The MAP kinase phosphatase LIP-1 is required for the meiotic cell-cycle arrest in developing oocytes

The RAS/MAP kinase signaling pathway plays an essential role at two steps during hermaphrodite germline development. First, RAS/MAP kinase (MPK-1) signaling is required for the progression of germ cells through the pachytene stage of meiotic prophase I (Lee et al. 2001, IWM abstract 991). After pachytene exit, MPK-1 is rapidly inactivated and the developing oocytes arrest in the diakinesis stage of meiotic prophase I. As the oocytes approach the spermatheca, the secreted sperm signal MSP causes the re-activation of MPK-1, which induces oocyte maturation and allows the meiotic cell cycle to progress (Miller et al. 2001).

We have previously shown that the dual specificity phosphatase LIP-1 acts as a negative regulator of MPK-1 during vulval induction (Berset et al. 2001). Here, we report an important role for LIP-1 in regulating MPK-1 activity during germline development. A lip-1 loss-of function-mutation (zh15) suppresses the pachytene arrest caused by reduction-of-function mutations in mpk-1 (oz140 or ga111). Consistent with this genetic interaction, we detected anti-LIP-1 antibody staining in pachytene stage germ cells but no staining at other stages of germline development. Furthermore, the proximal gonads of lip-1(zh15)mutants contain more but smaller oocytes than wild-type gonads (12.3±4.1 oocytes in lip-1(zh15) as opposed to 7.9±1.6 in wild-type gonads). The rate of oocyte maturation, on the other hand, is unchanged. Thus, germ cells in lip-1(zh15) mutants exit the pachytene stage at an increased rate, resulting in an overall acceleration of oocyte development. Moreover, by staining gonads with an antibody specific for the diphosphorylated, activated form of MAP kinase (anti-DP-ERK) we found that MPK-1 fails to be inactivated after germ cells exit the pachytene stage in lip-1(zh15)mutants. The increased levels of activated MPK-1 persist throughout the diakinesis stage until fertilization occurs. Interestingly, in 9% of lip-1(zh15) single mutants a few (< 5) unfertilized oocytes per gonad arm display an endomitotic (Emo) phenotype, they start multiple mitotic cell cycles without undergoing cytokinesis. This Emo phenotype is more penetrant (>80%) and stronger (usually >20 mitotic nuclei per gonad arm) in feminized (fem-2 or fog-1) lip-1(zh15) mutants that produce no sperm. Thus, unfertilized oocytes in lip-1(zh15) mutants often fail to arrest the meiotic cell-cycle.

Taken together, our results indicate that LIP-1 is responsible for the inactivation of MPK-1 after germ cells exit the pachytene stage. The inactivation of MPK-1 by LIP-1 is critical to allow the developing oocytes to arrest the meiotic cell cycle in the diakinesis stage until oocyte maturation is induced.