Positional cloning and characterization of Mei1, a vertebrate-specific gene required for normal meiotic chromosome synapsis in mice.

The mouse meiotic mutant Mei1 was isolated in a screen for infertile mice descended from chemically mutagenized embryonic stem cells. Homozygotes of both sexes are sterile due to meiotic arrest caused by defects in chromosome synapsis. Notably, RAD51 protein does not load onto Mei1 mutant meiotic chromosomes, suggesting that there is a defect in either recombinational repair or the production of double-strand breaks (DSBs) that require such repair. Here, we show that treatment of mutant males with cisplatin restores RAD51 loading, suggesting that mutant spermatocytes have intact recombinational repair mechanisms. Levels of histone H2AX phosphorylation (gammaH2AX) at leptonema are significantly reduced compared with wild-type controls but comparable to that seen in animals deficient for SPO11, the molecule required for catalyzing DSB formation during meiosis. These observations provide evidence that genetically programmed DSB induction is defective in Mei1 leptotene spermatocytes. We also report the positional cloning of Mei1, which encodes a product without significant homology to any known protein. Expressed almost exclusively in gonads, Mei1 has no apparent homologs in yeast, worms, or flies. However, Mei1 orthologs are present in the genomes of mammals, chickens, and zebrafish. Thus, Mei1 is required for vertebrate meiosis. To our knowledge, Mei1 is the first meiosis-specific mutation identified by forward genetic approaches in mammals.

Loss of srf-3-encoded nucleotide sugar transporter activity in Caenorhabditis elegans alters surface antigenicity and prevents bacterial adherence.

During the establishment of a bacterial infection, the surface molecules of the host organism are of particular importance, since they mediate the first contact with the pathogen. In Caenorhabditis elegans, mutations in the srf-3 locus confer resistance to infection by Microbacterium nematophilum, and they also prevent biofilm formation by Yersinia pseudotuberculosis, a close relative of the bubonic plague agent Yersinia pestis. We cloned srf-3 and found that it encodes a multitransmembrane hydrophobic protein resembling nucleotide sugar transporters of the Golgi apparatus membrane. srf-3 is exclusively expressed in secretory cells, consistent with its proposed function in cuticle/surface modification. We demonstrate that SRF-3 can function as a nucleotide sugar transporter in heterologous in vitro and in vivo systems. UDP-galactose and UDP-N-acetylglucosamine are substrates for SRF-3. We propose that the inability of Yersinia biofilms and M. nematophilum to adhere to the nematode cuticle is due to an altered glycoconjugate surface composition of the srf-3 mutant.

The mouse meiotic mutation mei1 disrupts chromosome synapsis with sexually dimorphic consequences for meiotic progression.

mei1 (meiosis defective 1) is the first meiotic mutation in mice derived by phenotype-driven mutagenesis. It was isolated by using a novel technology in which embryonic stem (ES) cells were chemically mutagenized and used to generate families of mice that were screened for infertility. We report here that mei1/mei1 spermatocytes arrest at the zygotene stage of meiosis I, exhibiting failure of homologous chromosomes to properly synapse. Notably, RAD51 failed to associate with meiotic chromosomes in mutant spermatocytes, despite evidence for the presence of chromosomal breaks. Transcription of genes that are markers for the leptotene and zygotene stages, but not genes that are markers for the pachytene stage, was observed. mei1/mei1 females are sterile, and their oocytes also show severe synapsis defects. Nevertheless, unlike arrested spermatocytes, a small number of mutant oocytes proved capable of progressing to metaphase I and attempting the first meiotic division. However, their chromosomes were unpaired and were not organized properly at the metaphase plate or along the spindle fibers during segregation. mei1 was genetically mapped to chromosome (Chr) 15 in an interval that is syntenic to human Chr 22q13. This region, which has been completely sequenced, contains no known homologs of genes specifically required for meiosis in model organisms. Thus, mei1 may be a novel meiotic gene.

Toward the genetics of mammalian reproduction: induction and mapping of gametogenesis mutants in mice.

The genetic control of mammalian gametogenesis is inadequately characterized because of a lack of mutations causing infertility. To further the discovery of genes required for mammalian gametogenesis, phenotype-driven screens were performed in mice using random chemical mutagenesis of whole animals and embryonic stem cells. Eleven initial mutations are reported here that affect proliferation of germ cells, meiosis, spermiogenesis, and spermiation. Nine of the mutations have been mapped genetically. These preliminary studies provide baselines for estimating the number of genes required for gametogenesis and offer guidance in conducting new genetic screens that will accelerate and optimize mutant discovery. This report demonstrates the efficacy and expediency of mutagenesis to identify new genes required for mammalian gamete development.