Isolation and characterization of a sperm-specific gene family in the nematode Caenorhabditis elegans

Hydrogen peroxide produced by superoxide dismutase SOD-2 activates sperm in Caenorhabditis elegans

Superoxide dismutase (SOD) is a ubiquitous antioxidant enzyme that catalytically converts the superoxide radical to hydrogen peroxide (H₂O₂). In mammals, high SOD activity is detectable in sperm and seminal plasma, and loss of SOD activity has been correlated with male infertility; however, the underlying mechanisms of sperm infertility remain to be clarified. Here we report that the deletion of two major SOD genes in Caenorhabditis elegans, sod-1 and sod-2, causes sperm activation defects, leading to a significant reduction in brood size. By examining the reactivity to the sperm activation signals Pronase and triethanolamine, we found that sod-1;sod-2 double mutant sperm cells display defects in pseudopod extension. Neither the content nor oxidative modification of major sperm protein, an essential cytoskeletal component for crawling movement, were significantly affected in sod-1;sod-2 mutant sperm. Surprisingly, H₂O₂, the dismutation product of SOD, could activate sod-1;sod-2 mutant sperm treated with Pronase. Moreover, the H₂O₂ scavenger ebselen completely inhibited pseudopod extension in wild-type sperm treated with Pronase, and H₂O₂ could directly induce pseudopod extension in wild-type sperm. Analysis of Pronase-triggered sperm activation in sod-1 and sod-2 single mutants revealed that sod-2 is required for pseudopod extension. These results suggest that SOD-2 plays an important role in the sperm activation of C. elegans by producing H₂O₂ as an activator of pseudopod extension.

Toward a physical map of the genome of the nematode Caenorhabditis elegans

A technique for digital characterization and comparison of DNA fragments, using restriction enzymes, is described. The technique is being applied to fragments from the nematode Caenorhabditis elegans (i) to facilitate cross-indexing of clones emanating from different laboratories and (ii) to construct a physical map of the genome. Eight hundred sixty clusters of clones, from 35 to 350 kilobases long and totaling about 60% of the genome, have been characterized.

Conservation of MAP kinase activity and MSP genes in parthenogenetic nematodes

Background

MAP (mitogen-activated protein) kinase activation is a prerequisite for oocyte maturation, ovulation and fertilisation in many animals. In the hermaphroditic nematode Caenorhabditis elegans, an MSP (major sperm protein) dependent pathway is utilised for MAP kinase activation and successive oocyte maturation with extracellular MSP released from sperm acting as activator. How oocyte-to-embryo transition is triggered in parthenogenetic nematode species that lack sperm, is not known.

Results

We investigated two key elements of oocyte-to-embryo transition, MSP expression and MAP kinase signaling, in two parthenogenetic nematodes and their close hermaphroditic relatives. While activated MAP kinase is present in all analysed nematodes irrespective of the reproductive mode, MSP expression differs. In contrast to hermaphroditic or bisexual species, we do not find MSP expression at the protein level in parthenogenetic nematodes. However, genomic sequence analysis indicates that functional MSP genes are present in several parthenogenetic species.

Conclusions

We present three alternative interpretations to explain our findings. (1) MSP has lost its function as a trigger of MAP kinase activation and is not expressed in parthenogenetic nematodes. Activation of the MAP kinase pathway is achieved by another, unknown mechanism. Functional MSP genes are required for occasionally emerging males found in some parthenogenetic species. (2) Because of long-term disadvantages, parthenogenesis is of recent origin. MSP genes remained intact during this short intervall although they are useless. As in the first scenario, an unknown mechanism is responsible for MAP kinase activation. (3) The molecular machinery regulating oocyte-to-embryo transition in parthenogenetic nematodes is conserved with respect to C. elegans, thus requiring intact MSP genes. However, MSP expression has been shifted to non-sperm cells and is reduced below the detection limits, but is still sufficient to trigger MAP kinase activation and embryogenesis.

Molecular characterization and real-time PCR transcriptional analysis of Dictyocaulus viviparus major sperm proteins

Major sperm proteins (MSPs) represent a protein family occurring in nematodes only. Identification of the 3' and 5' untranslated region (UTR) completed the so far partial msp complementary DNA sequences of the bovine lungworm Dictyocaulus viviparus. The full-length transcript contains sequence tracts consistent with the Kozak and polyadenylation consensus sequence. On genomic level, three full-length sequences differing in three nucleotides were determined containing a 65-bp phase zero intron. Conceptual translation inferred two MSP isoforms due to one substitution within the 126-amino acid polypeptide. Bioinformatic analysis predicted that bovine lungworm MSP folds into an immunoglobulin-like seven-stranded beta sandwich as known for Caenorhabditis elegans and Ascaris suum. Furthermore, bovine lungworm MSP is confidentially predicted to be N-terminal-acetylated and secreted via a non-classical pathway. Quantitative real-time polymerase chain reaction analysis using ten developmental lungworm stages showed that msp is transcribed mainly in adult male parasites and in some degree in hypobiotic L5. However, marginal msp transcription was detectable in all of the investigated developmental lungworm stages.

Genetic tests of ancient asexuality in root knot nematodes reveal recent hybrid origins

Background

The existence of "ancient asexuals", taxa that have persisted for long periods of evolutionary history without sexual recombination, is both controversial and important for our understanding of the evolution and maintenance of sexual reproduction. A lack of sex has consequences not only for the ecology of the asexual organism but also for its genome. Several genetic signatures are predicted from long-term asexual (apomictic) reproduction including (i) large "allelic" sequence divergence (ii) lack of phylogenetic clustering of "alleles" within morphological species and (iii) decay and loss of genes specific to meiosis and sexual reproduction. These genetic signatures can be hard to assess since it is difficult to demonstrate the allelic nature of very divergent sequences, divergence levels may be complicated by processes such as inter-specific hybridization, and genes may have secondary roles unrelated to sexual reproduction. Apomictic species of Meloidogyne root knot nematodes have been suggested previously to be ancient asexuals. Their relatives reproduce by meiotic parthenogenesis or facultative sexuality, which in combination with the abundance of nematode genomic sequence data, makes them a powerful system in which to study the consequences of reproductive mode on genomic divergence.

Results

Here, sequences from nuclear protein-coding genes are used to demonstrate that the first two predictions of ancient asexuality are found within the apomictic root knot nematodes. Alleles are more divergent in the apomictic taxa than in those species exhibiting recombination and do not group phylogenetically according to recognized species. In contrast some nuclear alleles, and mtDNA, are almost identical across species. Sequencing of Major Sperm Protein, a gamete-specific gene, from both meiotic and ameiotic species reveals no increase in evolutionary rate nor change in substitution pattern in the apomictic taxa, indicating that the locus has been maintained by selection.

Conclusion

The data strongly suggests the tropical root knot nematode apomicts have a recent origin and are not anciently asexual. The results support that interspecific hybridization has been involved in the origin of this asexual group and has played a role in shaping the patterns of genetic diversity observed. This study suggests that genetic signatures of ancient asexuality must be taken with caution due to the confounding effect of interspecific hybridization, which has long been implicated in the origins of apomictic species.

GenomeHistory: a software tool and its application to fully sequenced genomes

We present a publicly available software tool (http://www.unm.edu/~compbio/software/GenomeHistory) that identifies all pairs of duplicate genes in a genome and then determines the degree of synonymous and non-synonymous divergence between each duplicate pair. Using this tool, we analyze the relations between (i) gene function and the propensity of a gene to duplicate and (ii) the number of genes in a gene family and the family's rate of sequence evolution. We do so for the complete genomes of four eukaryotes (fission and budding yeast, fruit fly and nematode) and one prokaryote (Escherichia coli). For some classes of genes we observe a strong relationship between gene function and a gene's propensity to undergo duplication. Most notably, ribosomal genes and transcription factors appear less likely to undergo gene duplication than other genes. In both fission and budding yeast, we see a strong positive correlation between the selective constraint on a gene and the size of the gene family of which this gene is a member. In contrast, a weakly negative such correlation is seen in multicellular eukaryotes.

ERG30, a VAP-33-related protein, functions in protein transport mediated by COPI vesicles

Intracellular transport of newly synthesized and mature proteins via vesicles is controlled by a large group of proteins. Here we describe a ubiquitous rat protein-endoplasmic reticulum (ER) and Golgi 30-kD protein (ERG30)-which shares structural characteristics with VAP-33, a 33-kD protein from Aplysia californica which was shown to interact with the synaptic protein VAMP. The transmembrane topology of the 30-kD ERG30 corresponds to a type II integral membrane protein, whose cytoplasmic NH(2) terminus contains a predicted coiled-coil motif. We localized ERG30 to the ER and to pre-Golgi intermediates by biochemical and immunocytochemical methods. Consistent with a role in vesicular transport, anti-ERG30 antibodies specifically inhibit intra-Golgi transport in vitro, leading to significant accumulation of COPI-coated vesicles. It appears that ERG30 functions early in the secretory pathway, probably within the Golgi and between the Golgi and the ER.

Identification of a human homologue of the vesicle-associated membrane protein (VAMP)-associated protein of 33 kDa (VAP-33): a broadly expressed protein that binds to VAMP

We report the identification of a human homologue of the vesicle-associated membrane protein (VAMP)-associated protein (hVAP-33) that has been implicated in neuronal exocytosis in Aplysia californica. This hVAP-33 shared 50% amino acid identity with the A. californica form and had similar length, structural organization and VAMP-binding abilities. However, in contrast with the neuron-specific expression seen in A. californica, hVAP-33 was broadly expressed, suggesting possible roles in vesicle fusion in both neuronal and non-neuronal cells.

A trans-spliced leader sequence on actin mRNA in C. elegans

While determining the 5' ends of C. elegans actin mRNAs, we have discovered a 22 nucleotide spliced leader sequence. The leader sequence is found on mRNA from three of the four nematode actin genes. The leader also appears to be present on some, but not all, nonactin mRNAs. The actin mRNA leader sequence is identical to the first 22 nucleotides of a novel 100 nucleotide RNA transcribed adjacent, and in the opposite orientation, to the 5S ribosomal gene. The evidence suggests that the actin mRNA leader sequence is acquired from this novel nucleotide transcript by an intermolecular trans-splicing mechanism.

Monoclonal antibodies that recognize a polypeptide antigenic determinant shared by multiple Caenorhabditis elegans sperm-specific proteins

Four monoclonal antibodies that are directed against antigens present in sperm and absent from other worm tissues were characterized. Antibody TR20 is directed against the major sperm proteins, a family of small, abundant, cytoplasmic proteins that have been previously described (Klass, M. R., and D. Hirsh, 1981, Dev. Biol., 84:299-312; Burke, D. J., and S. Ward, 1983, J. Mol. Biol., 171:1-29). Three other antibodies, SP56, SP150, and TR11, are all directed against the same set of minor sperm polypeptides that range in size from 29 to 215 kD. More than eight different sperm polypeptides are antigenic by both immunotransfer and immunoprecipitation assays. The three antibodies are different immunoglobulin subclasses, yet they compete with each other for antigen binding so they are directed against the same antigenic determinant on the multiple sperm proteins. This antigenic determinant is sensitive to any of six different proteases, is insensitive to periodate oxidation or N-glycanase digestion, and is detectable on a polypeptide synthesized in vitro. Therefore, the antigenic determinant resides in the polypeptide chain. However, peptide fragments of the proteins are not antigenic, thus the determinant is likely to be dependent on polypeptide conformation. The antigenic determinant shared by these proteins could represent a common structural feature of importance to the localization or cellular specificity of these proteins.

The nucleotide sequence of a nematode vitellogenin gene

The nematode, Caenorhabditis elegans, contains a family of six genes that code for vitellogenins. Here we report the complete nucleotide sequence of one of these genes, vit-5. The gene specifies a mRNA of 4869 nucleotides, including untranslated regions of 9 bases at the 5' end and 51 bases at the 3' end. Vit-5 contains four short introns totalling 218 bp. The predicted vitellogenin, yp170A, has a molecular weight of 186,430. At its N terminus it is clearly related to the vitellogenins of vertebrates. However, the vit-5-encoded protein does not contain a serine-rich sequence related to the vertebrate vitellin, phosvitin. In fact, the amino acid composition of the nematode protein is very similar to that of the vertebrate protein without phosvitin. Vit-5 has a highly asymmetric codon choice dictionary. The favored codons are different from those favored in other organisms, but are characteristic of highly expressed C. elegans genes. The strong selection against rare codons is not as great near the 5' end of the gene; rare codons are 15 times more frequent within the first 54 bp than in the next 4.8 kb.

The C. elegans vitellogenin genes: short sequence repeats in the promoter regions and homology to the vertebrate genes

The nematode Caenorhabditis elegans contains a small family of vitellogenin genes which is expressed abundantly, but only in the intestine of the adult hermaphrodite worm. In order to identify possible regulatory elements, we have sequenced the DNA surrounding the 5' ends of five of the six genes. Contained within regions which have largely diverged from one another, two different heptameric sequences are found repeated within the first 200 bp upstream of each of the genes. The first sequence, TGTCAAT, is present as a perfect heptamer at least once upstream of each gene. It is repeated in both orientations four to six times in each 5' flanking region, allowing a one-base mismatch. The second sequence, CTGATAA, is also present as a perfect heptamer in a restricted region upstream of each gene. These two sequence elements may be involved in regulation of the vitellogenin genes. Remarkably, the CTGATAA sequence is present in a similar location in the promoter regions of vertebrate vitellogenin genes. In fact, our data reveal a surprising degree of similarity between the nematode and vertebrate vitellogenins.