Interstitial chiasmata and centromere orientation in heterozygotes for a translocation in rye

The meiotic behaviour of plants heterozygous for translocation T242W of rye (involving 2RL and 6RL) and an interstitial C-band in 2RL has been analyzed. Chain and frying pan quadrivalents predominate. The following results have further been obtained: (i) double chiasmata occur in the interstitial segment carrying the C-band; (ii) from the frequency of being bound at metaphase I and the frequency of recombinant chromosomes at anaphase I, estimates of chiasma frequencies (and chiasma interference) in interstitial segments have been derived; (iii) estimates of the recombination fraction between the interstitial C-band and the translocation breakpoint have been obtained from offspring analysis; (iv) there is a difference in the frequency of alternate orientation between configurations with and without interstitial chiasmata (adjacent-2 has not been observed and a small but significant excess of alternate vs. adjacent-1 coorientation appears). Without intersitial chiasmata, alternate orientation predominates. The possible reasons for these differences are discussed. Key words: Secale cereale, translocations, chiasma frequency, centromere orientation.

Metaphase position and orientation of an interchange quadrivalent of Allium triquetrum: II. Changes in position with changes in percentage alternate orientation

SummaryThe position and orientation of an interchange quadrivalent in flattened lateral views of metaphase I were studied in inflorescences of an interchange heterozygote of Allium triquetrum showing high, medium and low alternate orientation frequencies. As the overall frequency of alternate orientation changes in these different inflorescences so the positioning of the alternately oriented quadrivalents changes, in that it becomes more like that of the adjacent quadrivalents. The present data argue for the biological significance of the position/orientation data, and imply the presence of two components of alternately oriented quadrivalents with different positional characteristics.

The taxonomy of multivalent orientation: six modes of alternate or one?

Unlike adjacent I and II, alternate I and II orientations of interchange and tetrasome quadrivalents can be considered extremes within one particular population of orientations (alternate) and thus lack sufficient distinction to justify a separate taxonomic status. Within the population up to six types may be distinguished, but the biological significance of this distinction is small.Key words: multivalent, alternate orientation, adjacent orientation, chiasmata.

Position and orientation in the metaphase equator of an interchange quadrivalent of Allium triquetrum

SUMMARYThe position and orientation of an interchange quadrivalent in flattened, lateral views of metaphase I were studied in pollen-mother-cells of an interchange heterozygote of Allium triquetrum. The quadrivalent is most often located in marginal rather than central positions in the equator. Moreover, when positioned marginally the quadrivalent is more often than expected found in adjacent orientation, whilst when positioned centrally it is more often than expected found in alternate orientation. Consequently, the frequency of alternate (genetically balanced) orientation in the quadrivalent varies sharply according to whether data are obtained from more marginal or more central positions in the metaphase plate.

CENTROMERE ORIENTATION OF QUADRIVALENTS OF HETEROZYGOUS TRANSLOCATIONS AND AN AUTOPLOID OF GOSSYPIUM HIRSUTUM L

ABSTRACT Cytological observations of quadrivalents of heterozygous translocations in Gossypium hirsutum L. demonstrate that, in addition to alternate-1 and alternate-2 orientations, a third alternate orientation (alternate-3), which occurs as a three-dimensional, V-type configuration, can be identified.—Two additional types of disjunctions, the centromere orientations of which are rotational modifications of either adjacent or alternate configurations, were also observed in quadrivalents of a translocation heterozygote. These two types are rare, and both appear in the form of the Roman numeral X. The X and the alternate-3 types also occur in quadrivalents of an autoploid of G. hirsutum.—The two X types, along with adjacent-1, adjacent-2, alternate-1 and alternate-2 orientations, represent the six possible types of planar 2 x 2 random orientation of the four centromeres of a quadrivalent. Including the three-dimensional alternate-3 type, there are seven types of orientation.

ALTERNATE-1 AND ALTERNATE-2 DISJUNCTIONS IN HETEROZYGOUS RECIPROCAL TRANSLOCATIONS

ABSTRACT Alternate-1 and alternate-2 orientation of chromosomes, as well as the two types of adjacent orientation, were observed cytologically in the ring configurations of three reciprocal translocation heterozygotes of Gossypium hirsutum L. The observations indicate that the two types of alternate orientation should be characteristic of ring-forming translocations.