P-1 Detection and quantitative evaluation of chromosome loss induced in Drosophila wing primordial cells

Abstract

To detect and quantitatively characterize the abilities of environmental and genetic “agents” to induce chromosome loss in vivo we developed a simple, sensitive, inexpensive and rather fast genetic assay. It is based on the use of an engineered mwh+Y chromosome in Drosophila melanogaster. The mwh+ transgene in the mwh+Y chromosome completely rescues the mutant phenotype of mwh (multiple wing hairs), an outstanding recessive cell marker mutation. Loss of the mwh+Y chromosome in any cell of wing blade primordia leads to the formation of a cell without mwh+ gene function. (Importantly, the loss of the Y chromosome does not reduce cell viability.) Such a cell propagates its new trait during the subsequent rounds of mitoses along proliferation of the primordial cells. Its descending cells remain together and form an mwh mosaic spot (clone) on the developing wing blade, of course on mwh homozygous background. Strength of the studied environmental or genetic “agent” to induce chromosome loss is quantified based on the frequency and size of the mwh mosaic spots. Surveillance cellular mechanisms including spindle assembly checkpoint and mitotic catastrophe ensure the maintenance of chromosome/genome stability through the elimination of cells with e.g. unusual numbers of chromosomes. Reduced or depleted capacities of the surveillance mechanisms may lead to the formation of aneuploid cells. When surviving, such cells can subsequently be the source of mental retardation, miscarriage and cancer. Since components of the spindle assembly checkpoint and mitotic catastrophe appear to be evolutionary conserved, loss of the mwh+Y chromosome may well represent behavior of the chromosomes in general and thus the mwh+Y assay may be included some day in the battery of the so-called genetic toxicity testing procedures. In addition to a number of well known mutagens (X-rays, colchicine, ethyl methanesulfonate, formaldehyde, benzene), we characterized the effects of a parquet varnish and a synthetic thinner. These environmental components do induce chromosome losses, though with low efficiency. We also analyzed the chromosome destabilizing effects of gainand loss-of-function mutant lodestar Drosophila alleles that have been reported to induce chromosome instability. Lodestar, a member of the helicaserelated gene family, is a component of the mitotic catastrophe machinery and is engaged in placing the cohesin complex on the metaphase chromosomes. In Drosophila males, which carry the gain-of-function ldsHor-D mutant allele (that encodes the formation of sticky A777T-LDS molecules), (i) high frequencies of mostly equational nondisjunction occur during spermatogenesis and (ii) the chromosomes are rendered unstable such that they tend to be lost in the descending embryos. Loss of an unstable X chromosome, for example, leads to the formation of XX/X0, female/male mosaic (gynandromorph).