The Indian muntjac (Muntiacus muntjac): A model for clastogen evaluation, comparative gene mapping, and mammalian karyotype evolution.

Abstract

The Indian muntjac (Muntiacus muntjac), an asiatic deer, has the lowest diploid chromosome number among mammals (female 2N = 6; male 2N = 7). Each chromosome is uniquely identifiable in unbanded, Giemsa-stained metaphases, facilitating cytogenetic investigations. The Chinese muntjac (M. reevesi) is a closely related species whose diploid chromosome number is 46. These two species are phenotypically similar and can produce viable (but sterile) hybrids, suggesting evolutionary DNA sequence conservation. The goal of this project was to establish the relevance and utility of Indian muntjac cells for studies of induced chromosomal instability and of mammalian karyotype evolution by comparative gene mapping. The karyotypes of two cultured Indian muntjac fibroblast strains were stable over several generations, with 90% diploid, 8% tetraploid and 2% triploid cells. The Indian muntjac cellular DNA content was 93.8% of the human value, consistent with significant human/muntjac structural gene conservation. A detailed G-banded idiogram of muntjac metaphase chromosomes, combining ISCN-type nomenclature and relative band size/position measurements, was generated. This will prove useful for comparative gene mapping by in situ hybridization and for breakpoint determination in clastogenesis assays. The cytotoxic and cytogenetic effects of three known chemical clastogens (bleomycin, mitomycin C, and streptonigrin) were assayed in Indian muntjac fibroblasts. Cytotoxicity dose-response curves were similar to published human and rodent cell data. Spontaneous and induced chromosome breakage frequencies were comparable to those observed in control human fibroblasts. These data indicate that Indian muntjac fibroblasts are relevant for evaluation of chemical agents for potential human clastogenicity. For comparative gene mapping, muntjac chromosomes were purified flow-cytometrically after staining with propidium iodide (for DNA content) and fluoresceinated anti-kinetochore antibody (for centromere size). Southern hybridization of muntjac chromosome-specific DNA with human cDNA probes mapped the human chromosome #19-linked ERCC2 DNA repair gene and the human chromosome #16-linked protein kinase C {dollar}\beta{dollar} polypeptide gene to muntjac chromosome #2. The human ZFX/ZFY gene detected both male-specific and shared male/female bands in Indian muntjac DNA. The data indicate that Indian muntjac cells constitute a relevant system for investigation of induced clastogenesis and mammalian chromosome evolution.