Three-dimensional architecture of chromosome fibres in the crane fly: co-oriented autosomal bivalents and amphitelic sex univalents during prometaphase

Kinetochore-driven outgrowth of microtubules is a central contributor to kinetochore fiber maturation in crane-fly spermatocytes

During meiosis I in crane-fly spermatocytes, the increase in microtubule density in K-fibers clearly originates at the kinetochore. The increased density first appears at the kinetochore, then elongates toward the pole, probably by addition of tubulin at microtubule plus ends anchored in the kinetochore.

We use liquid crystal polarized light imaging to record the life histories of single kinetochore (K-) fibers in living crane-fly spermatocytes, from their origins as nascent K-fibers in early prometaphase to their fully matured form at metaphase, just before anaphase onset. Increased image brightness due to increased retardance reveals where microtubules are added during K-fiber formation. Analysis of experimentally generated bipolar spindles with only one centrosome, as well as of regular, bicentrosomal spindles, reveals that microtubule addition occurs at the kinetochore-proximal ends of K-fibers, and added polymer expands poleward, giving rise to the robust K-fibers of metaphase cells. These results are not compatible with a model for K-fiber formation in which microtubules are added to nascent fibers solely by repetitive “search and capture” of centrosomal microtubule plus ends. Our interpretation is that capture of centrosomal microtubules—when deployed—is limited to early stages in establishment of nascent K-fibers, which then mature through kinetochore-driven outgrowth. When kinetochore capture of centrosomal microtubules is not used, the polar ends of K-fibers grow outward from their kinetochores and usually converge to make a centrosome-free pole.

Relationship between the arrangement of microtubules and chromosome behaviour of syntelic autosomal univalents during prometaphase in crane fly spermatocytes

During meiotic prometaphase of crane fly spermatocytes, syntelic autosomal univalents are able to move between the spindle poles several times. The relationship between the behaviour of chromosomes and the arrangement of microtubules during this stage was studied using a fixation technique (Nicklas et al. 1979) which makes it possible to examine a certain cell in an electron microscope after live observation. After reorientation, when a syntelic univalent had started moving towards the opposite spindle pole, a short chromosome fibre was found. When a univalent had reached the equator, a chromosome fibre could be traced up to the spindle apex. During the movement towards the opposite spindle pole the degree of disorder in the chromosome fibre was high, whereas it was low in the fibre of a motionless univalent. The degree of disorder was determined by the relative proportion of skew fibre microtubules. At the beginning of reorientation a chromosome fibre was still present, but later, it was no longer possible to recognize such a fibre. Instead of a chromosome fibre, a bundle of microtubules laterally associated with the kinetochore was observed. Some microtubules of this bundle had a direct contact with the kinetochore. These observations strongly hint that the laterally associated microtubules have an important function in the reorientation of syntelic autosomal univalents.