A case of spontaneous chromosome breakage at a specific locus occurring at meiosis

Abstract

The undescribed species of grasshopper PW21 (Orthoptera: Eumastacidae: Morabinae) from northern Australia has been found to exhibit three different karyotypes in its natural populations. Two of these have 2nB = 13, the third has 2nB = 15. Males are invariably XO, but the X may be either acrocentric or metacentric. The relation between the 13-chromosome karyotypes and the 15-chromosome one is complex. In a single male individual of the 15-chromosome population (out of four examined cytologically) one of the large pair of metacentric chromosomes regularly undergoes fragmentation at a locus close to the centromere just before first metaphase. The acentric fragment is caught up in the spindle but frequently travels to the "wrong" pole at first anaphase. Possible causal explanations of this phenomenon are discussed. These include the one-locus hypothesis ("suicide" of a chromosome) and various two-locus hypotheses ("murder" of one chromosome by another).