Originally published as Genetics Published Articles Ahead of Print on September 15, 2004.

Genetics, Vol. 169, 137-147, January 2005, Copyright © 2005
doi:10.1534/genetics.104.031815

Non-Mendelian Inheritance Induced by Gene Amplification in the Germ Nucleus of Paramecium tetraurelia

Atsushi Matsuda1 and Mihoko Takahashi2

Institute of Biological Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan

2 Corresponding author: Institute of Biological Sciences, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8572, Japan.
E-mail: mihoko{at}biol.tsukuba.ac.jp

A genetic investigation of strain d4-95, which carries a recessive mutant allele (pwB95) of pawn-B, one of the controlling elements of voltage-dependent calcium channels in Paramecium tetraurelia, revealed a non-Mendelian feature. Progeny of the cross between d4-95 and wild type often expressed a clonally stable mutant phenotype, even when they had a wild-type gene. The mutant phenotype was also expressed after self-fertilization of theoretical wild-type homozygotes recovered from the cross. Our molecular analysis demonstrated that the copy number of the mutant pwB gene in the micro- and macronucleus of d4-95 was much greater than that of the wild type. Most of the amplified, extra pwB gene copies in d4-95 were heritable independently from the original pwB locus. Repeated backcrossing of d4-95 with the wild type to dilute extra pwB genes in the strain produced segregants with a completely normal Mendelian trait in testcrosses. These results strongly suggest that a non-Mendelian inheritance of d4-95 was induced by gene amplification in the micronucleus.