Dynamics and Ultrastructure of Monocentric Chromosome Movement

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Abstract
In all gall midges (Cecidomyiidae, Diptera) spermatogenesis is unorthodox. The case of Mycophila speyeri was investigated in vitro and by electron microscopy. In the first meiotic division in males three chromosomes (sperm chromosomes) segregate from the residual chromosomes by monocentric chromosome movement. The spermatogonial divisions prior to this are completely normal. In the interphase before meiosis I the three sperm chromosomes can be recognized: they are already condensed, in obvious contact with the nuclear envelope and separated from the residual chromosomes by lamellae of nucleolus-like material. In prophase these lamellae disappear and the residual chromosomes condense too. Near a pair of centrioles a spindle precursor forms. After the disappearance of the nuclear envelope a morphologically bipolar spindle appears, containing only the sperm chromosomes. These look like normal metaphase chromosomes each having two kinetochores which point to the poles. The chromosomal fibres are long individual bundles. In addition to microtubules they contain a lot of ER-like membranes which are connected to the microtubules by bridges. During anaphase the sperm chromosomes do not divide. Chromosome segregation involves three components: Cytokinesis separates the small spermatocyte with the three chromosomes from the large residual cell. An intercellular bridge persists. The spermatocyte nucleus as well as the residual cell nucleus go through interphase. The second meiotic division occurs only in the spermatocyte. It is normal and produces two spermatids. The chromosome velocity during monocentric division is much slower than the chromosome velocity during the spermatogonial division and the second meiotic division. The structural and functional peculiarities of the monocentric division are discussed.