Closely related species often differ karyotypically due to the fixation of chromosomal translocations and inversions which are generally deleterious when heterozygous but have normal fitnesses when homozygous (White, 1973; Muller, 1954, 1956). The fixation of such chromosomal rearrangements requires small population sizes, because they are selected against when in the minority but favored when in the majority, and hence they can become established in a local area only through random genetic drift. Naturalists have long known that most species exist in subdivided populations where each individual has a limited number of potential mates; and recent biogeographic studies, especially on islands, have emphasized the high turnover rates of local populations by extinction and colonization (Simberloff, 1974; Spight, 1974). The present paper develops population genetic models to determine how the fixation rates of negatively heterotic chromosomal rearrangements are influenced by the spontaneous occurrence rates, strengths of selection against heterozygotes, the effective size of demes (or local populations), interdeme migration rates, local extinction and colonization, and other factors. Data on chromosomal rearrangements concerning spontaneous occurrence, selection coefficients, and fixation rates in phylogeny are then used to draw inferences about the breeding structure of species during longterm evolution, particularly the effective size of demes. The effective size of a population through time, Ne, is the number of breed-

Effective deme sizes during long-term evolution estimated from rates of chromosomal rearrangement.

By combining elements of two Y-autosome translocations with displaced autosomal breakpoints, it is possible to produce zygotes heterozygous for a deficiency for the region between the breakpoints, and also, as a complementary product, zygotes carrying a duplication for precisely the same region. A set of Y-autosome translocations with appropriately positioned breakpoints, therefore, can in principle be used to generate a non-overlapping set of deficiencies and duplications for the entire autosomal complement.-Using this method, we have succeeded in examining segmental aneuploids for 85% of chromosomes 2 and 3 in order to assess the effects of aneuploidy and to determine the number and location of dosage-sensitive loci in the Drosophila genome (Figure 5). Combining our data with previously reported results on the synthesis of Drosophila aneuploids (see Lindsley and Grell 1968), the following generalities emerge.-1. The X chromosome contains no triplo-lethal loci, few or no haplo-lethal loci, at least seven Minute loci, one hyperploid-sensitive locus, and one locus that is both triplo-abnormal and haplo-abnormal. 2. Chromosome 2 contains no triplo-lethal loci, few or no haplo-lethal loci, at least 17 Minute loci, and at least four other haplo-abnormal loci. 3. Chromosome 3 contains one triplo-lethal locus that is also haplo-lethal, few or no other haplo-lethal loci, at least 16 Minute loci, and at least six other haplo-abnormal loci. 4. Chromosome 4 contains no triplo-lethal loci, no haplo-lethal loci, one Minute locus, and no other haplo-abnormal loci.-Thus, the Drosophila genome contains 57 loci, aneuploidy for which leads to a recognizable effect on the organism: one of these is triplo-lethal and haplo-lethal, one is triplo-abnormal and haplo-abnormal, one is hyperploid-sensitive, ten are haplo-abnormal, 41 are Minutes, and three are either haplo-lethals or Minutes. Because of the paucity of aneuploid-lethal loci, it may be concluded that the deleterious effects of aneuploidy are mostly the consequence of the additive effects of genes that are slightly sensitive to abnormal dosage. Moreover, except for the single triplo-lethal locus, the effects of hyperploidy are much less pronounced than those of the corresponding hypoploidy.

/pdf/segmental-aneuploidy-and-the-genetic-gross-structure-of-the-56ek8kudqc.pdf

Segmental aneuploidy and the genetic gross structure of the drosophila genome

Cryptic genetic variation is the dark matter of biology: it is variation that is not normally seen, but that might be an essential source of physiological and evolutionary potential. It is uncovered by environmental or genetic perturbations, and is thought to modify the penetrance of common diseases, the response of livestock and crops to artificial selection and the capacity of populations to respond to the emergence of a potentially advantageous macro-mutation. We argue in this review that cryptic genetic variation is pervasive but under-appreciated, we highlight recent progress in determining the nature and identity of genes that underlie cryptic genetic effects and we outline future research directions.

Uncovering cryptic genetic variation.

Plant leaves show pronounced plasticity of size and form. In the classical, partially dominant mutation Lanceolate (La), the large compound leaves of tomato (Solanum lycopersicum) are converted into small simple ones. We show that LA encodes a transcription factor from the TCP family containing an miR319-binding site. Five independent La isolates are gain-of-function alleles that result from point mutations within the miR319-binding site and confer partial resistance of the La transcripts to microRNA (miRNA)-directed inhibition. The reduced sensitivity to miRNA regulation leads to elevated LA expression in very young La leaf primordia and to precocious differentiation of leaf margins. In contrast, downregulation of several LA-like genes using ectopic expression of miR319 resulted in larger leaflets and continuous growth of leaf margins. Our results imply that regulation of LA by miR319 defines a flexible window of morphogenetic competence along the developing leaf margin that is required for leaf elaboration.

Regulation of LANCEOLATE by miR319 is required for compound-leaf development in tomato.

This review concern recent experimental information areas of animal cell molecular biology which are relevant to the mechanism of gene regulation. New data regarding interspersion and clustering of repetitive sequence elements in DNA are considered Molecular characteristics of
animal structural genes and mRNAs are discussed, with particular reference the frequency of structural gene sequences, mRNA turn over and the interpretation of dipteran complementation groups. The molecular characteristics of nuclear RNAs, the primary transcription products, are reviewed. Evidence for transcription level regulation is summarized and the relation of nuclear
and mRNA examined. The protein activator branch of the Britten-Davidson model for gene regulation is further developed and considered in light of current knowledge.

/pdf/organization-transcription-and-regulation-in-the-animal-2eajr4e362.pdf

Organization, transcription, and regulation in the animal genome.

An allelism matrix of ethyl methanesulphonate-induced recessive lethals found in a short chromosomal segment covered by the Y· mal + chromosome was constructed. When allelism tests for the lethals were extended to X-ray-induced lethals, a consistent linear map of 34 functional units could be constructed. 80% of the chemically induced lethals comprised single sections, which supports the view that most ethyl methanesulphonate-induced lethals were point-mutations. Most X-ray-induced lethals that were multi-section lethals were aberrations. The distribution of mutations in the affteced units was highly non-random; consequently any pooling of sensitivity patterns for whole chromosome segments would be misleading. There were some “hot-spots”, possibly due to the vicinity of intercalary heterochromatin. There was also an excess of chromosomes with more than one lethal. The possibility that variation in specificity of lethal and visible mutations obtained with different mutagenic agents is a secondary function of the number of “essential sites” in “vital” and “non-vital” genes is discussed.

Fine structure analysis of a chromosome segment in Drosophila melanogaster analysis of ethyl methanesulphonate-induced lethals

Allelism tests were performed between X-ray-induced lethals that were located on the X-chromosome of Drosophila at a region covered by the Y.ma-l + translocation. The complementation map which was constructed from the matrix of allelic combinations proved to be linear. So far 20 sections, or functional units, have been delineated. Further analysis should increase the number of sections until each section comprises one cistron. Practically all post-meiotic X-ray induced lethals in the region were aberrations. It is inferred that visible mutations induced by X-rays were deficiencies at non-essential loci. The breakage points of mutations in the segment were not distributed at random. The “hot spot” was attributed to an intercalary heterochromatic band, which by pairing with the adjacent proximal heterochromatin caused preference in breaks simultaneously at the “hot spot” and the proximal heterochromatin, and hence also caused preference for extensive aberrations. A correlation was found between male sterility and heterochromatic aberrations, but not between male sterility and X:A translocations. This kind of analysis suggests new approaches to the study of the organization of the genetic material in chromosomes and the nature of various mutational events.

Eliezer Lifschytz

Papers

Fine structure analysis of a chromosome segment in Drosophila melanogaster analysis of ethyl methanesulphonate-induced lethals

Fine structure analysis of a chromosome segment in Drosophila melanogaster. Analysis of x-ray-induced lethals.

A GENETIC ANALYSIS OF THE KILLER-PRUNE (K-pn) LOCUS OF DROSOPHILA MELANOGASTER

The action of the gene prune (pn) in Drosophila melanogaster.

A system for screening of rare events in genes of Drosophila melanogaster.