Identifying novel suppressors of memi-1(sb41) and their possible roles in meiosis
Creators: Herzog, Jens A
Publisher: name: University of Alberta Library
Year: 2018

<p>Meiosis consists of a highly regulated pair of cell divisions, which ensures haploidization of gametes. In C. elegans oogenesis, the first meiotic division (MI) proceeds only after the oocyte receives a diffusible signal from the sperm. Meiosis II (MII) proceeds only after fertilization; if the diffusible signal is received but the sperm do not fertilize, the oocytes abort MI, skip MII, and enter mitosis. This suggests that the sperm communicates with the oocyte both before and during fertilization which facilitates proper meiotic progression. The Srayko lab previously identified three highly-similar genes called memi-1, memi-2, and memi-3, which represent good candidates for “sensing” sperm entry. The memi genes are expressed in the female germline and are functionally redundant. When all three genes are targeted via memi-1/2/3(RNAi), oocytes are fertilized normally, but they abort MI, skip MII, and enter mitosis. Interestingly, a presumed hypermorphic mutation, memi-1(sb41), results in oocytes that become “stuck” in MII, until they eventually abort and progress into an abnormal mitosis. To find other components of the MEMI pathway important in the meiosis-to-mitosis transition, the Srayko lab conducted a mutagenic suppressor screen that yielded 27 suppressors of memi-1(sb41), of which 10 were found to be intragenic in initial screening. I investigated the remaining 17 memi-1(sb41) suppressors. Using classical genetic techniques, I identified 10 novel suppressor mutations of memi-1(sb41) and began characterization of the mutations. I also narrowed the number of possible suppressing mutations for the remaining 7 suppressor strains, from several thousand to 84. There is a problem though, in evaluating memi-1(sb41) because of the redundant action of memi-2/3. Previous experiments have shown that increasing the number of WT copies of memi-1 in a heterozygous memi-1(sb41) background increased the severity of the maternal-effect lethal (Mel) phenotype. This suggests that the Mel phenotype of memi-1(sb41) results in an increase in MEMI activity, which would make it a hypermorph. However, as the copies of memi act redundantly it is possible that memi-2/3 compensate for a loss of normal memi-1(sb41) function. To investigate if this was the case I lowered the activity of memi-2/3 through introducing deletions would also suppress memi-1(sb41), and tested the assumption that memi-1(sb41) continues to act redundantly in the absences of memi-2/3. I confirmed that deletions of different memi genes did improve embryonic viability in a memi-1(sb41) background. However, I also found that while memi-1(+) memi-2Δ memi-3Δ worms were viable, memi-1(sb41) memi-2Δ memi-3Δ were not. This indicated that MEMI-1(sb41) protein function, alone, is likely insufficient for meiotic processes, suggesting that it may be a neomorph. The memi-1(sb41) memi-2Δ memi-3Δ worms did show an improvement in embryonic viability when mated to males of the same genotype, suggesting that physiological differences between male and hermaphrodite sperm can impact the MEMI pathway, further strengthening the import of sperm derived factors in oocyte meiosis. Together this work provides multiple insights into the memi-1(sb41) mutation and potential interactors which will prove useful in future inquiries.</p>