Semilethal Mutations Research Articles

Influence of inbreeding on the morphobiological characteristics of meadow clover (Trifolium pratense L.)

The influence of inbreeding on the morphobiological characteristics of meadow clover was revealed. Self-pollination, repeated in the number of successive generations, leads to an increase in homozygosity and to inbred depression, which increases from I1 to I3 generation and stabilizes in the I4 generation. It was found that in the I1 generation, according to the main morphobiological characteristics, there is no inward depression, but the maximum release of recessive lethal mutations is manifested, which amounted to 6.2%, and the survival rate of seedlings decreases (91.7%). By the I4 generation, the number of chlorophyll-free seedlings decreases to 1.4%. All the main morpho-biological indicators that determine the productivity of plants decrease from generation I1 to generation I3 by 1.5-2 times and stabilize in generation I4. Obtaining hybrid F1 offspring by crossing a linear material with an I4 induction level leads to the restoration of plant productivity indicators. When creating a linear material, an increase in the number of highly self-compatible genotypes from I1 to I4 generation by 60% is clearly traced. The data obtained make it possible to take into account the survival rate of seedlings, the cleavage of lethal and semi-lethal mutations, the degree of inbred depression in the formation of sample volumes when creating a linear material of meadow clover.

Lethal and semi-lethal mutations in Holstein calves in Uruguay

ABSTRACT: Genetic disorders in Holstein cattle are a health problem that has grown worldwide in recent years, compromising the sustainability of modern dairy production. In Uruguay, Holstein-based milk production is one of the most important sectors of the country’s economy, but high levels of inbreeding have decreased the breed’s fertility in recent decades. This study investigated the presence and diffusion of lethal and semi-lethal alleles causing embryo death, abortions, fetal malformations, and neonatal diseases in Holstein calves. Using the GeneSeek® Genomic Profiler™ Bovine 50K BeadChip, we genotyped 383 calves (1-30 days-old) from 27 farms located in the main dairy region of Uruguay. Results showed a high prevalence of farms (85%) and carrier calves (21%), including one or more of the following semi-lethal or lethal alleles: Syndactylism (4.18%), brachyspina (3.39%), cholesterol deficiency haplotype (2.61%), complex vertebral malformation (2.09%), bovine leukocyte adhesion deficiency (1.04%s), and Holstein haplotypes HH1 (4.44%), HH3 (3.13%), HH4 (1.04%), and HH5 (0.26%). Most of these alleles had not been recognized previously in Uruguay. We concluded that lethal and semi-lethal mutations are widespread in the Holstein breed in Uruguay. More studies are required to determine their impact on dairy cattle fertility.

An investigation of fertility haplotypes in Holstein cows of Dutch origin under environments of the Rostov region

The intensification of animal husbandry in recent years has led to a significant increase in the productivity of farm animals. The use of modern breeding methods has led to a decrease in genetic diversity and the spread of recessive lethal and semi-lethal mutations in the animal genome. In the world population of cattle of Holstein rather high rates of fertility haplotypes have been identified that affect reproductive traits as well as those associated with embryonic and post-embryonic mortality at various stages. The results of an investigation of Holstein cows of Dutch origin (n=70) are housed in one of the breeding farms of the Rostov region for the presence of 4 fertility haplotypes: HCD (APOB), HH0 (FANCI), HH3 (SMC2) and HH5 (TFB1M) have been presented in the article. The researches have been performed as part of the task of the Federal Agency of Scientific Organizations No. AAAA18-118021590138-1. In order to diagnose mutations test systems developed in the laboratory of molecular principles of selection of the Department of biotechnology and molecular diagnostics of animals of the Federal Research Center of Animal Husbandry – All-Russian Institute of Animal Husbandry named after academician L. K. Ernst have been used. Hidden carriers of haplotypes HCD, HH0 and HH3 have been identified in the studied group of cows. The frequency of HCD, HH0, and HH3 haplotypes was 1,43 %, 7,14 and 4,29 %, respectively. Carriers of the HH5 haplotype have been not found in the studied sample of Holstein cows. The results of molecular genetic testing have been confirmed by the analysis of pedigrees. The development of methods for diagnosing genetic anomalies and identifying loci associated with the decrease in productive trait as well as continuous monitoring of breeding stock will allow selecting animals that have high productive traits and are not burdened with genetic defects and malformations.

New versions of evolutionary models with lethal mutations

In this paper, the impact of lethal mutations on evolutionary dynamics of asexual populations is analyzed. We suggest distinguishing different definitions of lethality, which lead to different mathematical formalizations of the microscopic model. Most of the studies focus on polyphasic lethality, meaning that individuals carrying lethal mutations have no offspring but consume common resources. In an alternative problem setting, monophasic lethal mutants die without giving offspring on the first stage of development. In the third case, semi-lethal mutations are considered when the lethal mutants survive with some probability. We suggest and investigate mathematical models for these cases, deriving the evolutionary characteristics of the steady state. We found that the peak sequence probability drastically depends on the version of lethality. The results obtained here can be used to solve the error threshold paradox at the origin of life.

Theoretical aspects of heterosis

The phenomenon of heterosis, known as the superior performance of hybrid organisms over their parents, has been exploited by agricultural practices in the production of various crops since the beginning of the last century; however, its genetic basis has remained obscure. With the experimental data obtained from the study of maize hybrids and mathematical calculations, some genetic models have been proposed to explain heterosis in terms of various types of gene interaction, such as dominance, overdominance, and epistasis. However, each of the proposed concepts has weak points, which impose limitations on the possibility of the overall interpretation of the heterotic response in F1. This review provides a critical assessment of the theoretical concepts of heterosis from the perspective of the currently accumulated data of genetics and molecular biology, which are focused on specific mechanisms acting for specific traits. In particular, the role of lethal and semi-lethal mutations in the formation of a heterotic phenotype in plants is shown. The issues of the interpretation of genetic effects in case of gene linkage, which are referred to as pseudo-overdominance, are considered. Particular attention is paid to nonallelic interactions of genes, which add new nuances when discussing the effects of dominance and overdominance. Information on combining ability and its practical use in the context of the concept of heterotic groups is presented. Some aspects of the genotype–environment interaction are shown. The analysis of the theoretical concepts of heterosis from the perspective of modern genetic data testifies to the important role of various types of gene action in the formation of an outstanding phenotype and confirms the need for a systematic approach to this complex and unique phenomenon.

The S4 Voltage Sensor Packs Against the Pore Domain in the KAT1 Voltage-Gated Potassium Channel

In voltage-gated ion channels, the S4 transmembrane segment responds to changes in membrane potential and controls channel opening. The local environment of S4 is still unknown, even regarding the basic question as to whether S4 is close to the pore domain. Relying on the ability of functional KAT1 channels to rescue potassium (K+) transport-deficient yeast, we have performed an unbiased mutagenesis screen aimed at determining whether S4 packs against S5 of the pore domain. Starting with semilethal mutations of surface-exposed S5 residues of the KAT1 pore domain, we have screened randomly mutagenized libraries of S4 or S1-S3 for second-site suppressors. Our study identifies two S4 residues that interact in a highly specific manner with two S5 residues in the middle of the membrane-spanning regions, supporting a model in which the S4 voltage sensor packs against the pore domain in the hyperpolarized, or "down," state of S4.

Genetic variants of the tuberous sclerosis 2 tumour suppressor gene in mouse t haplotypes.

The murine t complex on chromosome 17 contains a number of homozygous lethal and semi-lethal mutations that disrupt development of the mouse embryo. We recently characterized an embryonic lethality in the rat that results from a germ-line mutation in the tuberous sclerosis 2 (Tsc-2) tumour suppressor gene (the Eker mutation). Remarkably, mouse embryos homozygous for tw8 mutation display cranial defects reminiscent of those observed in rat embryos homozygous for the Eker mutation. To determine whether the Tsc-2 gene, which is in the t complex, is mutated in tw8 or other t haplotypes, we characterized this gene in a series of t haplotype mice. Four Tsc-2 polymorphisms were identified: three in the coding region and one intronic that appeared to be common to all t haplotypes analysed. No evidence was found to argue that the Tsc-2 gene is altered in tw8 haplotype mice. However, in the tw5 haplotype we found a G to T mutation in Tsc-2 that was present only in this t haplotype. In contrast to other polymorphisms within the Tsc-2 coding region which did not result in amino acid changes in Tsc-2 gene product tuberin, this mutation substituted a phenylalanine for a conserved cysteine in tw5 tuberin. Within the t complex, the Tsc-2 gene and the putative tw5 locus appeared to map to different positions, complicating identification of Tsc-2 as a candidate for the tw5 locus and suggesting that the G to T mutation in the Tsc-2 gene may have arisen independently of the tw5 functional mutation.

Ultrastructural Correlates of Neuromuscular Junction Development

Publisher Summary The larval and late embryonic neuromuscular junctions found on the abdominal muscles of Drosophila are a very useful experimental model system for examining the normal developmental processes underlying synapse formation and for examining the effects of lethal or semi-lethal mutations producing defects in proteins important to synaptic function. However, because some of the defects may be subtle, involving only quantitative changes in the sizes or number of pre- and postsynaptic organelles, it has been important to characterize the normal ultrastructure of the different types of nerve terminals that innervate these muscle fibers and to characterize the morphological responses that the muscle fiber makes to the presence of each of them in turn. Each of the abdominal muscle fibers is innervated by a unique combination of the subtypes of motor and neuromodulatory neurons. The secondary motor nerve branch containing their axons typically contacts each muscle fiber at a particular spot, and the type I terminals then give rise to a fairly consistent number and placement of branches that spread out from this point on each muscle fiber. Other terminal types diverge in a less consistent pattern. This region, referred to as the nerve entry point, is thus interesting from the point of view that it is the target for the ingrowing axons during development, as well as the point from which their terminals must disassociate from each other and form a specific branch pattern on the muscle fiber. The group of terminals from the various motor and neuromodulatory neurons that innervate each fiber is referred to as the junctional aggregate.

RISK OF POPULATION EXTINCTION FROM FIXATION OF NEW DELETERIOUS MUTATIONS.

The fixation of new deleterious mutations is analyzed for a randomly mating population of constant size with no environmental or demographic stochasticity. Mildly deleterious mutations are far more important in causing loss of fitness and eventual extinction than are lethal and semilethal mutations in populations with effective sizes, Ne , larger than a few individuals. If all mildly deleterious mutations have the same selection coefficient, s against heterozygotes and 2s against homozygotes, the mean time to extinction, t¯, is asymptotically proportional to e4Nes/Ne for 4Ne s > 1. Nearly neutral mutations pose the greatest risk of extinction for stable populations, because the magnitude of selection coefficient that minimizes t¯ is about ŝ = 0.4/Ne . The influence of variance in selection coefficients among mutations is analyzed assuming a gamma distribution of s, with mean s¯ and variance σs2. The mean time to extinction increases with variance in selection coefficients if s¯ is near ŝ, but can decrease greatly if s¯ is much larger than ŝ. For a given coefficient of variation of s, c=σs/s¯, the mean time to extinction is asymptotically proportional to Ne1+1/c2 for 4Nes¯>1. When s is exponentially distributed, (c = 1) t¯ is asymptotically proportional to Ne2. These results in conjunction with data on the rate and magnitude of mildly deleterious mutations in Drosophila melanogaster indicate that even moderately large populations, with effective sizes on the order of Ne = 103 , may incur a substantial risk of extinction from the fixation of new mutations.

Genetic analysis of the adenosine3 (Gart) region of the second chromosome of Drosophila melanogaster.

The Gart gene of Drosophila melanogaster is known, from molecular biological evidence, to encode a polypeptide that serves three enzymatic functions in purine biosynthesis. It is located in polytene chromosome region 27D. One mutation in the gene (ade3(1)) has been described previously. We report here forty new ethyl methanesulfonate-induced mutations selected aga!nst a synthetic deficiency of the region from 27C2-9 to ++28B3-4. The mutations were characterized cytogenetically and by complementation analysis. The analysis apparently identifies 12 simple complementation groups. In addition, two segments of the chromosome exhibit complex complementation behavior. The first, the 28A region, gave three recessive lethals and also contains three known visible mutants, spade (spd), Sternopleural (Sp) and wingless (wg); a complex pattern of genetic interaction in the region incorporates both the new and the previously known mutants. The second region is at 27D, where seven extreme semilethal mutations give a complex complementation pattern that also incorporates ade3(1). Since ade3(1) is defective in one of the enzymatic functions encoded in the Gart gene, we assume the other seven also affect the gene. The complexity of the complementation pattern presumably reflects the functional complexity of the gene product. The phenotypic effects of the mutants at 27D are very similar to those described for ade2 mutations, which also interrupt purine biosynthesis.

Lethal mutations flanking the 68C glue gene cluster on chromosome 3 of Drosophila melanogaster.

We have conducted a genetic analysis of the region flanking the 68C glue gene cluster in Drosophila melanogaster by isolating lethal and semilethal mutations uncovered by deficiencies which span this region. Three different mutagens were used: ethyl methanesulfonate (EMS), ethyl nitrosourea (ENU) and diepoxybutane (DEB). In the region from 68A3 to 68C11, 64 lethal, semilethal, and visible mutations were recovered. These include alleles of 13 new lethal complementation groups, as well as new alleles of rotated, low xanthine dehydrogenase, lethal(3)517 and lethal(3)B76. Six new visible mutations from within this region were recovered on the basis of their reduced viability; all proved to be semiviable alleles of lethal complementation groups. No significant differences were observed in the distributions of lethals recovered using the three different mutagens. Each lethal was mapped on the basis of complementation with overlapping deficiencies; mutations that mapped within the same interval were tested for complementation, and the relative order of the lethal groups within each interval was determined by recombination. The cytological distribution of genes within the 68A3-68C11 region is not uniform: the region from 68A2,3 to 68B1,3 (seven to ten polytene chromosome bands) contains at least 13 lethal complementation groups and the mutation low xanthine dehydrogenase; the adjoining region from 68B1,3 to 68C5,6 (six to nine bands) includes the 68C glue gene cluster, but no known lethal or visible complementation groups; and the interval from 68C5,6 to 68C10,11 (three to five bands) contains at least three lethal complementation groups and the visible mutation rotated. The developmental stage at which lethality is observed was determined for a representative allele from each lethal complementation group.

Transmission distortion of t-haplotypes is due to interactions between meiotic partners.

The T/t-complex of the mouse includes a series of recessive lethal and semi-lethal mutations but, despite such lethalities, mutant t-haplotypes are found in high frequency in wild mouse populations. This polymorphism is apparently maintained because heterozygous males preferentially transmit the t-bearing chromosome to their offspring. Despite many attempts to define the basis of the transmission ratio distortion, it has been unclear whether this is because t-bearing sperm have better than average fertilizing ability or whether +-bearing sperm in heterozygous males are rendered defective. To examine this point, we constructed male (XY in equilibrium XY) chimaeras containing +/+ and +/tw73 genotypes, marked respectively by albino and pigmented coat colours, and two isozyme variants. Such males produce a mixture of three different sperm types: +-bearing sperm from the +/+ genotype, +-bearing sperm from the +/t genotype, and t-bearing sperm from the +/t genotype. Appropriate matings can distinguish between these three types, and our data, reported here, show that t-bearing sperm in chimaeric mice maintain their advantage over their 'meiotic partners' but do not have any advantage over sperm from the +/+ genotype.

VIABILITY OF FEMALE GERM-LINE CELLS HOMOZYGOUS FOR ZYGOTIC LETHALS IN DROSOPHILA MELANOGASTER

We have analyzed the viability of different types of X chromosomes in homozygous clones of female germ cells. The chromosomes carried viable mutations, single-cistron zygotic-lethal and semi-lethal mutations, or small (about six chromosome band) deletions. Homozygous germ-line clones were produced by recombination in females heterozygous for an X-linked, dominant, agametic female sterile.All the zygotic-viable mutants are also viable in germ cells. Of 16 deletions tested (uncovering a total of 93 bands) only 2 (of 4 and 5 bands) are germ-cell viable. Mutations in 15 lethal complementation groups in the zeste-white region were tested. When known, the most extreme alleles at each locus were tested. Only in five loci (33%) were the mutants viable in the germ line. Similar studies of the same deletions and point-mutant lethals in epidermal cells show that 42% of the bands and 77% of the lethal alleles are viable. Thus, germ-line cells have more stringent cell-autonomous genetic requirements than do epidermal cells.The eggs recovered from clones of three of the germ-cell viable zw mutations gave embryos arrested early in embryogenesis, although genotypically identical embryos derived from heterozygous oogonia die as larvae or even hatch as adult escapers. For two genes, homozygosis of the mutations tested also caused embryonic arrest of heterozygous female embryos, and in one case, the eggs did not develop at all. Germ-line clones of one quite leaky mutation gave eggs that were indistinguishable from normal. The abundance of genes whose products are required for oogenesis, whose products are required in the oocyte, and whose activity is required during zygotic development is discussed.

Investigations on the organization of genetic loci in Drosophila melanogaster: lethal mutations affecting 6-phosphogluconate dehydrogenase and their suppression

The molecular nature of lethal and semilethal mutations in the Pgd locus of D. melanogaster coding for 6-phosphogluconate dehydrogenase (6PGD) was studied. All the 11 mutations affect the structural gene of the Pgd locus: 3 semilethal mutations resulted in altered 6PGD molecules with decreased catalytic activities; the rest 8 lethals were "null" alleles characterized by mutant polypeptides capable of reacting with antisera against highly purified 6PGD. "Null" or low activity alleles for glucose-6-phosphate dehydrogenase induced by ethyl methanesulfonate were shown to be suppressores for the lethal mutations in the Pgd locus. A monocistronic type of organization of the Pgd locus is suggested taking into account the biochemical mechanism of suppression of the Pgd-lethals and their location in the structural gene coding for 6PGD.

Fine genetic structure of the 2D3-2F5 region of the X-chromosome of Drosophila melanogaster.

97 lethal and semilethal mutations were induced by ethyl methanesulfonate, nitrosomethyl urea and gamma-irradiation in the 2D3-F5 region of the X-chromosome of D. melanogaster. Approximately 1 per cent of the tested X-chromosomes carried a lethal in the 2D3-2F5 region. The mutation frequencies per band or DNA content in the region and the whole X-chromosome are equal. Complementation analysis revealed at least 10 functionally independent essential loci in this region including about 10 bands. The data presented in this study support the one band--one gene hypothesis. The Pgd locus coding for 6-phosphogluconate dehydrogenase (6PGD) is mapped in the 2D3 (OR 2D4) band. Isolation of 11 lethal or semilethal point mutations with null or reduced 6PGD activity shows that the Pgd locus is a vital one.

Histidine regulation in Salmonella typhimurium. 8. Mutations of the hisT gene.

The hisT gene, one of six genes in which mutation causes derepression of the histidine operon in Salmonella typhimurium, is shown to code for a protein that is not essential for the growth of the bacteria. This is indicated by the characterization of particular classes of mutations in the hisT gene: amber mutations, frame-shift mutations, and temperature-sensitive mutations that affect repression but not growth. In addition, the class of semilethal mutations was selected for but not found.

Temperature-sensitive mutations in Drosophila melanogaster. 3. Dominant lethals and semilethals on chromosome 2.

Dominant temperature-sensitive lethal and semilethal (DTS-L) mutations induced by ethyl methanesulfonate have been recovered in chromosome 2 of Drosophila melanogaster. The 19 (0.39% of the chromosomes screened) characterized had greater than 65 per cent viability at 22 degrees +/- 2 degrees C and less than 5 per cent at 29 degrees +/- 0.5 degrees C. Fifteen of the DTS-L's map genetically as point mutants, one appears to be a synthetic lethal, and the remaining three yielded sterile females. Ten different DTS mutants had a high survival in triploid females at 29 degrees C. Eleven DTS-L's were found to map in the same genetic region (dp-b). All behaved as recessive lethals at 22 degrees C and failed to survive at 22 degrees C when combined with mutants 3 and 12. Crosses of the 11 mutants in all possible pairwise combinations yielded a circular complementation map with a side branch. The effective lethal phases and temperature-sensitive periods during development were determined for 15 of the mutants.

Temperature-sensitive mutations in Drosophila melanogaster. II. Frequency among second chromosome recessive lethals induced by ethyl methanesulfonate.

Recessive lethal and semilethal mutations induced in chromosome 2 by ethyl methanesulfonate were screened for temperature-sensitivity. Mutants which survive at 17 °C but die at 29° were recovered. At 0.005M EMS, 37.5% of the treated chromosomes were found to be lethal or semilethal from which it is estimated that 10.9% of all EMS-induced lethals are ts. This class of conditional lethals promises to be a useful tool in genetic studies.

Evidence of Physiological Mutations in the Cladoceran, Moina macrocopa

Extensive breeding experiments with Daphnia longispina (Banta and Wood, and unpublished data) give rather conclusive evidence that recessive mutations having physiological effects accumulate during parthenogenesis in this species. In order to learn whether recessive physiological mutations accumulate similarly in Moina macrocopa, 2 strains of this species were inbred (sexual reproduction). One strain, Banta's Line 1012, had a laboratory history of 1090 parthenogenetic generations before inbreeding, whereas the other line, Banta's Line 1705, had only a short history of 42 parthenogenetic generations before inbreeding. Of 151 ephippial (sexual) eggs obtained from Line 1012, 118 or 78.1% hatched. Of 321 ephippial eggs obtained from Line 1705, 282 or 87.9% hatched. Eggs in both of these groups include several series which fairly consistently showed the same differential rate of hatching as is represented by the totals given above. This is suggestive of the accumulation during long-continued parthenogenesis of semi-lethal mutations in Line 1012. Fifty-five females which hatched from the sexual eggs obtained by inbreeding Line 1012 were compared with 55 parthenogenetically-produced females of Line 1012 as to their viability, reproductive capacity, and other characteristics. Comparisons were made between the individuals of these 2 groups, on the basis of the following specific points: early mortality (died before maturity), sterility (attained maturity but died without producing young), longevity, number of broods of parthenogenetic young produced, and reproductive index (average number of young per mother per day of reproductive life).