Browsing by Author "Amit Bardhan"

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Reduced meiotic fitness in hybrids with heterozygosity for heterochromatin in the speciating Mus terricolor complex
(Indian Academy of Sciences, 2003) Tikaram Sharma; Amit Bardhan; Min Bahadur
Mus terricolor I, II and III are the three chromosomal species which differ in stable autosomal short-arm heterochromatin variations established in homozygous condition. Analysis of meiosis in the laboratory-generated F1 male hybrids from crosses (both ways) between M. terricolor I and II and between M. terricolor I and III shows high frequencies of pairing abnormalities at pachytene. The backcross (N3 generation) male hybrids between M. terricolor I and II have meiotic abnormalities as in the F1 male hybrids, though to a lesser extent. They show difference in pairing abnormalities in the different karyotypic forms; the backcross hybrids heterozygous for the heterochromatic short arms have more anomalies compared to the homokaryotypic hybrids. This suggests a negative influence of the heterochromatin heterozygosity in meiotic pairing. The results indicate a role for heterochromatin variations in the development of a reproductive barrier in the speciating M. terricolor complex.
Sequential meiotic prophase development in the pubertal Indian pygmy field mouse: Synaptic progression of the XY chromosomes, autosomal heterochromatin, and pericentric inversions
(National Research Council of Canada, 2000) Amit Bardhan; T. Sharma
Sequential meiotic prophase development has been followed in the pubertal male pygmy mouse Mus terricolor, with the objective to identify early meiotic prophase stages. The pygmy mouse differs from the common mouse by having large heterochromatic blocks in the X and Y chromosomes. These mice also show various chromosomal mutations; for example, fixed variations of autosomal short arms heterochromatin among different chromosomal species and pericentric inversion polymorphism. Identification of prophase stages was crucial to analyzing effects of heterozygosity for these chromosomal changes on the process of homologous synapsis. Here we describe identification of the prophase stages in M. terricolor, especially the pachytene substages, on the basis of morphology of the XY bivalent. Based on this substaging, we show delayed pairing of the heterochromatic short arms, which may be the reason for their lack of chiasmata. The identification of precise pachytene substages also reveals an early occurrence of 'synaptic adjustment' in the pericentric inversion heterobivalents, a mechanism that would prevent chiasma formation in the inverted segment and thereby would abate adverse effects of such heterozygosity. The identification of pachytene substages would serve as the basis to analyze the nature of synaptic anomalies met in M. terricolor hybrids (which will be the basis of a subsequent paper).
Sister chromatid cohesion during meiosis: Differential mechanisms for arms and centromere coherence
(1997) Amit Bardhan
Sister chromatid cohesion is a fundamental aspect of chromosome behaviour which ensures faithful disjunction of chromosomes. The differential release of centromeric- and arm-cohesion during meiosis suggests that the molecular basis of cohesion in the two domains might be different. A number of meiotic mutations differentially affect cohesion in the two domains. Recent identification of two proteins, ORD and mei-S332, shows that they are involved in maintaining cohesion in two spatially different locations. While mei-S332 acts to structurally hold the sister centromeres together until second meiotic anaphase, ORD is likely to time the release of cohesion along the arms during first meiotic division.