The chromosomal signal for sex determination in Caenorhabditis elegans.

In Caenorhabditis elegans, sex is determined by the number of X chromosomes which, in turn, determines the expression of the X-linked gene xol-1. Recent work has shown that xol-1 expression is controlled by least two distinct regulatory mechanisms, one transcriptional and another post-transcriptional. The transcriptional regulator is a repressor acting in XX embryos; although the specific gene has not been identified, the chromosome region has been defined. A previously defined regulator of xol-1, known as fox-1, maps to a different region of the X chromosome and works post-transcriptionally, consistent with the identification of the fox-1 gene product as a putative RNA binding protein.

Sex determination in polyploids of Caenorhabditis elegans.

In Caenorhabditis elegans triploid animals with two X chromosomes (symbolized 3A;2X) are males. However, these triploid males can be feminized by making them mutant for recessive dosage compensation mutations, by adding X chromosome duplications or by microinjecting particular DNA sequences termed feminizing elements. None of these treatments affects diploid males. This study explores several aspects of these treatments in polyploids. The dosage compensation mutants exhibit a strong maternal effect, such that reduction of any of the dosage compensation gene functions in the mother leads to sex reversal of 3A;2X animals. Likewise, all X chromosome duplications tested cause both sex reversal and intersexual development of many 3A;2X animals. Microinjected feminizing element DNA does not cause extensive sex reversal, but does result in intersexual development in 3A;2X animals. Neither X chromosome duplications nor microinjected feminizing elements show that extreme maternal effect of the dosage compensation mutants, although there is indirect evidence for a maternal effect of the feminizing elements. In particular, very little feminizing element DNA needs to be microinjected in order to feminize triploid males, far less than what is needed for stable inheritance, implying that feminizing elements can work within the mother's gonad. However, even very high concentrations of microinjected feminizing elements do not affect sex determination in diploid males, suggesting that they are not part of the numerator of the X/A ratio. In addition, no pair of X chromosome duplications feminizes diploid males, suggesting that none of these duplications contains a numerator of the X/A ratio. Instead, I infer that an X-linked locus, as yet undefined, must be present in two copies for hermaphrodite development to ensue or that the two X chromosomes might interact.

Sex determination and dosage compensation: lessons from flies and worms.

In both Drosophila melanogaster and Caenorhabditis elegans somatic sex determination, germline sex determination, and dosage compensation are controlled by means of a chromosomal signal known as the X:A ratio. A variety of mechanisms are used for establishing and implementing the chromosomal signal, and these do not appear to be similar in the two species. Instead, the study of sex determination and dosage compensation is providing more general lessons about different types of signaling pathways used to control alternative developmental states of cells and organisms.

Meiotic mutants that cause a polar decrease in recombination on the X chromosome in Caenorhabditis elegans.

Recessive mutations in three autosomal genes, him-1, him-5 and him-8, cause high levels of X chromosome nondisjunction in hermaphrodites of Caenorhabditis elegans, with no comparable effect on autosomal disjunction. Each of the mutants has reduced levels of X chromosome recombination, correlating with the increase in nondisjunction. However, normal or elevated levels of recombination occur at the end of the X chromosome hypothesized to contain the pairing region (the left end), with recombination levels decreasing in regions approaching the right end. Thus, both the number and the distribution of X chromosome exchange events are altered in these mutants. As a result, the genetic map of the X chromosome in the him mutants exhibits a clustering of genes due to reduced recombination, a feature characteristic of the genetic map of the autosomes in non-mutant animals. We hypothesize that these him genes are needed for some processive event that initiates near the left end of the X chromosome.

X-linked gene expression and sex determination in Caenorhabditis elegans.

The signal for sex determination in the nematode Caenorhabditis elegans is the ratio between the number of X chromosomes and the number of sets of autosomes (the X/A ratio). Animals with an X/A ratio of 0.67 (a triploid with two X chromosomes) or less are males. Animals with an X/A ratio of 0.75 or more are hermaphrodites. Thus, diploid males have one X chromosome and diploid hermaphrodites have two X chromosomes. However, the difference in X-chromosome number between the sexes is not reflected in general levels of X-linked gene expression because of the phenomenon of dosage compensation. In dosage compensation, X-linked gene expression appears to be 'turned down' in 2X animals to the 1X level of expression. An intriguing and unexplained finding is that mutations and X-chromosome duplications that elevate X-linked gene expression also feminize triploid males. One way that this relationship between sex determination and X-linked gene expression may be operating is discussed.

Multiple sites of action of volatile anesthetics in Caenorhabditis elegans.

The mechanism and site(s) of action of volatile anesthetics are unknown. In all organisms studied, volatile anesthetics adhere to the Meyer-Overton relationship--that is, a ln-ln plot of the oil-gas partition coefficients versus the potencies yields a straight line with a slope of -1. This relationship has led to two conclusions about the site of action of volatile anesthetics. (i) It has properties similar to the lipid used to determine the oil-gas partition coefficients. (ii) All volatile anesthetics cause anesthesia by affecting a single site. In Caenorhabditis elegans, we have identified two mutants with altered sensitivities to only some volatile anesthetics. These two mutants, unc-79 and unc-80, confer large increases in sensitivity to very lipid soluble agents but have little or no increases to other agents. In addition, a class of extragenic suppressor mutations exists that suppresses some altered sensitivities but specifically does not suppress the altered sensitivity to diethyl ether. There is much debate concerning the molecular nature of the site(s) of anesthetic action. One point of discussion is whether the site(s) consists of a purely lipid binding site or if protein is involved. The simplest explanation of our observations is that volatile anesthetics cause immobility in C. elegans by specifically interacting with multiple sites. This model is in turn more consistent with involvement of protein at the site(s) of action.

Microinjected DNA from the X chromosome affects sex determination in Caenorhabditis elegans.

The signal for sex determination in the nematode Caenorhabditis elegans is the ratio of the number of X chromosomes to the number of sets of autosomes (X/A ratio). By previous genetic tests, elements that feminized chromosomal males appeared to be widespread on the X chromosome, but the nature of these elements was not determined. In experiments to define a feminizing element molecularly, cloned sequences were added to chromosomally male embryos by microinjection into the mother. Three different X-chromosome clones, including part of an actin gene, part of a myosin heavy chain gene, and all of two myosin light chain genes, feminize chromosomal males. Both somatic and germline aspects of sex determination are affected. In contrast, about 40 kilobases of nematode autosomal DNA, phage lambda DNA, and plasmid pBR322 DNA do not affect sex determination. A feminizing region was localized to a maximum of 131 base pairs within an intron of the X-linked actin gene; a part of the gene that does not have this region is not feminizing. The results suggest that short, discrete elements found associated with many X-linked genes may act as signals for sex determination in C. elegans.

The effect of two genes on anesthetic response in the nematode Caenorhabditis elegans.

The authors studied the wild type strain, N2, and three mutant strains of the nematode, Caenorhabditis elegans, in order to measure genetically produced changes in responses to nine volatile anesthetics. They determined the anesthetic ED50s of N2 for thiomethoxyflurane, methoxyflurane, chloroform, halothane, enflurane, isoflurane, fluroxene, flurothyl, and diethylether. The log-log relationship of the oil-gas partition coefficients (O/G) and the ED50s of these agents for N2 yields a straight line with a slope of -.997 with a R2 of .98 over a range of O/G (at 37 degrees C) from 48 to 7230. When the O/Gs are corrected to 22 degrees C, the slope is -.964 with an R2 of .98. This relationship is similar to that found in other animals. Two mutant strains, unc-79 and unc-80, show altered responses to these anesthetics. These strains are two to three times more sensitive than N2 to anesthetics with an O/G greater than that of halothane (220 at 37 degrees C), yet they differ little from N2 in response to anesthetics with lower O/Gs. unc-79 and unc-80 are about 30% more sensitive than N2 to diethylether. The double mutant unc-79; unc-80 is more sensitive to halothane, isoflurane, and fluroxene than is either mutant alone. The authors believe these data indicate an alteration at the site of action of volatile anesthetics in unc-79 and unc-80. They also postulate that the interaction of unc-79 and unc-80 indicate these genes code for enzymes in a common pathway, and that unc-79 precedes unc-80 in this pathway.

X chromosome duplications affect a region of the chromosome they do not duplicate in Caenorhabditis elegans.

X chromosome duplications have been used previously to vary the dose of specific regions of the X chromosome to study dosage compensation and sex determination in Caenorhabditis elegans. We show here that duplications suppress and X-linked hypomorphic mutation and elevate the level of activity of an X-linked enzyme, although these two genes are located in a region of the X chromosome that is not duplicated. The effects do not depend on the region of the X chromosome duplicated and is stronger in strains with two doses of a duplication than in strains with one dose. This is evidence for a general elevation of X-linked gene expression in strains carrying X-chromosome duplications, consistent with the hypothesis that the duplications titrate a repressor acting on many X-linked genes.

Genetic analysis of X-chromosome dosage compensation in Caenorhabditis elegans.

We have shown that the phenotypes resulting from hypomorphic mutations (causing reduction but not complete loss of function) in two X-linked genes can be used as a genetic assay for X-chromosome dosage compensation in Caenorhabditis elegans between males (XO) and hermaphrodites (XX). In addition we show that recessive mutations in two autosomal genes, dpy-21 V and dpy-26 IV, suppress the phenotypes resulting from the X-linked hypomorphic mutations, but not the phenotypes resulting from comparable autosomal hypomorphic mutations. This result strongly suggests that the dpy-21 and dpy-26 mutations cause increased X expression, implying that the normal function of these genes may be to lower the expression of X-linked genes. Recessive mutations in two other dpy genes, dpy-22 X and dpy-23 X, increase the severity of phenotypes resulting from some X-linked hypomorphic mutations, although dpy-23 may affect the phenotypes resulting from the autosomal hypomorphs as well. The mutations in all four of the dpy genes show their effects in both XO and XX animals, although to different degrees. Mutations in 18 other dpy genes do not show these effects.

Genetic analysis of halothane sensitivity in Caenorhabditis elegans.

The nematode Caenorhabditis elegans appears to be a useful model for studying the action of volatile anesthetics. A mutant strain that is hypersensitive to the widely used anesthetic halothane was described earlier. The mutation is now shown to be an allele of unc-79. Other alleles of unc-79 are also associated with hypersensitivity to halothane. A strain with a mutation in a second gene, unc-80, is also hypersensitive to halothane. Nematodes bearing mutations in both unc-79 and unc-80 are slightly more sensitive to halothane than those bearing only one of these mutations. Mutations in a third gene, unc-9, suppress both unc-79 and unc-80. Nematodes bearing the suppressor mutations alone have normal sensitivity to halothane. These results show that sensitivity to halothane can be altered by mutations in several different genes.

An autosomal gene that affects X chromosome expression and sex determination in Caenorhabditis elegans.

Recessive mutant alleles at the autosomal dpy-21 locus of C. elegans cause a dumpy phenotype in XX animals but not in XO animals. This dumpy phenotype is characteristic of X chromosome aneuploids with higher than normal X to autosome ratios and is proposed to result from overexpression of X-linked genes. We have isolated a new dpy-21 allele that also causes partial hermaphroditization of XO males, without causing the dumpy phenotype. All dpy-21 alleles show hermaphroditization effects in XO males that carry a duplication of part of the X chromosome and also partially suppress a transformer (tra-1) mutation that converts XX animals into males. Experiments with a set of X chromosome duplications show that the defects of dpy-21 mutants can result from interaction with several different regions of the X chromosome. We propose that dpy-21 regulates X chromosome expression and may be involved in interpreting X chromosome dose for the developmental decisions of both sex determination and dosage compensation.

An acetylcholinesterase-deficient mutant of the nematode Caenorhabditis elegans.

Within a set of five separable molecular forms of acetylcholinesterase found in the nematode Caenorhabditis elegans, previously reported differences in kinetic properties identify two classes, A and B, likely to be under separate genetic control. Using differences between these classes in sensitivity to inactivation by sodium deoxycholate, a screening procedure was devised to search for mutants affected only in class A forms. Among 171 previously isolated behavioral and morphological mutant strains examined by this procedure, one (PR946) proved to be of the expected type, exhibiting a selective deficiency of class A acetylcholinesterase forms. Although originally isolated because of its uncoordinated behavior, this strain was subsequently shown to harbor mutations in two genes; one in the previously identified gene unc-3, accounting for its behavior, and one in a newly identified gene, ace-1, accounting for its selective acetylcholinesterase deficiency. Derivatives homozygous only for the ace-1 mutation also lacked class A acetylcholinesterase forms, but were behaviorally and developmentally indistinguishable from wild type. The gene ace-1 has been mapped near the right end of the X chromosome. Gene dosage experiments suggest that it may be a structural gene for a component of class A acetylcholinesterase forms.

Suppression and function of X-linked lethal and sterile mutations in Caenorhabditis elegans.

We have expanded our collection of recessive lethal and sterile mutants in the region of the X chromosome balanced by mnDp1(X;V), about 15% of the X linkage map, to a total of 54 mutants. The mutations have been mapped with respect to 20 overlapping deficiencies and five X duplications, and they have been assigned to 24 genes by complementation testing. Nine mutants are hermaphrodite-sterile: one of these is a sperm-defect mutant, two have abnormal gonadogenesis and six, in five genes, are maternally influenced mutants, producing inviable zygote progeny. One of the gonadogenesis mutants and two of the maternally influenced mutants are male fertile. All but one of the maternally influenced mutants give cross progeny when mated with wild-type males. Forty-three mutants were tested for suppression by homozygous sup-5 (e1464), which is believed to be specific for null alleles. Ten mutants that were judged by independent criteria not to be null mutants are not suppressed. Nine of the other 33 mutants, in nine genes, are suppressed, five in both heterozygous and homozygous suppressor stocks and four only in homozygous suppressor stocks.

Lethals, steriles and deficiencies in a region of the X chromosome of Caenorhabditis elegans.

Twenty-one X-linked recessive lethal and sterile mutations balanced by an unlinked X-chromosome duplication have been identified following EMS treatment of the small nematode, Caenorhabditis elegans. The mutations have been assigned by complementation analysis to 14 genes, four of which have more than one mutant allele. Four mutants, all alleles, are temperature-sensitive embryonic lethals. Twelve mutants, in ten genes, are early larval lethals. Two mutants are late larval lethals, and the expression of one of these is influenced by the number of X chromosomes in the genotype. Two mutants are maternal-effect lethals; for both, oocytes made by mutant hermaphrodites are rescuable by wild-type sperm. One of the maternal-effect lethals and two larval lethals are allelic. One mutant makes defective sperm. The lethals and steriles have been mapped by recombination and by complementation testing against 19 deficiencies identified after X-ray treatment. The deficiencies divide the region, about 15% of the X-chromosome linkage map, into at least nine segments. The deficiencies have also been used to check the phenotypes of hemizygous lethal and sterile hermaphrodites.