The three-dimensional architecture of chromosome fibres in the crane fly

Microtubules of amphitelically oriented sex univalent chromosome fibres were traced in longitudinal serial sections. The investigated chromosomes were from four different cells representing consecutive stages of anaphase segregation. A correlation was found between chromosome movement and a characteristic distribution of free microtubules (fMTs) oriented obliquely with respect to the kinetochore microtubules. During chromosome segregation the proportion of these skew fMTs (the proportion of skew fMTs is a measure of the degree of disorder in the fibre) is higher in the fibre pointing in the direction of movement than in the trailing fibre. The results are discussed in relation to spindle forces. Although the anaphase of amphitelic sex chromosomes is different in several respects (orientation of chromosome fibres, mutual connexion of chromosomes via kinetochore microtubules, spindle elongation occurring simultaneously), the observations on the distribution of fMTs in the chromosome fibres is, in principle, compatible with those previously made on syntelic autosomes.