Changes in the patterns of birefringence and filament deployment in the meiotic spindle of Nephrotoma suturalis during the first meiotic division

A study combining polarizing and Nomarski differential interference microscopes was carried out on Nephrotoma suturalis spermatocytes in order to correlate changes in the distribution of spindle birefringence with both the fibrillar organization of the spindle and movement of chromosomes within it. Records of fluctuations in birefringence obtained from measurements of retardation and analyses of ciné films show that there are significant transient fluctuations in birefringence during prometaphase (the so-called "northern lights phenomenon") and gradual changes in the magnitude and distribution of birefringence from prometaphase through telophase. A negative correlation was found between the distribution of birefringence at the various stages of division and the amount of fibrillar organization in the spindle observed with the Nomarski system. Neither the transient nor gradual birefringence changes in the Nephrotoma spindle can be explained adequately in terms of the number or distribution of spindle filaments. The possibility that these changes are associated with the forces developed within the spindle is discussed.