Somatic Mosaics Research Articles

The MEV Mouse Linkage Testing Stock: Mapping 30 Novel Proviral Insertions and Establishment of an Improved Stock

Continuing efforts to improve the MEV linkage testing stock are described. The MEV stock carries multiple copies of the ecotropic murine leukemia virus genome. These proviruses are individually detectable in Southern blots, making it possible to follow the segregation of many loci simultaneously in genetic crosses. About 50 novel proviral insertions have been detected in this stock and propagated to homozygosity in a number of sublines. Many of the proviruses were first observed in individuals that were somatic and germinal mosaics. From the minimum frequency of primary transmission of proviruses to offspring, it is estimated that about 7 embryonic cells contribute to the pool of germ cells that populate the gonads. Thirty of the proviruses have been mapped to 15 different chromosomes in intercrosses between MEV sublines and the inbred Mus musculus castaneus strain CAST/Ei or in other crosses. Mapped insertions appear to be randomly distributed among chromosomes, but possibly clustered within chromosomes. Several of the mapped insertions are located such that their inclusion in improved MEV stocks would permit the screening of additional regions of the genome. An improved MEV stock that is homozygous for 12 proviruses on 11 chromosomes, including novel insertions on Chromosomes 17 and 19 is described. The stock also carries four dominant visible markers on 4 additional chromosomes. This stock is estimated to sweep approximately 68% of the autosomal genome.

Pericentric intrachromosomal insertion responsible for recurrence of del(11)(p13p14) in a family

The combined use of qualitative and quantitative analysis of 11p13 polymorphic markers together with chromosomal in situ suppression hybridization (CISS) with biotin labeled probes mapping to 11p allowed us to characterize a complex rearrangement segregating in a family. We detected a pericentric intrachromosomal insertion responsible for recurrence of del(11)(p13p14) in the family: an insertion of brand 11p13-p14 carrying the genes for predisposition to Wilms' tumor, WT1, and for aniridia, AN2, into the long arm of chromosome 11 in 11q13-q14. Asymptomatic balanced carriers were observed over three generations. Classical cytogenetics had failed to detect this anomaly in the balanced carriers, who were first considered to be somatic mosaics for del(11)(p13). Two of these women gave birth to children carrying a deleted chromosome 11, most likely resulting from the loss of the 11p13 band inserted in 11q. Although in both cases the deletion encompassed exactly the same maternally inherited markers, there was a wide variation in clinical expression. One child, with the karyotype 46,XY, del(11)(p13p14), presented the full-blown WAGR syndrome with aniridia, mental retardation, Wilms' tumor, and pseudohermaphroditism, but also had proteinuria and glomerular sclerosis reminiscent of Drash syndrome. In contrast, the other one, a girl with the karyotype 46,XX,del(11)(p13), only had aniridia. Although a specific set of mutational sites has been observed in Drash patients, these findings suggest that the loss of one copy of the WT1 gene can result in similar genital and kidney abnormalities.

Independent guidance of retinal axons in the developing visual system of Drosophila

The development of the adult visual system of Drosophila requires the establishment of precise retinotopic connections between retinal photoreceptor cell axons and their synaptic partners in the optic lobe of the brain. To assess the role of axon-axon interactions in retinal axon guidance, we used genetic methods to disrupt the normal spatiotemporal order of retinal axon ingrowth. We examined retinal axon projections to the developing first optic ganglion, the lamina, in two mutants in which reduced numbers of ommatidia develop in the eye imaginal disk. We find that in the developing lamina of these mutants, sine oculis and Ellipse, retinal axons project to proper dorsoventral positions despite the absence of the usual array of neighboring retinal axons. In a second approach, we examined animals that were somatic mosaics for the mutation, glass. In glass- animals, retinal axons project aberrantly and the larval optic nerve is absent. We find that in the developing lamina of glass mosaic animals, wild-type retinal axons project to proper dorsoventral positions despite the misrouted projections of neighboring glass- retinal axons. In addition, wild-type retinal axons project normally in the absence of the larval optic nerve, indicating that the latter is not an essential pioneer for retinal axon navigation. Our observations support the proposal that axon fascicles can make at least some pathfinding decisions independently of other retinal axon fascicles. We suggest that positional guidance cues that might label axon pathways and target destinations contribute to retinotopic pattern formation in the Drosophila visual system.

Viviparous Leaves Produced by Somatic Activation of an Inactive Cytokinin-Synthesizing Gene

Tobacco plants that are somatic mosaics for expression of a cytokinin-synthesizing gene have viviparous leaves. Such a formation of shoots in an abnormal position represents a significant deviation from the usual organization of the plant body where a central axis produces shoots only in the axils of lateral leaf appendages and according to a precise phyllotactic pattern. This report links vivipary to the expression of a gene whose product is involved in the synthesis of the phytohormone cytokinin.

The plant oncogene rolC is responsible for the release of cytokinins from glucoside conjugates.

The rolC gene of Agrobacterium rhizogenes, which drastically affects growth and development of transgenic plants, codes for a cytokinin-beta-glucosidase. Indeed, rolC protein expressed in Escherichia coli as a fusion protein hydrolyses cytokinin glucosides, thus liberating free cytokinins. Furthermore, beta-glucosidase activity present in E. coli extracts expressing the rolC protein was inhibited by affinity-purified antibodies specific for the rolC protein. Finally, rolC proteins expressed in transgenic plants were shown to be responsible for cytokinin-beta-glucosidase activity. Morphological and phytohormonal analysis, performed on transgenic plants that are somatic mosaics for the expression of the rolC gene, extend and confirm our interpretation that the developmental, physiological and morphological alterations caused by rolC expression in transgenic plants are primarily due to a modification of the cytokinin balance. These observations shed new light on the control of growth and differentiation in plants by growth factors.

Rough, a Drosophila homeobox gene required in photoreceptors R2 and R5 for inductive interactions in the developing eye

The rough mutation in Drosophila disrupts an early stage of ommatidial assembly in the developing eye imaginal disc. Analysis of somatic mosaics suggests that rough gene function is required only in photoreceptors R2 and R5. However, these cells apparently differentiate normally in mutant eye discs and it is the cells that are subsequently added to the ommatidium that behave aberrantly. The rough gene was isolated by P-element transposon tagging, and the mutant can be rescued by transformation with an 8.6 kb genomic fragment. The rough transcription unit is 4.3 kb and consists of three exons that are joined in a 1.3 kb mRNA. The predicted rough protein is 350 amino acids long and contains a homeobox. Taken together, our results suggest that the role of the rough gene product may be to regulate the sending of signals by photoreceptors R2 and R5 to their neighbors in the developing ommatidia.

Rates of mutant and inherited structural cytogenetic abnormalities detected at amniocentesis: results on about 63,000 fetuses.

We report data on diagnoses made on amniotic fluid specimens from 1977 to 1984 as reported to the New York State Chromosome Registry. The rate of all de novo (presumed mutant) abnormalities was about 2 per 1,000 in about 61,000 fetuses in which results are unlikely to be biased by the reason for amniocentesis (except for maternal age). This includes about 0.5 per 1,000 de novo markers, about 0.5 per 1,000 other de novo unbalanced, and about 1.0 per 1,000 de novo balanced rearrangements. In about 55,000 fetuses in which rates of inherited abnormalities could be evaluated without apparent bias, the rate of all inherited rearrangement was about 2.9 per 1,000. This includes about 0.3 per 1,000 inherited markers, about 0.2 per 1,000 other inherited unbalanced rearrangements, and about 2.4 per 1,000 inherited balanced abnormalities. Only mutant markers showed a clear association with maternal age (37.6 +/- 2.7 in 24 cases v. 35.8 +/- 3.6 in controls). Inherited markers did not exhibit this trend (35.8 +/- 2.0 in 12 cases v. 36.4 +/- 2.8 in controls). Paternal age does not appear to account for the association. Among abnormalities of known origin, the ratio of mutant to inherited cases is for markers 64:36, for other unbalanced rearrangements 73:27, and for all balanced abnormalities 29:71. In a subgroup of about 55,000 fetuses, of 263 total abnormalities there were 8 instances of apparent true somatic mosaics (5 mutant and 3 of unknown origin but almost certainly mutant). There were also 20 instances of markers in which presumptive somatic loss had resulted in mosaicism (10 mutant, 6 of unknown origin and 4 inherited) and 13 other instances of mosaicism associated with apparent somatic loss (9 mutant, 3 of unknown origin, and 1 inherited). The sex ratio (Y to non-Y karyotypes) for all abnormalities detected was 228:210 (1.09), not different from controls. Only deletions (5:14) and 'other' unbalanced rearrangements (5:13) exhibited a suggestive deviation from this trend. The rates of mutant chromosome rearrangements reported from 1977 to 1983 showed no apparent time cluster, with the possible exception of a peak of markers in 1977, a trend that may be due to higher maternal age in this year. Among fetuses studied because of maternal exposure to putative mutagens there was a non-significant excess of mutants (2.9-5.7 per 1,000 v. 1.7-2.2 per 1,000) and a borderline significant excess of inherited rearrangements (8.6-11.5 per 1,000 v. 2.6-3.1 per 1,000).(ABSTRACT TRUNCATED AT 400 WORDS)

The distribution of and complementation relationships between spontaneous X-linked recessive lethal mutations recovered from crossing long-term laboratory stocks of Drosophila melanogaster

Males from a wild-type laboratory stock (M56i, Amherst), were mated with virgin females of the M-6 stock, In(1)sc 8 In(1)dl-49, y 31d sc 8 w a v Of f , and 149 spontaneous independent non-mosaically transmitted, as well as 8 incidentally detected, mosaically transmitted, X-linked recessive lethal mutations were recovered from 95704 F 2 cultures. 152 mutations were mapped over the entire length of the X-chromosome by complementation and/or crossover tests. Altogether 70 mutations, but no more than 63 mutations with lethal effects supposedly localized to single vital loci, fell into the 5 covered regions comprising about one-third of the X-chromosome that were studied in Lefevre's (1981) analysis of X-ray-induced lethal mutations. The distributions of spontaneous and X-ray mutations among the 5 regions were similar. Although there were far too few spontaneous mutations to make a meaningful comparison of relative mutability on a locus-by-locus basis, those loci displaying a relatively higher X-ray mutability, when taken as a group, tend to display a relatively higher spontaneous mutability, and those loci displaying a relatively lower X-ray mutability, when taken as a group, tend to display a relatively lower spontaneous mutability. Therefore, the data from lethal mutations are in agreement with the data from specific locus visible mutations and support the conclusion of Shukla et al. (1979) in being consistent with the assumption of proportionality between spontaneous and X-ray-induced mutation rates. Genetic analysis revealed that a minimum of 7.2% ( 11 152 ) of the were associated with chromosomal rearrangements. Within all covered regions, there were some 94 mutations that were tested for allelism, but only one possible case was found of a locus, l(1) L13, with as many as 3 independent intragenic mutations; it is unlikely that among the 152 mutations there were any other such loci. The locus most frequently involved in any kind of lethal mutation was the l(1) J1 locus near the distal tip of the X-chromosome with 5 mutations that included 3 ostensible multi-locus deficiencies. At equivalent entire X-chromosome recessive lethal mutation rates, the estimated per locus spontaneous mutation rate for non-mosaically transmitted X-chromosome lethals is about the same, or not lower than by a factor of 1 3 , as the estimated per locus spontaneous mutation rate for non-mosaically transmitted X-chromosome recessive visibles at 13 specific loci studies earlier by Schalet (1958, 1960) and re-evaluated here. In the earlier study of spontaneous visible mutations, at least 80% of the newly arisen non-clustered mutations (counting both transmitted and untransmitted), showed a mosaic distribution of mutant tissue by appearance and/or genetic test, but only 41% of the mutations that were germically completely mutant in transmission originally appeared in F 1 females as somatic mosaics. Upon reconsideration it is now appreciated that many newly arisen mutations would have been undetected because they were cryptic somatic mosaics. Accordingly, it is judged that the inclusion of an estimate of the numbers of undetected cryptic somatic mosaics would raise to at least 2 3 the proportion of newly arisen mutations that were somatic mosaics and germically complete in F 1 females. Among the 141 newly arisen non-clustered recessive lethal mutations that were germically complete, there were 9 mutations with dominant visible mutant effects and at least 3–5 of these were cryptic somatic or gonosomic mosaics. From the data on dominant and recessive visibles I infer that the majority of all spontaneous recessive lethal mutations were, although germinally complete, also mosaics of one type or another in F 1 females. Furthermore, if transposable elements were responsible for the mosaics observed or inferred to have occurred, then they must have been mobilized very early in cleavage in order to produce mutant cleavage nuclei more usually ancestral to somatic and germinal tissue or to somatic tissue alone, rather than germinal tissue alone.

Unequal mitotic sister chromatin exchange as the mechanism of ribosomal RNA gene magnification.

It is hypothesized that magnification of the gene coding for ribosomal RNA occurs by unequal mitotic sister chromatid exchange on the basis of five different lines of evidence. These are: (1) rDNA magnification occurs in mitotically active germ cells; (2) decreases in rDNA redundancy can be genetically produced, a phenomenon termed reduction; (3) magnification and reduction events are reversible and reciprocal; (4) it is possible to generate bb(+) and bb somatic bristle mosaics (bb mutants are partially deficient for rRNA genes); and (5) magnification of bb in a ring X chromosome is reduced. Implications of these results and the unequal sister exchange (USE) hypothesis are discussed.

Temperature-sensitive mutations in Drosophila melanogaster: X. Developmental analysis of the paralytic mutation, para ts

The recessive, sex-linked mutation, paralytic temperature-sensitive ( para ts ), causes a reversible temperature dependent paralysis in adult flies only. When adult para ts flies are shifted from 22°C to 29°C, they are paralyzed in less than 5 sec; a shift back to 22°C restores mobility almost instantly. Somatic cuticular mosaics of mutant and wild-type tissue were created by loss of an unstable ring X chromosome. The behavior of 310 (out of 371) of these mosaics was analyzed at 29°C. The relationship between the location of mutant tissue and the pattern of aberrant behavior at 29°C was analyzed in detail with a view to an anatomical localization of the defect caused by para ts . Many mosaics fell into one of the following ten categories whose behavior is discussed in detail: abdomen only, complete head, bilateral head, complete thorax, bilateral thorax, leg only, complete head-thorax, complete head-bilateral thorax, bilateral head-complete thorax, and bilateral head-thorax. The mosaic data suggest that the mutation is autonomous (i.e., mutant cells are not affected by surrounding wild-type tissue) and that it represents a lesion in nervous tissue. The results from electroretinograms and optomotor tests suggest that the mutation does not affect the ability of the eye to receive and transduce light into impulses in the retinula cells, but that the impulses either do not reach or cannot be interpreted by the brain. Thus, para ts affects the central nervous system. Survival of completely para ts flies with prolonged exposure to high temperature suggests that vital organs continue to function during paralysis. Indeed, the heart was observed to beat quite normally. This indicates that para ts does not effect the stomodeal ganglion of the autonomic nervous system. It is concluded that para ts affects at least two components of the central nervous system: (1) the head including the brain and subesophageal ganglion, and (2) the fused thoracic ganglia located in the ventral region of the thorax.

Behaviour and transmission of supernumerary chromosomes in diploid Dactylis glomerata

The somatic stability and the mode of transmission of B chromosomes were studied in diploid Orchard Grass: Dactylis glomerata L. ssp. judaica Steb. et Zoh. Somatic mosaics (different numbers of B's in different tillers) were detected in 4 out of 17 plants. Examination of the first and the second divisions of the male gametophyte revealed non-disjunction and preferential migration of the B's to the generative nucleus. Progeny from eight different cross-combinations were examined. The results obtained corroborate the direct evidence on accumulation of B's in the pollen grains, but the transmission patterns observed are too complex to be explained by this mechanism only.

SOMATIC MOSAICS IN THE DOMESTIC PIGEON

SOMATIC MOSAICS IN THE DOMESTIC PIGEON