Crossover suppression in mice heterozygous for tobacco mouse metacentrics.

Abstract

Evidence is presented to show that crossing-over between the house mouse acro-centric chromosomes 1, 4, and 5 and the homologous chromosomes of the tobacco mouse Robertsonian translocations, Rb(1.3)lBnr, Rb(4.6)2Bnr, and Rb(5.15)3Bnr, is suppressed in the proximal regions. The data on chromosomes 6 and 9 of Rb2(4.6)Bnr and Rb6(9.14)Bnr, respectively, were ambiguous, but no evidence of crossover suppression was found with the other arm of Rb3Bnr (chromosome 15), nor was any suppression detected with the other tobacco mouse metacentrics when tested with chromosomes 8, 11, 12, 13, 14, and 17. Equivalent tests with the two Robertsonian translocations of house mouse origin, Rb(6.15)lAld and Rb(9.19)163H, also failed to demonstrate crossover suppression. Consideration of the combined genetic and cytogenetic maps of the chromosomes investigated showed that where crossover suppression was found, it could be detected with gene markers located at least halfway down the lengths of the chromosomes. Moreover, probably all the tests employed markers that were located proximally enough to detect crossover suppression of this magnitude. It is concluded that crossover suppression is not typical of all Robertsonian translocations and that the suppression found with RblBnr, Rb2Bnr, and Rb3Bnr could be attributed to minor structural or genie differences between the tobacco mouse metacentrics and house mouse acrocentrics within the proximal regions of only those chromosomes affected. These findings clearly demonstrate that there is no direct relationship between crossover suppression and nondisjunction. Although both phenomena may be attributed to differences between the metacentric and acrocentric chromosomes of the two species, the differences responsible for each type of event need not be the same. There was a deficiency of progeny carrying a metacentric in many of the crosses, and this was more evident when the heterozygous parent was a female. There was no indication of any relationship between this event and either non-disjunction or crossover suppression.