Distorted Segregation Ratios Research Articles

FURTHER CHARACTERIZATION OF GENETIC ELEMENTS ASSOCIATED WITH THE SEGREGATION DISTORTER PHENOMENON IN DROSOPHILA MELANOGASTER

Segregation Distorter, SD, associated with the second chromosome of Drosophila melanogaster, is known to cause sperm bearing the non-SD homologue to dysfunction in heterozygous males. In earlier studies, using different, independently derived, SD chromosomes, three major loci were identified as contributing to the distortion of segregation ratios in males. In this study the genetic components of the SD-5 chromosome have been the subjects of further investigation, and our findings offer the following information. Crossover analysis confirms the mapping of the Sd locus to a position distal to but closely linked with the genetic marker pr. Spontaneous and radiation-induced recombinational analyses and deficiency studies provide firm support to the notion that the Rsp (Responder) locus lies within the proximal heterochromatin of chromosome 2, between the genetic markers lt and rl and most likely in the heterochromatin of the right arm. The major focus of this study, however, has been on providing a better definition of the genetic properties of the Enhancer of SD [E(SD)]. Our findings place this locus within the region of the two most proximal essential genes in the heterochromatin of the left arm of chromosome 2. Moreover, our analysis reveals a probable association of the E(SD) locus with a meiotic drive independent of that caused by Sd.

Evolution of mouse major histocompatibility complex genes borne by t chromosomes.

Virtually all wild mouse populations carry t haplotypes that cause embryonic lethality or semilethality, distortion of segregation ratios, suppression of crossing-over, and male sterility. The t complex of genes is located on chromosome 17, closely linked to the H-2, the major histocompatibility complex of the mouse. The t haplotypes differ from each other not only in lethal genes they carry but also in their linked H-2 haplotypes. In this study, we compared the class II H-2 genes present on 31 t chromosomes extracted from wild populations in different parts of the world. The comparison was based on the analysis of DNA fragments obtained after digestion with restriction endonucleases. The results reveal the existence of three major groups of class II alleles representing main branches on the evolutionary tree of the t chromosomes. Alleles within each group are similar if not identical, although they are borne by chromosomes that have been separated in time and space. The presence of similar alleles in Mus musculus and Mus domesticus suggests that some of them may have been separated for more than 1 million years. This must also be the minimal age of the t chromosomes but, because at least two of the three main branches appear to be related in their origin, the actual age of t chromosomes could be much greater. The observations support the proposal that H-2 genes evolve slowly.

A deficiency mutant of the Gc system.

In the course of a paternity investigation an apparent mother-child incompatibility was observed in the Gc system. An extensive family study was undertaken to test the hypothesis of a silent gene or null allele responsible for the contrary phenotypes: the mother had the type Gc 2, the son was GC 1. The apparent incompatibility was due to a "pseudo" silent allele, called Gc * 1, which controlled a group-specific component with extremely reduced serum concentrations. This double-band mutant could be differentiated from the Gc 1S bands by two-dimensional electrophoresis: isoelectric focusing (IEF)/6 M urea IEF. The allele Gc * 1 was found in 12 persons from this family, it was not associated with any apparent disease state. Also present in this family was the variant Gc 1C1. Pedigree analysis revealed a possible (not significant) distorted segregation ratio for the allele Gc * 1C1, which was found in 22 of 33 offspring from marriages with one parent heterozygous for Gc * 1C1.

Meiotic dysgenesis associated with behavioral mutants of Phycomyces

Phycomyces mutants, recently isolated for enhanced bending responses (hypertropic phenotype), have unusual genetic properties. In sexual crosses between hypertropic mutants and other strains, the progeny showed the following features: a) many incomplete tetrads, b) distortion of segregation ratios, c) progeny with nonparental phenotypes when hypertropic strains carrying mutations in the same gene or even the same allele were crossed, and d) morphologically abnormal progeny with phenotypes unrelated to those of the parents. In particular, the mesophorogenic colonies, which produced short sporangiophores, were genetically unstable; their mycelia produced sectors with normal morphology and segregated several alleles for different markers. Most of the phenomena (mutation, segregation distortion, and sterility) described in this paper resemble the "hybrid dysgenesis" syndrome in Drosophila. The results suggest that all seven hypertropic mutations affect the process of meiosis and thereby lead to unstable aneuploid progeny.

Cine photography and video recording of anterior segment fluorescein angiography.

A description is given of apparatus and technique for carrying out cine photography and video recording of anterior segment fluorescein angiography. We found cine best for single-frame analysis and video tape recording less expensive.

Copper tolerance and segregation distortion in aged Paramecium

The effect of age on copper tolerance and genetic phenomena in Paramecium has been investigated. Regression analysis of paired observations of log median tolerance limit to copper and age in fissions on four different days has shown that there is no reason to reject the null hypothesis that age has no effect on copper tolerance. Older cells are not, therefore, necessarily more susceptible to environmental stress as is often assumed. The isolation of starved cells from clones which are very old frequently results in inviability, but the progeny of 5 lines which had a high percentage of viability showed highly distorted segregation ratios at 4 marker loci. Eleven of the 19 ratios differed significantly from the expected 1 : 1. The distorted segregation was not consistently biased toward one allele nor were subclones of a single clone biased in the same direction. An interpretation is presented which suggests the distorted segregation is an artifact. It is proposed that segregation distortion results as a consequence of individual cells entering autogamy at different times rather than synchronously as they starve. If true this hypothesis has important technical implications for the measurement of viability at autogamy in aged lines.

Poisson analysis on the distribution of x-ray induced crossovers in spermatocytes of Drosophila.

T o explain the basis of distorted segregation ratios arising in a variety of situations in the Drosophilia male such as the Bar-Stone translocation (Novitslii and Sandler, 1957), heterozygotes for the X.Y and a fragment homologue (Lindsley and Sandler, 1958, heterozygotes for SD (segregation-distorter, Sandler, Hiraizumi, and Sandler, 1959), hnd the non-random behavior of X and Y in sc'-schmales (Gershenson, 1933; Sandler and Braver, 1954; Zimmering, 1963), Peacoclc and Ericlison (1965) have suggested the following hypothesis. They propose as a regular feature of meiosis the existence of a and a pole at the first division, two functional sperm being formed ultimately from the former and two non-functional sperm from the latter. Consequently, segregants included into cells arising froill the pole are recoverable, while those included into cells arising from the pole are not. Thus, distorted segregation ratios are visualized as arising from a non-random movement of segregants to these poles. It should be noted that the original suggestion that not all products of spermatogenesis may be functional was made by Novitslii and Sandler, 1957. Now, a prediction from this model postulatinga and pole, would be that from a meiotic exchange generating reciprocal products which then segregate at the first division, only one of these products is recoverable. This is so because one of the products will proceed to the pole. It would follow, then, that each recovered exchange will have arisen from an independent event. One line of evidence of independence would be that the distribution of crossovers does not deviate significantly from a Poisson distribution. Data bearing on this point are presented below.