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Causes of chiasma repatterning due to centric fusions

Claudio J. Bidau

Departamento de Genética, Facultad de Ciências Exactas, Químicas y Naturales, Universidad Nacional de Misiones, Félix de Azara 1552, 6° Piso, 3300 Posadas, Misiones, Republica Argentina. (CONICET)

ABSTRACT

In several grasshopper species, centric fusions (Robertsonian translocations) have been reported to produce a repatterning of chiasmata. Proximal chiasmata are eliminated, chiasma formation shifts to distal ends and overall chiasma frequency is reduced in fusion trivalents and bivalents. In this paper, this repatterning is explained on the basis of evidence gathered from the grasshopper Dichroplus pratensis. It is assumed that the chromosomes enter meiosis with a Rabl orientation and that chiasmata are formed preferentially in regions which include or arc close to zones that pair early in prophase I and have the most protracted period of synapsis. 'infused telo-acrocentric chromosomes have both ends attached to the nuclear envelope and initiate pairing from either one or both ends, which would determine a basically proximal-distal chiasma distribution. In fused chromosomes, four original ends are replaced by two, the number of early pairing regions is reduced and these are shifted to the ends. This explains both the elimination of proximal chiasmata and the observed reduction in chiasma frequency brought about by the fusions.

Keywords: Chiasma; Centric fusions.

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