became severely egg- laying defective (as is characteristic of animals lacking HSN function) . This result suggests that in seven of eight animals at least one HSN neuron properly innervated the sex muscles and could drive egg- laying. We infer that an HSN neuron can grow to its muscle targets and make proper synaptic connections even when its cell body is much further than normal from its target.