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A new method to detect potential genotoxic agents using mitotic crossing-over in diploid strains of Aspergillus nidulans

Lidia Terêsa de Abreu Pires^I; Tânia Maria Araújo Domingues Zucchi^II

^IDepartamento de Genética, Faculdade de Medicina de Ribeirão Preto, Av. Bandeirantes 3900, 14049-900 Ribeirão Preto, SP, Brasil
^IIInstituto de Ciências Biomédias, Av. Prof. Lineu Prestes 1374, Cidade Universitária, 05508-900 São Paulo, SP, Brasil. Fone: (011) 818-7264, FAX: (55) (011) 813-0845. Send correspondence to T.M.A.D.Z.

ABSTRACT

This paper presents a new method for detecting mitotic crossing-over in Aspergillus nidulans, based on the "homozygosity index" (HI) of recessive genes originally present in heterozygosis in diploid strains, which occurs after mitotic crossing-over between the marker in question and the centromere. Since homozygous diploids (-/-) for auxotrophic markers can not grow in MM, homozygotization can be demonstrated by distorted mitotic segregation of the alleles involved. Two similar diploid strains (UT 448//UT 184 and Z1//UT 184), which differ by a chromosomic duplicate segment transposed from chromosome II to I in the Z1 haploid strain, were used. This excess of genetic material confers to the Z1 mutant the uvs character and makes Z1//UT 184 more unstable and sensitive to genotoxic agents, as evidenced by its high spontaneous recombinational index.
After UV-treatment, the "HI" of both strains is significantly increased, especially for the Z1//UT 184 diploid strain.
Induced mitotic crossing-over detection through traditional methods requires that the event occurs in specific regions of the genome and involves only two markers. The novel aspect of the proposed method is that it abolishes such a restriction by rendering crossing-over a mere function of "HI" of any marker.

Keywords: genotoxic agents; mitotic crossing-over; diploid strains; Aspergillus nidulans.

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