Worm Breeder's Gazette 16(2): 43
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.
Johns Hopkins University School of Medicine, Baltimore, MD 21205
In C. elegans, A/P polarity is established after fertilization and is thought to be determined by the site of sperm entry, which defines the posterior pole of the embryo (Goldstein and Hird, 1996). The nature of the cue brought in by the sperm is not known. Using a collection of 1-cell arrested mutants, we have obtained evidence that the cue may be provided by microtubules emanating from the sperm asters.
From a screen for temperature sensitive embryonic lethal mutants, we isolated 30 mutants that arrest in the one-cell stage. In collaboration with the labs of Andy Golden and Diane Shakes, we found that a subset of these mutants arrest at the Metaphase to Anaphase Transition of meiosis I (25 mat mutants defining 5 loci). These mutants arrest with the maternal meiotic spindle at one end of the embryo ("maternal side") and with condensed sperm-derived chromatin at the other end ("paternal side"). To determine whether these embryos are polarized along the A/P axis, we stained mat embryos for PIE-1, a maternal factor that in wild-type segregates to the posterior (paternal side) shortly after the completion of meiosis. To our surprise, we found that in mat mutants PIE-1 segregates towards the maternal side rather than the paternal side.
Further examination of mat embryos led to the following observations:
- In addition to PIE-1, PAR-1, PAR-2, and PAR-3 are also present in reverse orientation in mat embryos. In contrast, P granules do not appear to segregate in these embryos.
- As is true in wild-type, the asymmetric segregation of PIE-1 in mat embryos requires par-1 and par-3, and the asymmetric segregation of PAR-3 does not require par-1. These observations suggest that, even though A/P polarity is reversed in mat embryos, the regulatory hierarchy of par genes is maintained.
- PIE-1 and PAR-2 segregation to the maternal side in mat embryos is sensitive to drugs that affect the integrity of the meiotic spindle (nocodazole and taxol). Indeed PAR-2 localization on the cortex correlates tightly with the position of the meiotic spindle.
- cdc2; mat-1 double mutants, which arrest before the onset of meiosis and do not form a meiotic spindle, fail to segregate PIE-1.
These data strongly suggest that in mat embryos the meiotic spindle defines the posterior end of the embryo. These observations suggest that a locally high concentration of microtubules may be sufficient to polarize the embryo. We propose that in wild-type, this high concentration is normally provided by the sperm-derived centrosomes that nucleate many microtubules after completion of meiosis. In contrast, in mat mutant embryos, where the sperm centrosomes never mature and the maternal meiotic spindle persists longer than normal, the meiotic spindle polarizes the embryo. This model is consistent with the findings of O’ Connell et al. who have shown that mutations in spd-2, which delay and attenuate sperm aster formation, eliminate A/P polarity (O’Connell, Maxwell and White, 1999 C. elegans meeting abstract 638).