Spontaneous interchanges in females of Drosophila melanogaster

Summary Herein is described an attempts to establish chromosome pairing-interchange relationships in Drosophila melanogaster female. For this purpose, the formation of half-translocations was studied in XXY and XX females bearing compounds of the second pair of autosomes. With respect to XXY females, it was expected that the free Y chromosome would pair with these compounds and that half-translocations involving 2L would arise. In as much as compound chromosomes in XX females had no partner for pairing, the formation of half-translocations involving 2L was not expected.Half-translocations were registered in the F₁ from crosses of XX and XXY females to b j pr cn/T(Y;2)C males. The cross was designed to permit the detection of very rarely occurring non-homologue interchanges.Offspring number was 335 in XX females and 550 in XXY females. The majority of offspring consisted of individuals arisen from the spontaneous restitution of compounds and the formation of 2n egg cells. Based on phenotype, the offspring of XX females contained 4 individuals with half-translocations involving 2L; there were 48 such flies among the offspring of XXY females. As confirmed by progeny analysis, 38 half-translocations occurred in XXY females and none in XX females. Of the 31 spontaneous interchanges in XXY females 28 were recorded between the Y and the left compound, one between the Y and the right compound, and one between the X and the left compound. Non-homologue interchanges were of oogonial origin judging by the fact that individuals with half-translocations arose in clusters. Unlike Y — left compound interchanges, the interchanges between autosomal compounds seem to be of meiotic origin.     

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.     

Genes Controlling Development: Morphoses, Phenocopies, Dimorphs, and Other Visible Expressions of Mutant Genes

We studied facultative dominant lethal mutations obtained earlier inDrosophila 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-Xfemales; and (3) 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 nonheritable. 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.     

Conditional lethal mutations shift the genome from stability to instability

The phenomenology of genomic destabilization is described in Drosophila melanogaster mutants containing radiation-induced conditional dominant lethals in the X chromosome and in autosome 2. Destabilization manifests itself as (1) the loss or decrease of lethality of previously lethal mutations; (2) the loss of expression of visible dominant mutations in an opposite homolog; (3) chromosomal instability resulting in the loss of the X chromosome in germline and somatic cells; (4) the occurrence of novel mutations (secondary mutagenesis); (5) the occurrence of single and mass modifications; (6) disturbances in individual development (formation of morphoses). The key event for the shift of the genome from the stable state into the unstable one is the occurrence of a conditional dominant lethal mutation.     

Further studies of spontaneous meiotic interchanges in Drosphila females Evidence for asynapsis of homologues in the interchange region

Spontaneous formation of half-translocations (HTs) of X · 2L and Y · 2L types in

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females were studied. The HTs were the result of interchange between the and C(2L) autosomal compound in their precentromeric heterochromatic regions. The HTs produced in previous experiments with females were also analysed.The great majority of spontaneous interchanges were of meiotic origin. Of 13 HT offspring yielded by

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females, 10 were X-cross-overs. 8 HT individuals among the offspring of females were X-crossovers. Based on the segregation pattern of chromosomes following interchange, it is concluded that interchange takes place during meiotic prophase. Interchange and crossing over are concomitant events giving rise to the trivalent. In this trivalent, the euchromatic region of compound pairs with the X euchromatic region, and the heterochromatic region with the C(2L). The heterochromatic regions of the X and in the trivalent are asynaptic. Two lines of evidence for this partial asynapsis were obtained: (1) the high rate of non-disjunction (34.1% in HT offspring of females); (2) the regular segregation of the X chromosome with C(2R). The crossing over in the X-euchromatic region, which was associated with interchange, was disturbed (a high proportion of multiple exchanges). Crossing-over disturbance and the high level of non-disjunction in the HT offspring were not caused by the presence of the and autosomal compounds in the stock investigated.It is concluded that the spontaneous asynapsis of the X and regions initiates pairing and interchange, thereby giving rise to abnormal crossing over and disjunction. Partial asynapsis of homologues as the sufficient cause for non-disjunction and non-homologue pairing is discussed.     

Changes in the energy status of Drosophila as a result of genetic mutation

Conditional dominant lethals (CDL) represent a special class of genetic mutations observed in Drosophila. Mutation manifests as a dominant allele in one genotype, but lethality is not expressed in another genotype. CDL mutants exhibit a set of traits discriminating them from classic mutations. We observed unusually high mobility of flies and high sexual activity of males carrying these mutations. We used special tests for evaluation of energy metabolism of CDL mutants. Indirect calorimetry (CO2 excretion measurement) has been used for estimation of energy exchange in four mutant and two control fly lines. A Special device has been used for evaluation of locomotor activity of these fly lines. Energy exchange and locomotor activity in CDL mutants were significantly higher than in control lines. We conclude that some genetic mutations are capable of increasing energy dissipation in their carriers.     

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.     

From Genetics of Intraspecific Differences to Genetics of Intraspecific Similarity

Based on the Mendelian approach to heredity, modern genetics describes inheritance of characters belonging to the category of intraspecific difference. the other large category of characters,intraspecific similarity, stays out of investigation. In this review, the genome part responsible for intraspecific similarity is considered as invariant and regulatory. An approach to studying the invariant part of the Drosophila melanogaster genome is formulated and the results of examining this genome part are presented. The expression of mutations at genes in the invariant genome part is different from that of Mendelian genes. We conclude that these genes are present in the genome in multiple copies and they are functionally haploid in the diploid genome. Severe abnormalities of development appearing in the progeny of mutant parents suggest that the mutant genes are genes regulating ontogeny. A hypothesis on an elementary ontogenetic event is advanced and the general scheme of ontogeny is presented. A concept on two types of gene allelism (cis- and trans-allelism) is formulated. This approach opens a possibility for studying genetic material responsible for the formation of intraspecific similarity characters at different taxonomic levels on the basis of crossing individuals of the same species.