Evidence from mosaic analysis of the masculinizing gene her-1 for cell interactions in C. elegans sex determination

Suppressors of a lin-12 hypomorph define genes that interact with both lin-12 and glp-1 in Caenorhabditis elegans

The lin-12 gene of Caenorhabditis elegans is thought to encode a receptor which mediates cell-cell interactions required to specify certain cell fates. Reversion of the egg-laying defective phenotype caused by a hypomorphic lin-12 allele identified rare extragenic suppressor mutations in five genes, sel-1, sel-9, sel-10, sel-11 and sel(ar40) (sel = suppressor and/or enhancer of lin-12). Mutations in each of these sel genes suppress defects associated with reduced lin-12 activity, and enhance at least one defect associated with elevated lin-12 activity. None of the sel mutations cause any obvious phenotype in a wild-type background. Gene dosage experiments suggest that sel-1 and sel(ar40) mutations are reduction-of-function mutations, while sel-9 and sel-11 mutations are gain-of-function mutations. sel-1, sel-9, sel-11 and sel(ar40) mutations do not suppress amorphic lin-12 alleles, while sel-10 mutations are able to bypass partially the requirement for lin-12 activity in at least one cell fate decision. sel-1, sel-9, sel-10, sel-11 and sel(ar40) mutations are also able to suppress the maternal-effect lethality caused by a partial loss-of-function allele of glp-1, a gene that is both structurally and functionally related to lin-12. These sel genes may therefore function in both lin-12 and glp-1 mediated cell fate decisions.

In search of new mutants in cell-signaling systems of the nematode Caenorhabditis elegans. Review

Development of multicellular organisms is controlled mainly by cell-signaling systems. In this review I first discuss methods of genetic analysis and properties of mutants of cell-signaling systems in general and in the nematode C. elegans. Then, I describe two of our approaches to isolating new mutants in cell-signaling of C. elegans. The first approach is to select for mutants that have the same visible phenotype as those in known cell-signaling genes. In a survey of larval lethal mutations we found that there are quite a few mutants in which the inner surface of the body wall is detached from the outer surface of the intestine. Some of them map in genes that are known to act in cell-signaling systems in vulval induction or sex myoblast migration, which are not essential to the growth and survival of C. elegans. Therefore, we think many of the mutations of the above phenotype disrupt cell-signaling in an unidentified essential function, and also cell-signaling in the non-essential functions. The second approach is to isolate mutants resistant to a drug expected to disturb cell-signaling. As the drug we have chosen sodium fluoride, which depletes calcium ion, activates G-proteins and inactivates some phosphatases. The mutants are grouped into two classes (three and two genes, respectively) according to degree of fluoride-resistance and growth rate of larvae. Although there is so far no direct evidence that these mutants are related to cell-signaling, they show complex epistasis that can be explained by a model consisting of a cell-signaling pathway.

Sex determining gene expression during embryogenesis

The Y-linked gene Sry acts during a critical period of gonadal differentiation to divert the normal or default pathway of gene activity that would otherwise lead to the development of ovaries into one that leads to the development of testes. It acts cell autonomously, probably within the cell lineage that gives rise to Sertoli cells in the testis or follicle cells in the ovary. The remaining cell types within the gonad, each of which has a developmental choice, then become fated to follow the testicular pathway. This process must depend on cell-cell interactions as Sry is not required within these other cell types for their differentiation. Subsequent male development of the animal as a whole is dependent on the production of testosterone and other factors by the testis. Sry encodes a DNA binding protein of the HMG box class, and presumably acts to regulate the expression of other genes which then confer cellular phenotype. However, rather than operating like other classes of transcription factor, it has been shown to induce a dramatic bend in its DNA binding sites, and may not directly affect transcription of target genes. Instead, it may permit other factors to interact, which in turn either activate or repress transcription. Sequence comparisons between Sry genes from various species suggest that the HMG box is the only functional part of the protein. This part is responsible for DNA binding, and both mouse and human SRY bind the same consensus sequence at high affinity in vitro. However, the human gene fails to cause female to male sex reversal in transgenic mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular characterization of the her-1 gene suggests a direct role in cell signaling during Caenorhabditis elegans sex determination

We have characterized two transcripts from the male-determining her-1 locus in Caenorhabditis elegans. The larger transcript, which appears more important for male development, is predicted to encode a novel 175-amino-acid, cysteine-rich polypeptide with an apparent amino-terminal signal sequence and potential cleavage and glycosylation sites. Expression of a full-length cDNA construct for the larger transcript driven by a body-wall-myosin promoter causes extensive masculinization of all sexually dimorphic tissues in XX (normally hermaphrodite) animals. This activity is dependent on the presence of the her-1 signal sequence or a substitute synthetic signal sequence in the encoded polypeptide. These results suggest that a secreted product of the her-1 gene dictates male development.

Testis determination: soft talk and kinky sex

The activity of the Y-linked Sry gene during a critical period of gonadal differentiation is the normal trigger for testis determination and subsequent male development in mammals. This gene encodes a DNA-binding protein of the HMG-box class. It has been shown to induce a dramatic kink in target DNA-binding sites, which allows for much speculation on how the gene functions to regulate testis-specific gene expression. It is also clear that cell interactions are vital to its mode of action, and generally in the process of gonadal differentiation.

Molecular analysis of the C. elegans sex-determining gene tra-1: a gene encoding two zinc finger proteins

The tra-1 gene is the terminal control gene for somatic sex determination in the nematode Caenorhabditis elegans. Here we identify two tra-1 mRNAs: one is a 1.5 kb transcript that peaks in abundance in the second larval stage, and the other is a 5 kb transcript that is present at relatively constant abundance throughout development. Both RNAs occur at similar levels in both sexes, suggesting that regulation of tra-1 is posttranscriptional. Neither RNA is germline restricted. The two RNAs are colinear at their 5' ends: the shorter RNA encodes a protein with two zinc finger motifs, and the longer RNA encodes a protein with five zinc fingers. The identification of eight nonsense mutations confirms that these are authentic tra-1 RNAs and demonstrates that the longer one is essential for tra-1 activity. The transcription pattern reveals that alternative mRNA processing governs the number of zinc fingers in the resulting tra-1 protein. The tra-1 fingers are strikingly similar to those of three other proteins, the products of the human GLI and GLI3 and Drosophila cubitus interruptus Dominant (ciD) genes.

Molecular genetics of sex determination in C. elegans

Sexual fate in the nematode Caenorhabditis elegans is controlled by a group of genetically well-characterized genes. Several of these sex-determining genes have now been analysed at the molecular level. Transcriptional regulation is likely to control both commitment to a single sexual fate and maintenance of that decision; in addition, intercellular signalling appears to coordinate the sexual fates of cells throughout the animal to adopt a single sexual fate.

Genetic sex determination mechanisms and evolution

Different animal groups exhibit a surprisingly diversity of sex determination systems. Moreover, even systems that are superficially similar may utilize different underlying mechanisms. This diversity is illustrated by a comparison of sex determination in three well-studied model organisms: the fruitfly Drosophila melanogaster, the nematode Caenorhabditis elegans, and the mouse. All three animals exhibit male heterogamety, extensive sexual dimorphism and sex chromosome dosage compensation, yet the molecular and cellular processes involved are now known to be quite unrelated. The similarities must have arisen by convergent evolution. Studies of sex determination demonstrate that evolution can produce a variety of solutions to the same basic problems in development.

tra-2 encodes a membrane protein and may mediate cell communication in the Caenorhabditis elegans sex determination pathway

The Caenorhabditis elegans sex-determining gene, tra-2, promotes female development in XX animals. In this paper we report the cDNA sequence corresponding to a 4.7 kb tra-2 mRNA and show that it is composed of 23 exons, is trans-spliced to SL2, and contains a perfect direct repeat in the 3' untranslated region. This mRNA is predicted to encode a 1475 amino acid protein, named pTra2A, that has a secretory signal and several potential membrane-spanning domains. The molecular analysis of tra-2 loss-of-function mutations supports our open reading frame identification and suggests that the carboxy-terminal domain is important for tra-2 activity. We propose that in XX animals the carboxy-terminal domain of pTra2A negatively regulates the downstream male promoting fem genes. In XO animals, tra-2 is negatively regulated by her-1, which acts cell nonautonomously. Because hydropathy predictions suggest that pTra2A is an integral membrane protein, pTra2A might act as a receptor for the her-1 protein. We propose that in XO animals, the her-1 protein promotes male development by binding and inactivating pTra2A. The role of cell communication in C. elegans sex determination might be to ensure unified sexual development throughout the animal. If so, then regulation of sexual fate by her-1 and tra-2 might provide a general model for the coordination of groups of cells to follow a single cell fate.