STUDIES ON THE CHROMOSOME SPIRALIZATION CYCLE INTRILLIUM

Abstract

Studies of the major, relic, and relational coils and of chromosome, chromatid, and chromonema length changes in meiotic, microspore, and root tip chromosomes of Trillium erectum L. and T. grandiflorum Salisb. have led to the following conclusions:—Elongation of the chromonema occurs between early diakinesis and first anaphase and between second anaphase and microspore prophase. Contraction occurs between zygotene and early diakinesis, between first and second anaphase, and during microspore prophase. Chromonema elongation between early diakinesis and anaphase is associated with the formation of the major coil and its transition into the relic coil of the microspore prophase is accompanied by a further elongation. The tertiary split results in the half-chromatids being associated in the form of a plectonemic spiral, which persists as such to microspore prophase. The gyres of the plectonemic relic coil become partially straightened out to form a relational coil, whose twists are in the same direction as their antecedent relic coils. During microspore prophase some relational twists are apparently eliminated at intrabrachial changes of direction, others by contraction and untwisting of the chromatids.Changes of direction fall into three categories: (1) those associated with attachments, (2) those associated with chiasmata, and (3) the remainder, the frequency of which is proportional to the number of gyres. Intrabrachial changes of direction are more numerous in microspore prophase than in root tip chromosomes, presumably because Factor (2) is inoperative in the latter. There are very few data that could possibly be taken to indicate that chromosomes may have an inherent directional pattern of coiling.To the tentative hypothesis of Wilson and Huskins that the major coil of meiosis is formed by an elongation of the chromonema within a restricted space, the pellicle, the following may be added: the half-chromatids at metaphase are wound in the form of a plectonemic spiral. The straightening out of this spiral results in the relationally twisted chromatids of microspore prophase. These twists are eliminated both by cancellation at the points at which changes of direction have occurred in the major coil and also by contraction and untwisting of the chromatids.

A mechanical basis for chromosome function

“…Thus, chromatin expansion forces first promote axis extension; when no further extension is possible, compression stress develops along the axes and promotes axis coiling; when expansion forces are alleviated, axes contract back to their original lengths but now in their new expansional stress-promoted shape. The same progression was described for diakinesis coiling in higher plant meiosis, leading to a closely related model, 50 years ago (29,31). Notably, also, mitotic prometaphase, an expansion phase (Fig.…”

Cine Micrographic Studies on Mitotic Spiralization Cycle

“…The stcindcird or somatic coil is often said to leave the chromatid as a plectonemic, hollow, wire-spring formed coil, apparently singlestranded. This opinion is based upon observations on fixed material from the first or both post-meiotic divisions in microspores and pollen- tubes of Trillium (SPARROW, HUSKINS and WILSON, 1941, SPARROW, 1942, WILSON, SPARROW and POND, 1959 and from root-tip mitoses in AlIium (WILSON and COLEMAN, 1952). However, KUWADA (1939), NE-BEL (1941) , MANTON (1945), GIMENEZ-MARTIN et a[.…”

“…The origin of the plectonemic relational coil is believed to trace back to the relic coil in the same division, the relational coil thus being formed during condensation of the relic coil (SPARROW, HUSKINS andWILSON, 1941, SPARROW, 1942). In about 25 cells analysed, both relic gyres and relational twists are present in the chromosomes when they become visible in early prophase (cf.…”

Variability in Length and Arm Ratio of the Pachytene Chromosomes of Corn

“…This technique is known to produce a considerable swelling of the chromo some, but since relative measurements remain comparable to those from other techniques it has been thought that the swelling per unit length can be considered constant without serious error (Manton 1937, Sparrow, Huskins andWilson 1941). Any tendency for the various chromosomes to respond differently in different cells to these chemicals would be inseparable from other effects operating at the cellular level in this experiment.…”