The Effect of Temperature and Parental Age on Recombination and Nondisjunction in CAENORHABDITIS ELEGANS

The effect of temperature and parental age on recombination frequency in C. elegans was studied between pairs of closely linked markers on linkage groups I and V. In the regions studied, recombination frequency varied three-fold over the temperature range 13.5 degrees to 26 degrees . Temperature-shift experiments indicated that a temperature-sensitive recombination event occurs approximately 50 oocytes prior to fertilization. Recombination frequency was observed to decrease with maternal age. The greatest decrease was observed in the first 24 hours of egg production. The frequency of male progeny, a measure of X-chromosome nondisjunction was also studied. This frequency increased with elevated temperature and age of the parent.

Caenorhabditis elegans expresses three functional profilins in a tissue-specific manner

Profilins are actin binding proteins, which also interact with polyphosphoinositides and proline-rich ligands. On the basis of the genome sequence, three diverse profilin homologues (PFN) are predicted to exist in Caenorhabditis elegans. We show that all three isoforms PFN-1, PFN-2, and PFN-3 are expressed in vivo and biochemical studies indicate they bind actin and influence actin dynamics in a similar manner. In addition, they bind poly(L-proline) and phosphatidylinositol 4,5-bisphosphate micelles. PFN-1 is essential whereas PFN-2 and PFN-3 are nonessential. Immunostainings revealed different expression patterns for the profilin isoforms. In embryos, PFN-1 localizes in the cytoplasm and to the cell-cell contacts at the early stages, and in the nerve ring during later stages. During late embryogenesis, expression of PFN-3 was specifically detected in body wall muscle cells. In adult worms, PFN-1 is expressed in the neurons, the vulva, and the somatic gonad, PFN-2 in the intestinal wall, the spermatheca, and the pharynx, and PFN-3 localizes in a striking dot-like fashion in body wall muscle. Thus the model organism Caenorhabditis elegans expresses three profilin isoforms and is the first invertebrate animal with tissue-specific profilin expression.

The sterol modifying enzyme LET-767 is essential for growth, reproduction and development in Caenorhabditis elegans

The let-767 gene encodes a protein that is similar to mammalian steroid enzymes that are responsible for the reduction of 17-beta hydroxysteroid hormones. Caenorhabditis elegans is incapable of the de novo synthesis of cholesterol. Therefore, this free-living nematode must extract cholesterol from its environment and modify it to form steroid hormones that are necessary for its survival. C. elegans is unable to survive in the absence of supplemental cholesterol, and is therefore sensitive to cholesterol limitation. We show that a mutation in let-767 results in hypersensitivity to cholesterol limitation, supporting the hypothesis that LET-767 acts on a sterol derivative. Furthermore, let-767 mutants exhibit defects in embryogenesis, female reproduction and molting. Although ecdysone is the major molting hormone in insects, there is as yet no evidence for ecdysone synthesis in C. elegans, suggesting that a different hormone is required for molting in C. elegans. Our results suggest that LET-767 modifies a sterol hormone that is required both for embryogenesis and for later stages of development.

Changes in gene expression associated with developmental arrest and longevity in Caenorhabditis elegans

Gene expression in a developmentally arrested, long-lived dauer population of Caenorhabditis elegans was compared with a nondauer (mixed-stage) population by using serial analysis of gene expression (SAGE). Dauer (152,314) and nondauer (148,324) SAGE tags identified 11,130 of the predicted 19,100 C. elegans genes. Genes implicated previously in longevity were expressed abundantly in the dauer library, and new genes potentially important in dauer biology were discovered. Two thousand six hundred eighteen genes were detected only in the nondauer population, whereas 2016 genes were detected only in the dauer, showing that dauer larvae show a surprisingly complex gene expression profile. Evidence for differentially expressed gene transcript isoforms was obtained for 162 genes. H1 histones were differentially expressed, raising the possibility of alternative chromatin packaging. The most abundant tag from dauer larvae (20-fold more abundant than in the nondauer profile) corresponds to a new, unpredicted gene we have named tts-1 (transcribed telomere-like sequence), which may interact with telomeres or telomere-associated proteins. Abundant antisense mitochondrial transcripts (2% of all tags), suggest the existence of an antisense-mediated regulatory mechanism in C. elegans mitochondria. In addition to providing a robust tool for gene expression studies, the SAGE approach already has provided the advantage of new gene/transcript discovery in a metazoan.

MUP-4 is a novel transmembrane protein with functions in epithelial cell adhesion in Caenorhabditis elegans

Tissue functions and mechanical coupling of cells must be integrated throughout development. A striking example of this coupling is the interactions of body wall muscle and hypodermal cells in Caenorhabditis elegans. These tissues are intimately associated in development and their interactions generate structures that provide a continuous mechanical link to transmit muscle forces across the hypodermis to the cuticle. Previously, we established that mup-4 is essential in embryonic epithelial (hypodermal) morphogenesis and maintenance of muscle position. Here, we report that mup-4 encodes a novel transmembrane protein that is required for attachments between the apical epithelial surface and the cuticular matrix. Its extracellular domain includes epidermal growth factor-like repeats, a von Willebrand factor A domain, and two sea urchin enterokinase modules. Its intracellular domain is homologous to filaggrin, an intermediate filament (IF)-associated protein that regulates IF compaction and that has not previously been reported as part of a junctional complex. MUP-4 colocalizes with epithelial hemidesmosomes overlying body wall muscles, beginning at the time of embryonic cuticle maturation, as well as with other sites of mechanical coupling. These findings support that MUP-4 is a junctional protein that functions in IF tethering, cell-matrix adherence, and mechanical coupling of tissues.

Genes and mechanisms related to RNA interference regulate expression of the small temporal RNAs that control C. elegans developmental timing

RNAi is a gene-silencing phenomenon triggered by double-stranded (ds) RNA and involves the generation of 21 to 26 nt RNA segments that guide mRNA destruction. In Caenorhabditis elegans, lin-4 and let-7 encode small temporal RNAs (stRNAs) of 22 nt that regulate stage-specific development. Here we show that inactivation of genes related to RNAi pathway genes, a homolog of Drosophila Dicer (dcr-1), and two homologs of rde-1 (alg-1 and alg-2), cause heterochronic phenotypes similar to lin-4 and let-7 mutations. Further we show that dcr-1, alg-1, and alg-2 are necessary for the maturation and activity of the lin-4 and let-7 stRNAs. Our findings suggest that a common processing machinery generates guide RNAs that mediate both RNAi and endogenous gene regulation.

Molecular and functional characterization of a family of rat brain T-type calcium channels

Voltage-gated calcium channels represent a heterogenous family of calcium-selective channels that can be distinguished by their molecular, electrophysiological, and pharmacological characteristics. We report here the molecular cloning and functional expression of three members of the low voltage-activated calcium channel family from rat brain (alpha(1G), alpha(1H), and alpha(1I)). Northern blot and reverse transcriptase-polymerase chain reaction analyses show alpha(1G), alpha(1H), and alpha(1I) to be expressed throughout the newborn and juvenile rat brain. In contrast, while alpha(1G) and alpha(1H) mRNA are expressed in all regions in adult rat brain, alpha(1I) mRNA expression is restricted to the striatum. Expression of alpha(1G), alpha(1H), and alpha(1I) subunits in HEK293 cells resulted in calcium currents with typical T-type channel characteristics: low voltage activation, negative steady-state inactivation, strongly voltage-dependent activation and inactivation, and slow deactivation. In addition, the direct electrophysiological comparison of alpha(1G), alpha(1H), and alpha(1I) under identical recording conditions also identified unique characteristics including activation and inactivation kinetics and permeability to divalent cations. Simulation of alpha(1G), alpha(1H), and alpha(1I) T-type channels in a thalamic neuron model cell produced unique firing patterns (burst versus tonic) typical of different brain nuclei and suggests that the three channel types make distinct contributions to neuronal physiology.

LET-413 is a basolateral protein required for the assembly of adherens junctions in Caenorhabditis elegans

Epithelial cells are polarized, with apical and basal compartments demarcated by tight and adherens junctions. Proper establishment of these subapical junctions is critical for normal development and histogenesis. We report the characterization of the gene let-413 which has a critical role in assembling adherens junctions in Caenorhabditis elegans. In let-413 mutants, adherens junctions are abnormal and mislocalized to more basolateral positions, epithelial cell polarity is affected and the actin cytoskeleton is disorganized. The LET-413 protein contains one PDZ domain and 16 leucine-rich repeats with high homology to proteins known to interact with small GTPases. Strikingly, LET-413 localizes to the basolateral membrane. We suggest that LET-413 acts as an adaptor protein involved in polarizing protein trafficking in epithelial cells.

UNC-60B, an ADF/cofilin family protein, is required for proper assembly of actin into myofibrils in Caenorhabditis elegans body wall muscle

The Caenorhabditis elegans unc-60 gene encodes two functionally distinct isoforms of ADF/cofilin that are implicated in myofibril assembly. Here, we show that one of the gene products, UNC-60B, is specifically required for proper assembly of actin into myofibrils. We found that all homozygous viable unc-60 mutations resided in the unc-60B coding region, indicating that UNC-60B is responsible for the Unc-60 phenotype. Wild-type UNC-60B had F-actin binding, partial actin depolymerizing, and weak F-actin severing activities in vitro. However, mutations in UNC-60B caused various alterations in these activities. Three missense mutations resulted in weaker F-actin binding and actin depolymerizing activities and complete loss of severing activity. The r398 mutation truncated three residues from the COOH terminus and resulted in the loss of severing activity and greater actin depolymerizing activity. The s1307 mutation in a putative actin-binding helix caused greater activity in actin-depolymerizing and severing. Using a specific antibody for UNC-60B, we found varying protein levels of UNC-60B in mutant animals, and that UNC-60B was expressed in embryonic muscles. Regardless of these various molecular phenotypes, actin was not properly assembled into embryonic myofibrils in all unc-60 mutants to similar extents. We conclude that precise control of actin filament dynamics by UNC-60B is required for proper integration of actin into myofibrils.

Identification and characterization of a serine hydroxymethyltransferase isoform in Caenorhabditis briggsae

In the nematode Caenorhabditis elegans, the maternal effect lethal gene mel-32 encodes a serine hydroxymethyltransferase isoform. Since interspecies DNA comparison is a valuable tool for identifying sequences that have been conserved because of their functional importance or role in regulating gene activity, mel-32(SHMT) genomic DNA from C. elegans was used to screen a genomic library from the closely related nematode Caenorhabditis briggsae. The C. briggsae genomic clone identified fully rescues the Mel-32 phenotype in C. elegans, indicating functional and regulatory conservation. Computer analysis reveals that CbMEL-32(SHMT) is 92% identical (97% similar) to CeMEL-32(SHMT) at the amino acid level over the entire length of the protein (484 amino acids), whereas the coding DNA is 82.5% identical (over 1455 nucleotides). Several highly conserved non-coding regions upstream and downstream of the mel-32(SHMT) gene reveal potential regulatory sites that may bind trans-acting protein factors.

Functional genomics in Caenorhabditis elegans: An approach involving comparisons of sequences from related nematodes

Comparative genomic analysis was used to investigate the gene structure of the bli-4 locus from two related Caenorhabditis species, C. elegans and C. briggsae. In C. elegans, bli-4 is a complex gene encoding a member of the kex2/subtilisin-like family of proprotein convertases. Genomic sequence comparisons coupled with RT-PCR analysis identified five additional coding exons that had not been identified previously using standard recombinant DNA techniques. The C. briggsae gene was able to rescue both viable blistered and developmentally arrested mutants of C. elegans bli-4, demonstrating functional conservation. In addition, deletion analysis of conserved sequences outside of coding regions, combined with phenotypic rescue experiments, identified regulatory elements that alter the expression of the bli-4 gene. These results demonstrate the utility of genomic sequence comparisons of homologous genes in related species as an effective tool with which to dissect the functional information of complex genes.

THE CAENORHABDITIS ELEGANS GENOME: A Guide in The Post Genomics Age

The completion of the entire genome sequence of the free-living nematode, Caenorhabditis elegans is a tremendous milestone in modern biology. Not only will scientists be poring over data mined from this resource, but techniques and methodologies developed along the way have changed the way we can approach biological questions. The completion of the C. elegans genomic sequence will be of particular importance to scientists working on parasitic nematodes. In many cases, these nematode species present intractable challenges to those interested in their biology and genetics. The data already compared from parasites to the C. elegans database reveals a wealth of opportunities for parasite biologists. It is likely that many of the same genes will be present in parasites and that these genes will have similar functions. Additional information regarding differences between free-living and parasitic species will provide insight into the evolution and nature of parasitism. Finally, genetic and genomic approaches to the study of parasitic nematodes now have a clearly marked path to follow.

Lethal mutations defining 112 complementation groups in a 4.5 Mb sequenced region of Caenorhabditis elegans chromosome III

The central gene cluster of chromosome III was one of the first regions to be sequenced by the Caenorhabditis elegans genome project. We have performed an essential gene analysis on the left part of this cluster, in the region around dpy-17III balanced by the duplication sDp3. We isolated 151 essential gene mutations and characterized them with regard to their arrest stages. To facilitate positioning of these mutations, we generated six new deficiencies that, together with preexisting chromosomal rearrangements, subdivide the region into 14 zones. The 151 mutations were mapped into these zones. They define 112 genes, of which 110 were previously unidentified. Thirteen of the zones have been anchored to the physical sequence by polymerase chain reaction deficiency mapping. Of the 112 essential genes mapped, 105 are within these 13 zones. They span 4.2 Mb of nucleotide sequence. From the nucleotide sequence data, 920 genes are predicted. From a Poisson distribution of our mutations, we predict that 234 of the genes will be essential genes. Thus, the 105 genes constitute 45% of the estimated number of essential genes in the physically defined zones and between 2 and 5% of all essential genes in C. elegans.

Serine hydroxymethyltransferase is maternally essential in Caenorhabditis elegans

The mel-32 gene in the free living soil nematode Caenorhabditis elegans encodes a serine hydroxymethyltransferase (SHMT) isoform. Seventeen ethylmethanesulfonate (EMS)-induced mutant alleles of mel-32(SHMT) have been generated, each of which causes a recessive maternal effect lethal phenotype. Animals homozygous for the SHMT mutations have no observable mutant phenotype, but their offspring display an embryonic lethal phenotype. The Mel-32 phenotype has been rescued with a transgenic array containing only mel-32(SHMT) genomic DNA. Heteroduplex analysis of the 17 alleles allowed 14 of the mutations to be positioned to small regions. Subsequent sequence analysis has shown that 16 of the alleles alter highly conserved amino acids, while one allele introduces a stop codon that truncates two thirds of the predicted protein. mel-32(SHMT) has a 55-60% identity at the amino acid level with both isoforms of SHMT found in yeast and humans and a 50% identity with the Escherichia coli isoform. The C. elegans mel-32 mutation represents the first case where SHMT has been shown to be an essential gene.

The PAX gene egl-38 mediates developmental patterning in Caenorhabditis elegans

Mutations in the C. elegans gene egl-38 result in a discrete set of defects in developmental pattern formation. In the developing egg-laying system of egl-38 mutant hermaphrodites, the identity of four uterine cells is disrupted and they adopt the fate of their neighbor cells. Likewise, the identity of two rectal epithelial cells in the male tail is disrupted and one of these cells adopts the fate of its neighbor cell. Genetic analysis suggests that the egl-38 functions in the tail and the egg-laying system are partially separable, as different egl-38 mutations can preferentially disrupt the different functions. We have cloned egl-38 and shown that it is a member of the PAX family of genes, which encode transcription factors implicated in a variety of developmental patterning events. The predicted EGL-38 protein is most similar to the mammalian class of proteins that includes PAX2, PAX5 and PAX8. The sequence of egl-38 mutant DNA indicates that the tissue-preferential defects of egl-38 mutations result from substitutions in the DNA-binding paired domain of the EGL-38 protein. egl-38 thus provides the first molecular genetic insight into two specific patterning events that occur during C. elegans development and also provides the opportunity to investigate the in vivo functions of this class of PAX proteins with single cell resolution.

Interpreting a sequenced genome: toward a cosmid transgenic library of Caenorhabditis elegans

We have generated a library of transgenic Caenorhabditis elegans strains that carry sequenced cosmids from the genome of the nematode. Each strain carries an extrachromosomal array containing a single cosmid, sequenced by the C. elegans Genome Sequencing Consortium, and a dominate Rol-6 marker. More than 500 transgenic strains representing 250 cosmids have been constructed. Collectively, these strains contain approximately 8 Mb of sequence data, or approximately 8% of the C. elegans genome. The transgenic strains are being used to rescue mutant phenotypes, resulting in a high-resolution map alignment of the genetic, physical, and DNA sequence maps of the nematode. We have chosen the region of chromosome III deleted by sDf127 and not covered by the duplication sDp8(III;I) as a starting point for a systematic correlation of mutant phenotypes with nucleotide sequence. In this defined region, we have identified 10 new essential genes whose mutant phenotypes range from developmental arrest at early larva, to maternal effect lethal. To date, 8 of these 10 essential genes have been rescued. In this region, these rescues represent approximately 10% of the genes predicted by GENEFINDER and considerably enhance the map alignment. Furthermore, this alignment facilitates future efforts to physically position and clone other genes in the region. [Updated information about the Transgenic Library is available via the Internet at http://darwin.mbb.sfu.ca/imbb/dbaillie/cos mid.html.]

An essential ubiquitin-conjugating enzyme with tissue and developmental specificity in th nematode Caenorhabditis elegans

The ubc-2 gene in Caenorhabditis elegans encodes a ubiquitin-conjugating enzyme (E2) homologous to yeast UBC4 and UBC5. UBC4 and UBC5 are individually dispensable class I E2 enzymes involved in the degradation of short-lived and abnormal proteins. Transgenic analysis using ubc-2-lacZ fusions and in situ immunofluorescence indicate that ubc-2 is abundantly expressed in most tissues of embryos and early larvae, but becomes specific to the nervous system in L4 larvae and adults. This suggests that the functions of this type of E2 are developmentally regulated in C.elegans. This hypothesis is supported by antisense analysis, which shows that blocking the expression of ubc-2 has a more severe effect in early developmental stages than in later stages. Through complementation of previously identified essential genes in the vicinity of ubc-2, we demonstrate that ubc-2 corresponds to let-70, a gene essential for C.elegans larval development. One let-70(ubc-2) allele contains a His75-->Tyr substitution, while another has an altered splice donor site.

Characterization of the let-653 gene in Caenorhabditis elegans

A mutation in the let-653 gene of Caenorhabditis elegans results in larval death. The lethal arrest is concurrent with the appearance of a vacuole anterior to the lower pharyngeal bulb. The position of the vacuole is consistent with a dysfunction of the secretory/excretory apparatus. Germline transformation rescue experiments were able to position the let-653 gene to two overlapping cosmid subclones. Sequence data generated from both cDNA and genomic DNA subclones indicated that let-653 encodes a mucin-like protein. Our characterization suggests that a mucin-like protein is essential for effective functioning of the secretory/excretory apparatus within C. elegans.

Molecular cloning and characterization of the dpy-20 gene of Caenorhabditis elegans

We describe the molecular analysis of the dpy-20 gene in Caenorhabditis elegans. Isolation of genomic sequences was facilitated by the availability of a mutation that resulted from insertion of a Tc1 transposable element into the dpy-20 gene. The Tc1 insertion site in the m474::Tc1 allele was identified and was found to lie within the coding region of dpy-20. Three revertants (two wild-type and one partial revertant) resulted from the excision of this Tc1 element. Genomic dpy-20 clones' were isolated from a library of wild-type DNA and were found to lie just to the left of the unc-22 locus on the physical map, compatible with the position of dpy-20 on the genetic map. Cosmid DNA containing the dpy-20 gene was successfully used to rescue the mutant phenotype of animals homozygous for another dpy-20 allele, e1282ts. Sequence analysis of the putative dpy-20 homologue in Caenorhabditis briggsae was performed to confirm identification of the coding regions of the C. elegans gene and to identify conserved regulatory regions. Sequence analysis of dpy-20 revealed that it was not similar to other genes encoding known cuticle components such as collagen or cuticulin. The dpy-20 gene product, therefore, identifies a previously unknown type of protein that may be directly or indirectly involved in cuticle function. Northern blot analysis showed that dpy-20 is expressed predominantly in the second larval stage and that the mRNA is not at all abundant. Data from temperature shift studies using the temperature-sensitive allele e1282ts showed that the sensitive period also occurs at approximately the second larval stage. Therefore, expression of dpy-20 mRNA and function of the DPY-20 protein are closely linked temporally.

Recovery of duplications by drug resistance selection in Caenorhabditis elegans

We have devised a scheme that facilitates rapid screening for duplications of essential loci. Our scheme takes advantage of the lev-1(x22) mutation, which confers resistance in a recessive fashion to the potent anthelmintic levamisole. We have tested our methodology by recovering two gamma ray induced duplications of let-56, the first essential gene to the left of unc-22. One of the duplications is attached to the fourth chromosome. The other duplication is attached to the X chromosome. This duplication contains a functional copy of the unc-22 gene, as well as functional copies of several essential loci adjacent to unc-22. Results we have obtained during analysis of this duplication are compatible with the notion that the copy of the unc-22 gene located on the duplication is subject to X chromosome dosage compensation.

The Caenorhabditis elegans unc-60 gene encodes proteins homologous to a family of actin-binding proteins

Mutations in the unc-60 gene of the nematode Caenorhabditis elegans result in paralysis. The thin filaments of the muscle cells are severely disorganized and not bundled with myosin into functional contractile units. Here we report the cloning and sequence of unc-60. Two unc-60 transcripts, 1.3 and 0.7 kb in size, were detected. The transcripts share a single exon encoding only the initial methionine, yet encode proteins with homologous sequences. The predicted protein products are 165 and 152 amino acids in length and their sequences are 38% identical. Both proteins are homologous to a family of actin depolymerizing proteins identified in vertebrate, plant and protozoan systems. We propose that the unc-60 locus encodes proteins that depolymerize growing actin filaments in muscle cells, and that these proteins are required for the assembly of actin filaments into the contractile myofilament lattice of C. elegans muscle. unc-60 has an essential function in development, since one unc-60 allele, s1586, has a recessive lethal phenotype. Our characterization of s1586 has shown that it is a small deletion which disrupts both coding regions.

The generation and genetic analysis of suppressors of lethal mutations in the Caenorhabditis elegans rol-3(V) gene

The Caenorhabditis elegans rol-3(e754) mutation is a member of a general class of mutations affecting gross morphology, presumably through disruption of the nematode cuticle. Adult worms homozygous for rol-3(e754) exhibit rotation about their long axis associated with a left-hand twisted cuticle, musculature, gut and ventral nerve cord. Our laboratory previously isolated 12 recessive lethal alleles of rol-3. All these lethal alleles cause an arrest in development at either early or mid-larval stages, suggesting that the rol-3 gene product performs an essential developmental function. Furthermore, through the use of the heterochronic mutants lin-14 and lin-29, we have established that the expression of rol-3(e754)'s adult specific visible function is not dependent on the presence of an adult cuticle. In an attempt to understand rol-3's developmental role we sought to identify other genes whose products interact with that of rol-3. Toward this end, we generated eight EMS induced and two gamma irradiation-induced recessive suppressors of the temperature sensitive (ts) mid-larval lethal phenotype of rol-3(s1040ts). These suppressors define two complementation groups srl-1 II and srl-2 III; and, while they suppress the rol-3(s1040) lethality, they do not suppress the adult specific visible rolling phenotype. Furthermore, there is a complex genetic interaction between srl-2 and srl-1 such that srl-2(s2506) fails to complement all srl alleles tested. These results suggest that srl-1 and srl-2 may share a common function and, thus, possibly constitute members of the same gene family. Mutations in both srl-1 and srl-2 produce no obvious hermaphrodite phenotypes in the absence of rol-3(s1040ts); however, males homozygous for either srl-1 or srl-2 display aberrant tail morphology. We present evidence suggesting that the members of srl-2 are not allele specific with respect to their suppression of rol-3 lethality, and that rol-3 may act in some way to influence proper posterior morphogenesis. Finally, based on our genetic analysis of rol-3 and the srl mutations, we present a model whereby the wild-type products of the srl loci act in a concerted manner to negatively regulate the rol-3 gene.

The use of deficiencies to determine essential gene content in the let-56-unc-22 region of Caenorhabditis elegans

We have investigated the possibility of using the polymerase chain reaction to detect deletions of coding elements in the unc-22-let-56 interval on chromosome IV in the nematode Caenorhabditis elegans. Our analysis of approximately 13 kb of genomic sequence immediately to the left of the unc-22 gene resulted in the identification of four possible genes. Partial cDNAs have been identified for three of them. To determine whether any of these coding elements are essential for development, we required a method for the induction and selection of mutations in these elements. Our approach was to identify a set of formaldehyde and gamma radiation induced unc-22 mutations that mapped to the unc-22-let-56 region, and then employ polymerase chain reaction methodology to identify deficiencies that affected one or more of the four identified coding elements. Two small deficiencies were identified in this manner. Characterization of these deficiencies shows that there are no coding elements between unc-22 and let-56 (the nearest mutationally identified gene to the left of unc-22), which are required in development under laboratory conditions. We conclude that the polymerase chain reaction is a practical tool for the detection of deletions of coding elements identified in this region, and that characterization of such deficiencies provides a method for assessing whether or not these elements are required for development.

Cloning and characterization of the C. elegans histidyl-tRNA synthetase gene

In this paper, we report the cloning and sequencing of the C. elegans histidyl-tRNA synthetase gene. The complete genomic sequence, and most of the cDNA sequence, of this gene is now determined. The gene size including flanking and coding regions is 2230 nucleotides long. Three small introns (45-50 bp long) are found to interrupt the open reading frame. The open reading frame translates to 523 amino acids. This putative protein sequence shows extensive homology with the human and yeast histidyl-tRNA the histidyl-tRNA synthetase gene is a single copy gene. Hence, it is very likely that it encodes both the cytoplasmic and the mitochondrial histidyl-tRNA synthetases. It is likely to be trans-spliced since it contains a trans-splice site in its 5' untranslated region.

Molecular analysis of two genes between let-653 and let-56 in the unc-22(IV) region of Caenorhabditis elegans

A previous study of genomic organization described the identification of nine potential coding regions in 150 kb of genomic DNA from the unc-22(IV) region of Caenorhabditis elegans. In this study, we focus on the genomic organization of a small interval of 0.1 map unit bordered on the right by unc-22 and on the left by the left-hand breakpoints of the deficiencies sDf9, sDf19 and sDf65. This small interval at present contains a single mutagenically defined locus, the essential gene let-56. The cosmid C11F2 has previously been used to rescue let-56. Therefore, at least some of C11F2 must reside in the interval. In this paper, we report the characterization of two coding elements that reside on C11F2. Analysis of nucleotide sequence data obtained from cDNAs and cosmid subclones revealed that one of the coding elements closely resembles aromatic amino acid decarboxylases from several species. The other of these coding elements was found to closely resemble a human growth factor activatable Na+/H+ antiporter. Paris of oligonucleotide primers, predicted from both coding elements, have been used in PCR experiments to position these coding elements between the left breakpoint of sDf19 and the left breakpoint of sDf65, between the essential genes let-653 and let-56.

Genetic analysis and complementation by germ-line transformation of lethal mutations in the unc-22 IV region of Caenorhabditis elegans

The subject of this study is the organization of essential genes in the 2 map-unit unc-22 IV region of the Caenorhabditis elegans genome. With the goal of achieving mutational saturation of essential genes in this region, 6491 chromosomes mutagenized with ethyl methanesulfonate (EMS) were screened for the presence of lethal mutations in the unc-22 region. The genetic analysis of 21 lethal mutations in the unc-22 region resulted in the identification of 6 new essential genes, making a total of 36 characterized to date. A minimum of 49 essential genes are estimated to lie in this region. A set of seven formaldehyde-induced deficiencies of unc-22 and surrounding loci were isolated to facilitate the positioning of essential genes on the genetic and physical maps. In order to study essential genes at the molecular level, our approach was to rescue lethal mutations by the injection of genomic DNA in the form of cosmid clones into the germ-line of balanced heterozygotes carrying a lethal mutation. The cosmid clones containing let-56 and let-653 were identified by this method.

Genetic analysis of a major segment [LGV(left)] of the genome of Caenorhabditis elegans

From 10,900 F1 progeny of ethyl methanesulfonate (EMS)-mutagenized Caenorhabditis elegans nematodes, we isolated 194 lethal mutations on the left arm of LGV, a region balanced by the reciprocal translocation of eT1. The analysis of 166 of those mutations resulted in the identification of one deficiency and alleles of 78 genes including 38 new genes, thus increasing the number of identified essential genes to 101. We estimate that there are a minimum of 120 essential genes in this region, which comprises approximately 7% of the recombinational distance, although only about 4.2% of the genes, in C. elegans. We calculate that there are a minimum of 2850 essential genes in the genome. The left arm of LGV has two recombinational gene clusters separated by a high-recombination and/or essential gene-sparse region. One gene in this region, let-330, is the largest EMS target on the left arm of LGV, with twice as many alleles (16) as the next most EMS-mutable genes, let-332 and rol-3. Another gene in the sparse region, lin-40, and the region near lin-40 are major targets for Tc1 mobilization-induced mutagenesis. The analysis of essential genes in large regions should help to define C. elegans in terms of all its genes and aid in the understanding of the relationship of genome structure to genome function.

Most ultraviolet irradiation induced mutations in the nematode Caenorhabditis elegans are chromosomal rearrangements

In this study we have determined the utility of 254-nm ultraviolet light (UV) as a mutagenic tool in C. elegans. We have demonstrated that irradiation of adult hermaphrodites provides a simple method for the induction of heritable chromosomal rearrangements. A screening protocol was employed that identifies either recessive lethal mutations in the 40 map unit region balanced by the translocation eT1(III;V), or unc-36(III) duplications. Mutations were recovered in 3% of the chromosomes screened after a dose of 120 J/m2. This rate resembles that for 1500 R gamma-ray-induced mutations selected in a similar manner. The mutations were classified either as lethals [mapping to Linkage Group (LG)III or LGV] or as putative unc-36 duplications. In contrast to the majority of UV-induced mutations analysed in microorganisms, we found that a large fraction of the C. elegans UV-induced mutations are not simple intragenic lesions, but are deficiencies for more than one adjacent gene or more complex events. Preliminary evidence for this conclusion came from the high frequency of mutations that had a dominant effect causing reduced numbers of adult progeny. Subsequently 6 out of 9 analysed LGV mutations were found to be deficiencies. Other specific rearrangements also identified were: one translocation, sT5(II;III), and two unc-36 duplications, sDp8 and sDp9. It was concluded that UV irradiation can easily be used as an additional tool for the analysis of C. elegans chromosomes, and that C. elegans should prove to be a useful organism in which to study the mechanisms whereby UV acts as a mutagen in cells of complex eukaryotes.

Evolutionarily conserved regions in Caenorhabditis transposable elements deduced by sequence comparison

In this paper we present the sequence of an intact Caenorhabditis briggsae transposable element, Tcb2. Tcb2 is 1606 base pairs in length and contains 80 base pair imperfect terminal repeats and a single open reading frame. We have identified blocks of T-rich repeats in the regions 150-200 and 1421-1476 of this element which are conserved in the Caenorhabditis elegans element Tc1. The sequence conservation of these regions in elements from different Caenorhabditis species suggests that they are of functional importance. A single open reading frame corresponding to the major open reading frame of Tc1 is conserved among Tc1, Tcb1, and Tcb2. Comparison of the first 550 nucleotides of the sequence among the three elements has allowed the evaluation of a model proposing an extension of the major open reading frame. Our data support the suggestion that Tc1 is capable of producing a 335 amino acid protein. A comparison of the sequence coding for the amino and carboxy termini of the 273 amino acid transposase from Caenorhabditis Tc1-like elements and Drosophila HB1 showed different amounts of divergence for each of these regions, indicating that the two functional domains have undergone different amounts of selection. Our data are not compatible with the proposal that Tc1-related sequences have been acquired via horizontal transmission. The divergence of Tc1 from the two C. briggsae elements, Tcb1 and Tcb2, indicated that all three elements have been diverging from each other for approximately the same amount of time as the genomes of the two species.

Functional elements and domains inferred from sequence comparisons of a heat shock gene in two nematodes

Caenorhabditis elegans and Caenorhabditis briggsae are two closely related nematode species that are nearly identical morphologically. Interspecific cross-hybridizing DNA appears to be restricted primarily to coding regions. We compared portions of the hsp-3 homologs, two grp 78-like genes, from C. elegans and C. briggsae and detected regions of DNA identity in the coding region, the 5' flanking DNAs, and the introns. The hsp-3 homologs share approximately 98% and 93% identity at the amino acid and nucleotide levels, respectively. Using the nucleotide substitution rate at the silent third position of the codons, we have estimated a lower limit for the date of divergence between C. elegans and C. briggsae to be approximately 23-32 million years ago. The 5' flanking DNAs and one of the introns contain elements that are highly conserved between C. elegans and C. briggsae. Some of the regions of nucleotide identity in the 5' flanking DNAs correspond to previously detected identities including viral enhancer sequences, a heat shock element, and an element present in the regulatory regions of mammalian grp78 and grp94 genes. We propose that a comparison of C. elegans and C. briggsae sequences will be useful in the detection of potential regulatory and structural elements.

Genetic and fine structure analysis of unc-26(IV) and adjacent regions in Caenorhabditis elegans

The genetic organization of unc-26(IV) and adjacent regions was studied in Caenorhabditis elegans. We constructed a fine structure genetic map of unc-26(IV), a gene that affects locomotion and pharyngeal muscle movement but not muscle structure. Eleven alleles were positioned relative to each other recombinationally and were classified according to phenotypic severity. The unc-26 gene spans at least 0.026 map units, which is exceptionally large for a C. elegans gene. All but one allele, e205, are amorphic alleles. Interestingly, e205 is hypomorphic but also suppressible by the amber suppressor sup-7. Nineteen lethal mutations in the unc-26 region were isolated and characterized. The unc-26 region is subdivided into four zones by five deficiency breakpoints. These mutations fall into 15 complementation groups. The stages of development affected by these mutations were determined.

Pairing for recombination in LGV of Caenorhabditis elegans: a model based on recombination in deficiency heterozygotes

The effect of deficiencies on recombination was studied in Caenorhabditis elegans. Heterozygous deficiencies in the left half of linkage group V [LGV(left)] were shown to inhibit recombination to their right. Fourteen deficiencies, all to the left of unc-46, were analyzed for their effect on recombination along LGV. The deficiencies fell into two groups: 10 "major inhibitors" which reduce recombination to less than 11% of the expected rate between themselves and unc-46; and four "minor inhibitors" which reduce recombination, but to a much lesser extent. All four minor inhibitors delete the left-most known gene on the chromosome, while six of the ten major inhibitors do not (i.e., these are "internal" deficiencies). Where recombination could be measured on both sides of a deficiency, recombination was inhibited to the right but not to the left. In order to explain these results we have erected a model for the manner in which pairing for recombination takes place. In doing so, we identify a new region of LGV, near the left terminus, that is important for the pairing process.

Analysis of lethal mutations induced in a mutator strain that activates transposable elements in Caenorhabditis elegans

A screen was conducted for lethal mutations in the nematode Caenorhabditis elegans in a strain containing the mutator mut-4 (st700)I to examine the nature of mutator-induced lethal mutations within two large chromosomal regions comprising a total of 49 map units (linkage group IV (right) and linkage group V (left)). The genetic analysis of 28 lethal mutations has revealed that the mutator locus mut-4(st700)I causes both putative single-gene mutations and deficiencies. We have identified lethal mutations in three different genes, in addition to seven deficiencies. There is a mutational hot spot on linkage group V (left) around the lin-40 locus. Six mutations appear to be alleles of lin-40. In addition, 5 of 7 deficiencies have breakpoints at or very near lin-40. All seven deficiencies delete the left-most known gene on linkage group V (left) and thus appear to delete the tip of the chromosome. This is in contrast to gamma ray and formaldehyde induced deficiencies, which infrequently delete the closest known gene to the tip of a chromosome.

The HSP70 multigene family of Caenorhabditis elegans

1. The heat shock response of the nematode Caenorhabditis elegans has been characterized. 2. There are at least nine genes in the hsp70 multigene family of C. elegans. 3. Five of the hsp70 genes have been characterized and assigned to one of at least three hsp70 gene subfamilies. One of the subfamilies consists of an hsp70 protein that has the potential to be translocated into the endoplasmic reticulum and another subfamily consists of a protein that has the potential to be translocated into the mitochondria. 4. The C. elegans hsp70 multigene family has several unique characteristics including introns in the heat inducible hsp70 genes, at least one trans-spliced hsp70 mRNA and two grp78 related genes, one of which is highly heat inducible. 5. The identification and characterization of C. elegans hsp70 multigene family is the basis for a genetic characterization of the regulation and function of a gene family during the development of a multicellular eukaryote.

Evolutionarily conserved coding sequences in the dpy-20-unc-22 region of Caenorhabditis elegans

Caenorhabditis elegans provides an excellent opportunity to study the organization of a complex genome. The alignment of the genetic and molecular maps over a large stretch of the genome is an essential part of this study. The objective of this paper was the identification and characterization of coding regions in four cosmids containing DNA from the interval between dpy-20 and unc-22 on linkage group IV. These cosmids were characterized with regard to the map position and the developmental patterns of expression of coding sequences. Since an extensive genetic map already exists for this region, this detailed description of the coding sequences in the dpy-20-unc-22 region will make possible alignment of the molecular and genetic maps for this portion of the C. elegans genome. In this study, we have used interspecies cross-hybridization to localize and identify potential coding elements. We have investigated four cosmids containing approximately 150 kb of C. elegans genome adjacent to the well-characterized muscle gene, unc-22(IV). Fragments subcloned from the four cosmids were hybridized at moderate stringency to the genome of the related species, Caenorhabditis briggsae. In this way nine potential coding regions were identified. Seven of these nine fragments also hybridized to mRNA transcripts on Northern blots. Five of the seven showed maximal hybridization to RNA from L2-stage animals, a pattern that resembles that of actin transcription. It is speculated that the functions of these five may in some way be related to one another, and perhaps also to that of unc-22, which is itself a muscle gene.

Characterization of the hsp70 multigene family of Caenorhabditis elegans

Our laboratory has been characterizing the hsp70 multigene family from the nematode Caenorhabditis elegans as the first step to the genetic characterization of the heat shock response in a relatively simple multicellular eukaryote. Two gene members, hsp-1 and hsp-2ps have already been characterized (Snutch et al., 1988; Heschl and Baillie, 1989). The third gene member, hsp-3, is expressed constitutively and is non-heat inducible; its mRNA is most abundant at the L1 larval stage. The hsp-3 protein (hsp70C) shares a high degree of identity with the rat grp78 protein and has a long, hydrophobic leader sequence. The carboxyl terminus of hsp70C has the putative ER-retention signal, KDEL. The fourth gene member, hsp-6 is expressed constitutively and moderately heat inducible. A partial hsp-6 protein (hsp70F) sequence shares a higher degree of identity with the Escherichia coli dnaK protein than with eukaryotic hsp70 proteins. The predicted amino-terminal half of the hsp70F polypeptide also contains a long, amphiphilic leader sequence similar to mitochondrial import leader sequences. These two genes encode proteins that potentially cross intracellular membranes. We compared the 5'-flanking DNA from the C. elegans hsp-3 gene to fragment containing enhancer activity from the rat grp78 gene regulatory region (Lin et al., 1986). A 23-nucleotide sequence was conserved between the two promoter regions. This sequence shares approximately 80% identity between these two evolutionary distant organisms. A comparison to other hsp70 genes did not reveal any conservation of this 23-nucleotide sequence. We propose that this sequence may be involved in a unique aspect of the regulation of the C. elegans' grp78-like gene and the rat grp78 gene.

Identification of a heat-shock pseudogene from Caenorhabditis elegans

While characterizing the hsp70 gene family from Caenorhabditis elegans we encountered an unusual member of this family. Sequence data reveal that the hsp-2ps gene is a pseudogene of the constitutively expressed, heat-inducible hsp-1 gene. Two stop codons generated near the 5' end of the sequence as well as several frameshift mutations and a large internal deletion confirm the identification of hsp-2ps as a pseudogene. The nucleotide substitution rate of the third codon position was twice that of the first and second codon positions, suggesting that the hsp-2ps gene was nonfunctional since the time of the duplication event. The hsp-2ps gene duplicates a region of the hsp-1 gene that lies exclusively within the transcribed region and retains the introns. We feel that the hsp-2ps gene was produced by a transpositional duplication event, which occurred approximately 8.5 million years ago.

Formaldehyde mutagenesis of the eT1 balanced region in Caenorhabditis elegans: dose-response curve and the analysis of mutational events

In this study we have generated a dose-response curve for the formaldehyde induction of recessive lethal mutations in the eT1(III;V)-balanced region of C. elegans. We have mapped 96 out of 112 formaldehyde-induced lesions to either LGIII or LGV and genetically analyzed 31 lesions that mapped to LGV. Our findings showed that a 4-h treatment with 0.1% formaldehyde gave the best mutation induction frequency with the least side effects. We found that formaldehyde induced putative point mutations, deficiencies and more complex lesions in C. elegans. We isolated 11 putative point mutations, 3 of which defined new genes and 8 were alleles of known genes. One of the new genes, let-450, is currently the left-most known gene on LGV. We also isolated 5 deficiencies. Our formaldehyde-induced lesions increased the number of zones in the eT1-balanced region of LGV from 22 to 34.

Sequence identity between an inverted repeat family of transposable elements in Drosophila and Caenorhabditis

The Tc1-like transposable elements, originally described in Caenorhabditis elegans, have a much wider phylogenetic distribution than previously thought. In this paper, we demonstrate that Tc1 shares sequence identity in its open reading frame and terminal repeats with a new transposable element Barney (also known as TCb1-Transposon Caenorhabditis briggsae 1). Barney was detected and isolated by Tc1 hybridization from the closely related nematode species, Caenorhabditis briggsae. The conserved open reading frames of Tc1 and Barney share identity with a structurally similar family of elements named HB found in Drosophila melanogaster, after the introduction of 3 small centrally located deletions in HB1. These reading frames would code for proteins with 30% amino acid identity (42% when conservative changes are included). Tc1, Barney and HB1 contain highly conserved blocks of amino acids which are likely to be in the functional domains of the putative transposase.

The unc-22(IV) region of Caenorhabditis elegans: genetic analysis of lethal mutations

The organization of essential genes in the unc-22 region, defined by the deficiency sDf2 on linkage group IV, has been studied. Using the balancer nT1 (IV;V), which suppresses recombination over 49 map units, 294 lethal mutations on LGIV(right) and LGV(left) were recovered using EMS mutagenesis. Twenty-six of these mutations fell into the unc-22 region. Together with previously isolated lethal mutations, there is now a total of 63 lethal mutations which fall into 31 complementation groups. Mutations were positioned on the map using eight overlapping deficiencies in addition to sDf2. The lethal alleles and deficiencies in the unc-22 region were characterized with respect to their terminal phenotypes. Mapping of these lethal mutations shows that sDf2 deletes a minimum of 1.8 map units and a maximum of 2.5 map units. A minimum estimate of essential gene number for the region using a truncated Poisson calculation is 48. The data indicate a minimum estimate of approximately 3500 essential genes in the Caenorhabditis elegans genome.

The Caenorhabditis elegans hsp70 gene family: a molecular genetic characterization

We have isolated genomic clones representing six distinct members of the Caenorhabditis elegans 70-kDa heat-shock protein gene (hsp70) family. Each member exists as a single copy element in the C. elegans genome. Transcripts of four of the hsp70 genes have been detected by Northern-blot analysis. One member, hsp70C, appears to be a heat-shock-cognate hsp70 gene (hsc70) since its transcription is developmentally regulated and is not increased in response to heat shock. Transcripts of another gene, hsp70A, are abundant in control worms and are also increased (two- to six-fold) upon heat shock. Nucleotide sequencing of genomic and cDNA clones of hsp70A reveals that it is highly homologous to Drosophila and yeast heat-shock-inducible and heat-shock-cognate hsp70 genes. Three DNA elements homologous to the heat-shock promoter, 5'-C--GAA--TTC--G-3' are located upstream from the Hsp70A-coding region. We find that hsp70A contains three introns, one of which is in a similar position with an intron in the Drosophila hsc1 and hsc2 genes. Finally, utilizing strain-specific restriction fragment length differences, we have mapped the chromosomal position of hsp70A to the far right of chromosome IV.

Genetic organization of the unc-60 region in Caenorhabditis elegans

We have investigated the chromosomal region around unc-60 V, a gene affecting muscle structure, in the nematode Caenorhabditis elegans. The region studied covers 3 map units and lies at the left end of linkage group (LG) V. Compared to the region around dpy-11 (at the center of LGV), the unc-60 region has relatively few visible genes per map unit. We found the same to be true for essential genes. By screening simultaneously for recessive lethals closely linked to either dpy-11 or unc-60, we recovered ethyl methanesulfonate-induced mutations in 10 essential genes near dpy-11 but in only two genes near unc-60. Four deficiency breakpoints were mapped to the unc-60 region. Using recombination and deficiency mapping we established the following gene order: let-336, unc-34, let-326, unc-60, emb-29, let-426. Regarding unc-60 itself, we compared the effect of ten alleles (including five isolated during this study) on hermaphrodite mobility and fecundity. We used intragenic mapping to position eight of these alleles. The results show that these alleles are not distributed uniformly within the gene, but map to two groups approximately 0.012 map unit apart.

Genetic organization of the unc-22 IV gene and the adjacent region in Caenorhabditis elegans

The genetic organization of the region immediately adjacent to the unc-22 IV gene in Caenorhabditis elegans has been studied. We have identified twenty essential genes in this interval of approximately 1.5-map units on Linkage Group IV. The mutations that define these genes were positioned by recombination mapping and complementation with several deficiencies. With few exceptions, the positions obtained by these two methods agreed. Eight of the twenty essential genes identified are represented by more than one allele. Three possible internal deletions of the unc-22 gene have been located by intra-genic mapping. In addition, the right end point of a deficiency or an inversion affecting the adjacent genes let-56 and unc-22 has been positioned inside the unc-22 gene.

Cloning within the unc-43 to unc-31 interval (linkage group IV) of the Caenorhabditis elegans genome using Tc1 linkage selection

The region around the twitcher gene, unc-22, flanked by unc-43 on the left and by unc-31 on the right, has been intensively studied in our laboratory over the period of the last 8 years. In this paper we describe the identification and isolation of probes specific for several restriction fragment length differences (RFLDs) which lie within this region. Many RFLDs in Caenorhabditis elegans are caused by the insertion of a transposable element, Tc1. The method we used involved the isolation of Tc1-containing genomic fragments. These were recovered from a lambda gt 10 library of DNA from a specially constructed genetic strain containing the unc-43 to unc-31 interval from the BO strain and the rest of the genome from N2. Because the BO strain is rich in Tc1 insertion sites and the N2 strain has few, the majority of Tc1-bearing genomic fragments in the constructed strain were derived from the unc-22 region. Of nine such Tc1-bearing genomic fragments isolated, six were found which mapped within the region of interest. The 350 kilobases of genomic sequences isolated as a result of these studies are being used to study the molecular organization of this region. The method described here for Tc1 linkage selection is one that is rapid, general, and may be targeted to any genetically characterized region of the C. elegans genome.

Mutagenesis in Caenorhabditis elegans. II. A spectrum of mutational events induced with 1500 r of gamma-radiation

We previously established a gamma-ray dose-response curve for recessive lethal events (lethals) captured over the eT1 balancer. In this paper we analyze the nature of lethal events produced, with a frequency of 0.04 per eT1 region, at a dose of 1500 r. To do so, we developed a protocol that, in the absence of cytogenetics, allows balanced lethals to be analyzed for associated chromosomal rearrangements. A set of 35 lethal strains was chosen for the analysis. Although the dosage was relatively low, a large number of multiple-break events were observed. The fraction of lethals associated with rearrangements was found to be 0.76. Currently most X- and gamma-ray dosages used for mutagenesis in C. elegans are 6000-8000 r. From our data we conservatively estimated that 43% of rearrangements induced with 8000 r would be accompanied by additional chromosome breaks in the genome. With 1500 r the value was 5%. The 35 lethals studied were derived from 875 screened F1's. Among these lethals there were (1) at least two unc-36 duplications, (2) at least four translocations, (3) at least six deficiencies of chromosome V (these delete about 90% of the unc-60 to unc-42 region) and (4) several unanalyzed rearrangements. Thus, it is possible to recover desired rearrangements at reasonable rates with a dose of only 1500 r. We suggest that the levels of ionizing radiation employed in most published C. elegans studies are excessive and efforts should be made to use reduced levels in the future.

Meiotic pairing behavior of two free duplications of linkage group I in Caenorhabditis elegans

In this paper we describe the meiotic pairing behavior of two free duplications in Caenorhabditis elegans. sDp1 is a duplication of approximately 30 map units of the right portion of linkage group I including unc-74 to unc-54. This duplication pairs, recombines, and apparently segregates from one of the normal homologues. A second duplication, sDp2, is a duplication of approximately 15 map units of the left portion of the linkage group. sDp2 was not observed to recombine with the normal homologue but did suppress exchange between the two normal homologues in a sDp2/ ++/ dpy-5 unc-35 heterozygote. Although a number of free duplications have been described previously in Caenorhabditis elegans, none of these have been shown to pair with normal homologues. The meiotic behavior of the duplications described in this paper can be understood assuming the existence in C. elegans chromosomes of pairing sites of the type described in D. melanogaster chromosomes (I. Sandler 1956; Hawley 1980).