Genes controlling ontogenesis: morphosis, phenocopies, dimorphs, and other visible expressions of mutant genes

We studied facultative dominant lethal mutations obtained earlier in Drosophila melanogaster. In some genotypes, these mutations were expressed as lethals, but in other genotypes they lacked this expression. The mutations were maintained in the following cultures: (1) females Muller-5 heterozygous for the mutation; (2) males crossed to attached-X females; and females and males homozygous for the mutation. During culturing, many mutations were found to give rise to phenotypically abnormal progeny. Generally, these abnormalities were morphoses involving various body parts; they were mostly asymmetric and non-heritable. Maternal and paternal effects in the formation of morphoses were observed. In four cases, dimorphic mutations were recorded: a female homozygous for the mutation had mutant phenotype whereas its male counterpart was phenotypically normal. The mutations were recessive with regard to the norm. New phenotypes behaving as mutations with incomplete penetrance arose during culturing. In cultures of mutant homozygotes phenocopies would appear en masse; they would persist for one or two generations and disappear. One wave of phenocopies succeeded another. Visible phenotypes appeared, which further behaved as ordinary recessive mutations. We concluded that these visible manifestations are characteristic for regulatory mutations controlling ontogeny. Their appearance is explained by the activation of new regulatory scenarios caused by blocking standard regulatory pathways.     

Towards a male-only release system for SIT with the Queensland fruit fly,Bactrocera tryoni,using a genetic sexing strain with a temperature-sensitive lethal mutation

Flies that are homozygous for the recessive autosomal mutation bent wingshave a limited ability to fly and are less tolerant of high temperatures than normal flies in both the egg and puparial stages. The differences between the mutant and normal flies were found sufficient to be the basis of a genetic sexing strain. Genetic sexing strains were created using translocations of the autosome bearing the wild-type allele of bent wings(chromosome 2) to the Y chromosome, and crossing male flies carrying the translocation to mutant bent wingsfemales. In the resulting strain, the females were homozygous for the bent wingsmutation and the males were phenotypically normal for wing characters. Several translocations were recovered after irradiation, but only one translocation involving chromosome 2 was both stable and expressed in a stock that was vigorous enough for long-term viability. Unfortunately, all stocks containing the translocation showed high levels of temperature-dependent lethality, including, inexplicably, both males and females. Translocation stocks showing this effect included bent wings, another second chromosome mutation, white marks, and an otherwise normal stock. This phenomenon is probably rare, as it has not been reported before. It is likely that bent wingscould be suitably used with another translocation.     

Battle of the Xs

Females and males often exhibit conspicuous morphological, physiological and behavioral differences. Similarly, gene expression profiles indicate that a large portion of the genome is sex‐differentially deployed, particularly in the germ line. Because males and females are so fundamentally different, each sex is likely to have a different optimal gene expression profile that is never fully achieved in either sex because of antagonistic selection in females versus males. Males are hemizygous for the X chromosome, which means that recessive male‐favorable de novo mutations on the X chromosome are subject to immediate selection. In females, a recessive female‐favorable mutation on one of two X chromosomes is not available for selection until it becomes frequent enough in the local population to result in homozygous individuals. Given that most mutations are recessive, one would expect that genes or alleles favoring males should accumulate on the X chromosome. Recent microarray work in Drosophila and C. elegans clearly shows the opposite. Why is the X chromosome a highly disfavored location for genes with male‐biased expression in these animals? BioEssays 26:543–548, 2004. Published 2004 Wiley Periodicals, Inc.     

Parental effects of conditional mutations and their explanations

A study of the properties of conditional dominant and recessive lethals in Drosophila melanogaster has demonstrated parental effects in the inheritance and manifestation of these mutations. Maternal and paternal effects are present when conditional mutations interact with (1) one another, (2) the Y chromosome, or (3) chromosomal rearrangements, as well as (4) when the visual expression of a conditional mutation is inherited or (5) during the formation of morphoses (monstrosities) in mutant offspring. The maternal and paternal effects do not exclude one another: the same mutation can display both patterns. The characters manifesting themselves at late developmental stages (morphoses) are inherited according to a parental effect pattern. A general concept of the parental effect is proposed and its types are classified.     

Enhanced longevity in tau mutant Syrian hamsters,Mesocricetus auratus

The single-gene mutation tau in the Syrian hamster shortens the circadian period by about 20% in the homozygous mutant and simultaneously increases the mass-specific metabolic rate by about 20%. Both effects might be expected to lead to a change in longevity. To test such expectations, the life span of male and female hamsters from three genotypes (wild-type, heterozygous, and homozygous tau mutants, all derived from heterozygote crosses to randomize the genetic background) was recorded in constant darkness. Male hamsters lived significantly longer than females: the overall average life span was 96.9 weeks (SE = 2.5, n = 118) for males and 82.0 weeks (SE = 2.1, n = 99) for females. To our surprise, male and female homozygous mutant hamsters lived significantly longer rather than shorter compared to wild-types. For males, the difference between the two genotypes was on average 14%; for females, the difference was 16%. The mortality rate of wild-type males was significantly different from that of homozygous tau males but not different from that of heterozygotes. Overall, survival of wild-type females was statistically distinguishable from both heterozygous and homozygous mutant females. Male and female wild-type hamsters were heavier than homozygote mutants throughout the entire life span, and heterozygous mutants had intermediate weights. There was no correlation between body mass and life span, and the causes of the extended life span in tau mutant hamsters remain unresolved.     

Butyrate sensitive suppressor of position-effect variegation mutations in drosophila melanogaster

Summary Mutations at a locus on chromosome II of D. melanogaster suppressing position-effect variegation mutations have been identified which display recessive butyrate sensitivity. Survival of homozygous mutant flies is significantly reduced on medium containing sodium n-butyrate. The butyrate sensitive suppressor mutations are further characterized by recessive female sterility and reduced survival of homozygotes. Complementation analysis showed their allelism. The locus of these mutations, Su-var (2) 1, has been localized to 40.5±0.2 and, by using interstitial duplications, to region 31CD on the cytogenetic map. Moreover, the mutant alleles of the Su-var (2) 1 locus display a lethal interaction with the heterochromatic Y chromosome. The presence or absence of a Y chromosome in males or females has a strong influence on the viability of homozygous or transheterozygous suppressor flies. All the genetic properties of Su-var (2) 1 mutants suggest strongly that this locus affects chromosome condensation.     

Biparental inheritance of chloroplast DNA and the existence of heteroplasmic cells in alfalfa

Summary Mapping of chloroplast DNA (ctDNA) restriction fragment patterns from a chlorophyll deficient mutant and two phenotypically normal alfalfa genotypes (Medicago sativa L.) has demonstrated the existence of a distinct ctDNA genotype from each source. These unique restriction fragment patterns were utilized to identify maternal or paternal origin of ctDNA in hybrid plants from crosses involving the normal alfalfa genotypes as females and the yellow-green chlorophyll deficient sectors as males. Progeny from these crosses expressing the yellow-green sectored phenotypes contained paternal ctDNA in the chlorophyll deficient sectors and maternal ctDNA in the normal sectors, confirming biparental plastid inheritance. The existence of mixed cells containing both mutant and normal plastids at various stages of sorting-out was observed by transmission electron microscopy of mesophyll cells in mosaic tissue from hybrid plants. This observation verified the biparental transmission of plastids in alfalfa.     

Mutations affecting skeletal muscle myofibril structure in the zebrafish

We describe embryonic lethal mutations in the zebrafish, Brachydanio rerio, which affect organization of skeletal muscle myofibrils. The mutations, fub-1(b45) and fub-1(b126), were independently isolated from progeny of gamma-irradiated females. Each segregates as a single recessive gene: b45 is located about 23 map units from its centromere. The b126 mutation has a similar but slightly larger apparent gene-centromere distance and a less severe phenotype. The two mutations fail to complement, suggesting that they are allelic. Homozygous b45 mutant embryos are paralyzed, and their axial skeletal muscle cells are unstriated, containing severely disorganized myofibrillar components. Gel-electrophoretic comparisons of b45 mutant and wild-type muscle proteins failed to reveal absent or altered major myofibrillar proteins. Embryos genetically mosaic for b45 were also phenotypically mosaic, suggesting that the defect is cell-autonomous. We suggest that these mutations identify a gene required for proper organization of skeletal muscle myofibrils, and that the more severe mutation may represent a null allele.     

Genetic and Molecular Analysis of Spe-27, a Gene Required for Spermiogenesis in Caenorhabditis Elegans Hermaphrodites

Hermaphrodites with mutations in the spe-27 gene are self-sterile, laying only unfertilized eggs; mutant males are fertile. Hermaphrodites make spermatids that fail to activate to crawling spermatozoa so passing oocytes sweep them out of the spermatheca. These spermatids do activate and produce self-progeny if young mutant hermaphrodites are mated by fertile (or sterile) males. Spermatids isolated from either mutant males or hermaphrodites initiate activation in vitro when treated with proteases, but then arrest with spiky membrane projections that resemble those of a normal intermediate in pseudoped formation. These phenotypes are identical to spe-8 and spe-12 mutants. They can be explained if males and hermaphrodites have distinct pathways for spermatid activation, and these three genes are necessary only for the hermaphrodite pathway. Consistent with this model, when spe-27 mutant male spermatids without seminal fluid are artificially inseminated into hermaphrodites, they fail to activate. The spe-27 gene has been isolated, sequenced and its regulatory regions identified. The sequence predicts a 131 amino acid polypeptide that has no striking structural motifs and no resemblance to known proteins. Two of the mutations in spe-27 alter mRNA splicing; a third mutation is a temperature-sensitive missense mutation.     

Coupled Instability of Two X-Linked Genes in DROSOPHILA MAURITIANA: Germinal and Somatic Mutability

A highly unstable allele has been isolated at the white locus of Drosophila mauritiana, a sibling species of D. melanogaster. This allele, white-peach (wpch), mutates spontaneously in males and females to give both wild-type and bleached-white derivatives. The mutation frequency is about 10(-3) mutations/generation. There is no evidence for clustering among mutant progeny, and phenotypically wpch flies with mosaic patches of wild-type tissue in the eyes are frequently recovered. Another X-linked locus, plum, is destabilized when wpch is on the same X chromosome.     

Spontaneous and Ethyl Methanesulfonate-Induced Mutations Controlling Viability in DROSOPHILA MELANOGASTER. II. Homozygous Effect of Polygenic Mutations

Polygenic mutations affecting viability were accumulated on the second chromosome of Drosophila melanogaster by treating flies with EMS in successive generations. The treated chromosomes were later made homozygous and tested for their effects on viability by comparison of the frequency of such homozygotes with that of other genotypes in the same culture. The treated wild-type chromosomes were kept heterozygous in Pm/+ males by mating individual males in successive generations to Cy/Pm females. The number of generations of accumulation was 1 to 30 generations, depending on the concentration of EMS. A similar experiment for spontaneous polygenic mutations was also conducted by accumulating mutations for 40 generations. The lower limit of the spontaneous mutation rate of viability polygenes is estimated to be 0.06 per second chromosome per generation, which is about 12 times as high as the spontaneous recessive lethal mutation rate, 0.005. EMS-induced polygenic mutations increase linearly with the number of treated generations and with the concentration of EMS. The minimum mutation rate of viability polygenes is about 0.017 per 10(-4)m, which is only slightly larger than the lethal rate of 0.013 per 10(-4) m. The maximum estimate of the viability reduction of a single mutant is about 6 to 10 percent of the normal viability. The data are consistent with a constant average effect per mutant at all concentrations, but this is about three times as high as that for spontaneous mutants. It is obvious that one can obtain only a lower limit for the mutation rate, since some mutants may have effects so near to zero that they cannot be detected. The possibility of measuring something other than the lower limit is discussed. The ratio of the load due to detrimental mutants to that caused by lethals, the D/L ratio, is about 0.2 to 0.3 for EMS-induced mutants, as compared to about 0.5 for spontaneous mutants. This is to be expected if EMS treatment produces a large fraction of small deletions and other chromosome rearrangements which are more likely to be lethal.     

Male-Specific Lethal Mutations of DROSOPHILA MELANOGASTER

A total of 7,416 ethyl methanesulfonate (EMS)-treated second chromosomes and 6,212 EMS-treated third chromosomes were screened for sex-specific lethals. Four new recessive male-specific lethal mutations were recovered. When in homozygous condition, each of these mutations kills males during the late larval or early pupal stages, but has no detectable effect in females. One mutant, mle^ts, is a temperature sensitive allele of maleless, mle (Fukunaga, Tanaka and Oishi 1975), while the other three mutants identify two new loci: male-specific lethal-1 (msl-1) (two alleles) at map position 2-53.3 and male-specific lethal-2 (msl-2) at 2-9.0.——The male-specific lethality associated with these mutants is not related to the sex per se of the mutant flies, since sex-transforming genes fail to interact with these mutations. Moreover, the presence or absence of a Y chromosome in males or females has no influence on the male-specific lethal action of these mutations. Finally, no single region of the X chromosome, when present as a duplication, is sufficient to rescue males from the lethal effects of msl-1 or msl-2. These results suggest that the number of complete X chromosomes determines whether a fly homozygous for a male-specific lethal mutation lives or dies.     

Evidence of a Dual Function in Fl(2)d, a Gene Needed for Sex-Lethal Expression in Drosophila Melanogaster

In Drosophila melanogaster, the female sexual development of the soma and the germline requires the activity of the gene Sxl. The somatic cells need the function of the gene fl(2)d to follow the female developmental pathway, due to its involvement in the female-specific splicing of Sxl RNA. Here we report the analysis of both fl(2)d1 and fl(2)d2 mutations: (1) fl(2)d1 is a temperature-sensitive mutation lethal in females and semilethal in males; (2) fl(2)d2 is lethal in both sexes; (3) the fl(2)d1/fl(2)d2 constitution is temperature-sensitive and lethal in females, while semilethal in males. The temperature-sensitive period of fl(2)d1 in females expands the whole development. SxlM1 partially suppresses the lethality of fl(2)d1 homozygous females and that of fl(2)d1/fl(2)d2 constitution, whereas it does not suppress the lethality of fl(2)d2 homozygous females. The addition of extra Sxl+ copies does not increase the suppression effect of SxlM1. The fl(2)d1 mutation in homozygosis and the fl(2)d1/fl(2)d2 constitution, but not the fl(2)d2 in homozygosis, partially suppress the lethality of SxlM1 males. This suppression is not prevented by the addition of extra Sxl+ copies. The semilethality of both fl(2)d1 and fl(2)d1/fl(2)d2 males, and the lethality of fl(2)d2 males, is independent of Sxl function. There is no female synergistic lethality between mutations at fl(2)d and neither at sc or da. However, the female synergistic lethality between mutations at Sxl and either sc or da is increased by fl(2)d mutations. We have analyzed the effect of the fl(2)d mutations on the germline development of both females and males. For that purpose, we carried out the clonal analysis of fl(2)d1 in the germline. In addition, pole cells homozygous for fl(2)d2 were transplanted into wild-type host embryos, and we checked whether the mutant pole cells were capable of forming functional gametes. The results indicated that fl(2)d mutant germ cells cannot give rise to functional oocytes, while they can form functional sperm. Moreover, SxlM1 suppresses the sterility of the fl(2)d1 homozygous females developing at the permissive temperature. Thus, with respect to the development of the germline the fl(2)d mutations mimic the behavior of loss-of-function mutations at the gene Sxl. Females double heterozygous for fl(2)d and snf1621 are fully viable and fertile. fl(2)d2 in heterozygosis partially suppresses the phenotype of female germ cells homozygous for snf1621; however, this is not the case with the fl(2)d1 mutation. The fl(2)d mutations partially suppress the phenotype of the female germ cells homozygous for ovoDIrSI.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of gibberellin mutations on tolerance to apical meristem damage in Arabidopsis thaliana

To examine the role of gibberellin hormones (GAs) in tolerance to apical meristem damage (AMD), we characterized the reaction norms of several GA-deficient and insensitive mutants of Arabidopsis thaliana in response to AMD and compared them to those of the wild type, Landsberg, from which they were derived. We included 'natural' genotypes of A. thaliana--accessions with shorter lab histories--in order to evaluate how representative Landsberg is of other genotypes. The GA mutations did not alter the level of tolerance to AMD, which was consistent with equal compensation for all genotypes. Generally, the reaction norms to AMD did not differ among the GA mutants themselves, or between the GA mutants and Landsberg. The GA mutations did affect the overall phenotypes of the plants, but these effects were not simply related to whether the mutation was early or late in the biochemical pathways. The GA-insensitive mutant was phenotypically different from the GA-deficient mutants and from Landsberg. The natural populations differed significantly from Landsberg, particularly in attributes related to size and inflorescence production, one more example of the need for researchers to be careful when generalizing the results of studies based upon laboratory strains. Our results indicate that early-flowering genotypes of A. thaliana can be remarkably tolerant to AMD, and that GA deficiency/insensitivity does not hinder tolerance to AMD, at least in this genetic background. Moreover, we confirm that mutations at regulatory loci can have noncatastrophic effects on fitness, as recently found by other investigators.     

Two new male-sterile mutants of Zea mays (Poaceae) with abnormal tapetal cell morphology

Two new recessive male-sterile mutants of Zea mays (Poaceae), or maize, were studied to identify the timing of pollen abortion and to examine the involvement of anther wall cell layers. The results of test crosses indicated that these mutants were not allelic with any known male-sterile mutants of maize. Light and transmission electron microscopy were used to compare pollen development in homozygous male-sterile mutants to that in fertile heterozygous siblings. In both mutants, microspores abort soon after release from the meiotic tetrad. However, the two mutations have strikingly different phenotypes. Large lipid bodies accumulate in the tapetal cells as the microspores vacuolate and die in the mutant ms25. Large vacuoles appear in both the tapetal cells and the young microspores as they begin to disintegrate in the mutant ms26. Because abnormal tapetal cell morphology is detected in both mutants, it is possible that both of these mutations affect the expression of genes in tapetal cells.     

A Recessive Founder Mutation in Regulator of Telomere Elongation Helicase 1, RTEL1, Underlies Severe Immunodeficiency and Features of Hoyeraal Hreidarsson Syndrome

Dyskeratosis congenita (DC) is a heterogeneous inherited bone marrow failure and cancer predisposition syndrome in which germline mutations in telomere biology genes account for approximately one-half of known families. Hoyeraal Hreidarsson syndrome (HH) is a clinically severe variant of DC in which patients also have cerebellar hypoplasia and may present with severe immunodeficiency and enteropathy. We discovered a germline autosomal recessive mutation in RTEL1, a helicase with critical telomeric functions, in two unrelated families of Ashkenazi Jewish (AJ) ancestry. The affected individuals in these families are homozygous for the same mutation, R1264H, which affects three isoforms of RTEL1. Each parent was a heterozygous carrier of one mutant allele. Patient-derived cell lines revealed evidence of telomere dysfunction, including significantly decreased telomere length, telomere length heterogeneity, and the presence of extra-chromosomal circular telomeric DNA. In addition, RTEL1 mutant cells exhibited enhanced sensitivity to the interstrand cross-linking agent mitomycin C. The molecular data and the patterns of inheritance are consistent with a hypomorphic mutation in RTEL1 as the underlying basis of the clinical and cellular phenotypes. This study further implicates RTEL1 in the etiology of DC/HH and immunodeficiency, and identifies the first known homozygous autosomal recessive disease-associated mutation in RTEL1.     

GENETICS OF GRACILARIA TIKVAHIAE (RHODOPHYCEAE). XI. FURTHER CHARACTERIZATION OF A BISEXUAL MUTANT1,2

A spontaneous bisexual mutant of Gracilaria tikvahiae McLachlan has been further characterized. Female plants that are carriers for the mutation, but do not themselves express bisexuality, have been identified among progeny derived from the original bisexual male plant. In crosses to normal males these carrier females yielded normal tetrasporophytes which in the subsequent gametophyte generation produced a 2 female: 1 male: 1 bisexual segregation. In crosses to bisexual males the carrier females produced unusual tetrasporophytes that formed cystocarps in addition to tetraspores. The gametophyte generation obtained from the tetraspores of these tetrasporophytes included only females and bisexuals, these being present in a 1:1 ratio. Other crosses, using bisexual male progeny, indicated that these have the same characteristics as the original bisexual mutant. All of the results are consistent with the genetic interpretation made previously that bisexuality results from a single recessive mutation, designated bi, in a gene distinct from the mt locus controlling male vs. female differentiation. From the phenotypes that have been observed it appears that the mutation does not cause bisexuality per se but rather results in unregulated female expression in males and tetrasporophytes where female-specific genes are normally repressed. It is suggested that the normal bi⁺ allele plays an important role in that repression process. The origin of the bisexual mutation has been re-examined by studying the progenitor stock of the mutation and stocks related to it. It appears that the bisexual mutation arose in a two-step process, first to a low level of expression that is found in the progenitor stocks and then to the high level of expression found in mutant clone 1045(bi) and its descendants.     

Defective transfer of seminal-fluid materials during matings of semi-fertile fruitless mutants in Drosophila

In context of the semi-sterility exhibited by Drosophila males expressing certain mating-enabling fruitless (fru) mutant genotypes, we examined the transfer of seminal fluid using a transgene that encodes the Sex Peptide (SP) oligopeptide fused to Green Fluorescent Protein (GFP). We found that this fusion construct expresses SP-GFP in a valid manner within accessory glands of the male reproductive system in normal and fru-mutant males. Transfer of SP-GFP to live females was readily detectable during and after copulation. With respect to the pertinent combinations of fru mutations, we demonstrated that these abnormal genotypes cause males to transmit mating-related materials in two aberrant ways: one involving whether any seminal-fluid entities are transferred at all during a given mating; the other revealing an intriguing aspect of these fruitless effects, such that the mutations in question cause males to transfer female-affecting materials in a manner that varies among copulations. In this regard, certain mutant males that do not transfer SP nevertheless are able to transfer sperm: a fru-mated female possessing no GFP who was not fecund initially could produce progeny when seminal-fluid proteins were subsequently supplied by mating with a male that was spermless owing to the effects of a tudor mutation.     

Nuclear-extranuclear interactions affecting oligomycin resistance in Aspergillus nidulans.

Summary The extranuclear mitochondrial oligomycin-resistant mutation ofAspergillus nidulans, (oliA1), was transferred asexually into four nuclear oligomycin-resistant strains of different phenotypes. In all four cases, the possession of the nuclear plus extranuclear mutation led to an increase in the in vivo level of oligomycin resistance. In two cases, the altered cytochrome spectrum and impaired growth ability determined by (oliA1) were suppressed by the nuclear mutations. In the third case, the in vitro oligomycin resistance of the double mutant ATPase was dramatically increased above that of either of the component single mutant strains, indicating a synergystic interaction between the nuclear and extranuclear gene products. In the fourth case, the double mutant became cold-sensitive.A new extranuclear mitochondrial oligomycin-resistant mutation (oliB332) is described. This mutant is phenotypically similar to, though not identical with, (oliA1) but is separable by recombination.A range of nuclear oligomycin-resistant mutants have been mapped. Despite presenting five distinctly different phenotypes, they all map at the same locus.