Worm Breeder's Gazette 13(3): 42 (June 1, 1994)

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.

Somatic Regulation of Germ-line Development Part III; Oocyte Meiotic Prophase Arrest

Jim McCarter[1], Tim Schedl[2]

[1]Dept. of Genetics, Washington Univ. School of Medicine, St. Louis, MO 63110, jim@wugenmail.wustl.edu
[2]Dept. of Genetics, Washington Univ. School of Medicine St. Louis, MO 63110, jim@wugenmail.wustl.edu

Meiotic arrest at prophase of MI in oocytes is a nearly universal phenomenon among metazoans. In C. elegans, oocytes in the proximal gonad are arrested at diakinesis of MI prophase until entry into the spermatheca and fertilization. In unmated females, large numbers of arrested oocytes stack in the arm until mating provides sperm.

Oocyte Meiotic Prophase Arrest Defective (Oar) Elimination of somatic cells can result in failure of oocyte meiotic arrest in the gonad arm. Laser ablation of 1 SS cell (see figure in first abstract) produces oocytes which leave diakinesis arrest and undergo multiple cycles of DNA synthesis (44% Oar, n=98). While karyokinesis and cytokinesis do not occur, presumably because C. elegans oocytes lack centrioles, multiple cycles of chromosome condensation and decondensation take place. This endomitotic process results in a greatly enlarged polyploid nucleus in the oocyte.

We postulate that the Oar phenotype arises in oocytes following ablation because of a reduction in a soma to oocyte signal for arrest. Alternatively, oocytes could fail to maintain arrest because the myoepithelial sheath, which contracts to push them into the spermatheca has been partially eliminated; as a result, oocytes stay in the gonad 'too long' and leave arrest. The ability of oocytes in unmated females to maintain arrest indefinitely, however, argues against this alternative hypothesis. Females do not appear to have additional mechanisms for arrest not found in hermaphrodites. Feminized gonad arms display a similar frequency of polyploid oocytes following ablation (from part II, and ablations in fog mutants). This also indicates that the Oar phenotype is independent of the presence of sperm.

The Oar phenotype, as observed following ablation and in the oar mutants (described below), is distinct from that of polyploid unfertilized oocytes found in the uterus of hermaphrodites that have exhausted their sperm, and in spe or fer mutants. Such oocytes complete meiosis I upon entry into the spermatheca and become polyploid in the uterus(1), whereas polyploid oocytes in Oar animals are found in the gonad arm itself. Employing the fully penetrant oar-1 ( oz1 )mutant (see below), Oar endomitosis has been observed without any evidence of the meiosis I division having occurred.(2)

Ablations are in progress to further localize the somatic cells responsible for maintenance of arrest. The Oar phenotype does not result from ablation of DUs, the AC, or DTCs, or control ablations of germ cells. Ablation of proximal sheath (and possibly distal spermatheca) in LA, can result in Oar, whereas ablation of the distal sheath pair, proximal spermatheca, or the spermatheca-uterine valve has no effect. This suggests that the cells needed for maintenance of arrest are in the immediate vicinity of the arrested oocytes and that their time of signalling may be late (IA or adult).

Conclusions Our ablation results indicate that somatic gonad cells signal oocytes for the maintenance of meiotic arrest. The situation may be similar in some other metazoans; mammalian oocytes removed from the surrounding follicle cells also leave arrest.(3)

Somatic Gonad Genetics

Mutations that have one or more of the germ-line phenotypes described in Parts I, II, and III may identify genes that are directly or indirectly involved in somatic gonad to germ-line signalling. R. Francis and MT. Le in the lab have characterized lin( oz128 )II which transforms SS cells (Z1.ap and Z4 .pa)into extra DTCs.(4) lin( oz128 )animals often have four gonad arms (a Shiva phenotype). The lin( oz128 )germline mimics all three phenotypes seen in the SS ablations. Iin( oz128 )arms missing the descendants of 1 SS can be Oar and/or Fog. lin( oz128 )arms established by extra DTCs lack all other somatic cells and are Glp-Miniature. ceh-18 ,a POU-domain gene characterized by D. Greenstein et. al., is expressed in the sheath and DTCs, and may be involved in specifying the fate of these somatic gonad cells. ceh-18 mutants show incompletely penetrant Oar and Glp-Miniature germ-line phenotypes.(5)

Oar Genetics

Mutations have also been isolated which show solely a completely penetrant Oar phenotype. Oar-l( oz1 )V was characterized by K. Iwasaki.(6) Oar-2 ( oz136 )III, Oar-3 ( oz138 )IV, Oar-4 ( oz145 )V, Oar-5 ( oz148 )II, and Oar-6 ( oz154 )are being characterized. We postulate that oar mutants can arise by: disrupting somatic cell fate or differentiation, blocking generation of an arrest signal in the somatic gonad or its reception in the oocyte, or bypassing a oocyte meiotic cell cycle checkpoint.

Literature Cited:

(1) Ward and Carrel. Dev. Bio. 73:304 321,1979.

(2) Iwasaki, Francis, and Schedl. Unpublished Observations, 1993.

(3) reviewed in Current Topics in Dev. Bio. 28: 125- 153, 1993., and Int. Review of

Cytology. 57: 185-282, 1979.,

(4) McCarter, et. al. 1993 Worm Meeting, p304 .,and Francis, Le, and Schedl. Unpublished Observations, 1993.

(5) Greenstein, et. al. Submitted, 1994.

(6) Iwasaki and Schedl. 1993 Worm Meeting, p.214.