Class II Analphoid Chromosome in a Child with Aberrant Chromosome 7: A Rare Cytogenetic Association

A neocentromere is a functional centromere that has arisen within a region not known to have a centromere. We present a case with a very rarely reported class II neocentromere formation in an aberrant chromosome 7. A 22-month-old male was referred because of dysmorphic features. Banding cytogenetics was performed, and a ring 7 and a supernumerary marker chromosome along with a normal chromosome 7 were found. In situ hybridization using a centromeric probe revealed 46 signals, of which 2 signals for chromosome 7 were observed, one on the normal and one on the ring chromosome. Further analysis using FISH revealed that the linear acentric fragment was part of the 7q region, which suggests that there could be a possible McClintock mechanism.

Induced partially sterile inversion heterozygote in Ornithogalum virens L.

Normal looking plants of Ornithogalum virens L. (Liliaceae) raised from γ-ray irradiated seeds were partially sterile. Meiotic studies carried out on this plant for two consecutive years revealed the frequent occurrence of a dicentric chromosome bridge accompanied by an acentric fragment at anaphase I (AI). Paracentric inversion followed by crossing over in the inverted region might have resulted in this type of consequence at AI. Normal plants of O. virens (2n = 6) showed regular presence of 3 bivalents at metaphase I (MI) and equal 3 : 3 separation of chromosomes at AI. Presence of 2 to 6 univalents in the pollen mother cells (PMCs) of the treated plant indicated the occurrence of occasional asynapsis possibly due to induced inversion. Early movement, lagging and unequal separations of chromosomes were encountered rather infrequently at AI. Pollen sterility of this inversion heterozygote plant was 44.80 and 54.64% in the first and second year, respectively compared to only 1.12 to 3.90% in the control plants. Key words: Ornithogalum virens; γ-ray; Paracentric inversion; Heterozygote; Sterility DOI: 10.3329/bjb.v38i1.5116 Bangladesh J. Bot. 38(1): 33-37, 2009 (June)

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

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).