Linear orientation of trivalents and quadrivalents in late metaphase I pollen mother cells of pearl millet

Abstract

Robertsonian trivalents, trisomic trivalents, and tetrasomic ring quadrivalants showed linear or indifferent orientation frequencies at late metaphase I of 1/3. This fits a model assuming sequential centromere activation in the multivalent with random order of activation and sufficiently long interval to permit stable coorientation of any two early centromeres before the following centromere is activated. A late centromere trapped between two coorienting early ones is prevented from orientation. Reorientation in pearl millet, when it occurs, is late in these configurations. Chain quadrivalents have less linear but more indifferent orientations than the expected 1/6 and 1/3, respectively. Possible reasons for this are given. For rye the situation is very different: linear orientation of Robertsonian trivalents is practically absent, presumably owing to insufficiently long intervals between centromere activations, and to early reorientation. A reduction in interval length between centromere activations is assumed to accompany the stabilization of autotretraploids.Key words: pearl millet, trivalents, quadrivalents, centromere activation, chromosome orientation.

Prevalence of Linear Configuration among Chain Trivalents at Metaphase I in Pollen Mother Cells of Petunia axillaris (LAM.) B.S.P.

“…The data drastically deviate from the 33.3 percent limit for "non-neighbour co-orientation", suggested by Narasinga Rao and Sybenga (1984). Although the suggested model shifts the emphasis of orientation studies from quadri to tri-valents, it bypasses the question concerning the interaction between the midcentromere which is trapped and the spindle, if any.…”

“…It is regarded as ineffective (Narasinga Rao, unpublished: Cf. Sybenga and Rickards 1987) whether or not it is "denied effective activity" by being late (Narasinga Rao and Sybenga 1984) and whether or not it is assumed to be "inactivated" (Sybenga 1975) and whether or not it is believed to be "amphitelic" (Rickards 1983 Apart from the adjacent and virtually vertical linears (Figs . 1, 2) two flexed forms of the linear chain trivalent are also occasionally seen.…”

Non-random patterns of non-disjunctional orientation in trivalents of multiple Robertsonian heterozygotes of Dichroplus pratensis (Acrididae)

“…The observed frequencies are very different from those predicted by the model of centromeric orientation of multivalents developed by Narasinga Rao and Sybenga (1984). The model implies an interdependence between centromeric activity and orientation and assumes: 1) a temporal sequence of centromere activation (each centromere activation being an independent event); 2) inactivation of the 'central' centromere by stretching through tension exerted by the two adjacent active centromeres; and 3) absence of multivalent reorientation.…”

“…Considering that nondisjunctional orientation could result from activation and coorientation of the centromeres belonging to the telocentrics and inactivation of the centromere of the fusion metacentric (Narasinga Rao & Sybenga, 1984;Sybenga, 1975), the interaction can be explained as a simultaneous inactivation of centromeres of metacentrics which are now closer within the nucleus. This hypothesis is supported by the excess of metaphases I in which all the trivalents are nondisjunctionally oriented (irrespective of number and quality of the fusions) while the frequency of cells in which all are disjunctionally oriented (through the coorientation of the central centromere with the other two) do not depart from the expected values.…”

The Cytogenetic Analysis of a Robertsonian Rearrangement in Lolium multiflorum.

“…This is what happens in bivalents and the opposing forces on the co-orientated centromeres stabilise the configuration. Narasinga Rao and Sybenga (1984) considered the timing of activation of the centromeres in a chain trivalent. Applied to the present data, the order of activation of the centromeres of the Robertsonian trivalent, if random, would be any of the following: centric chromosomes and st that of the standard chromosome, which is always positioned in the middle of the trivalent.…”