Reconstruction of the Neurospora crassa pachytene karyotype from serial sections of synaptonemal complexes

Serial sections from isolated asci were used to reconstruct the seven pachytene bivalents of Neurospora crassa. The synaptonemal complex could be traced for its whole length in each bivalent, being attached to the nuclear envelope at both ends in six. The satellite end of the nucleolar chromosome did not appear to be attached to the nuclear envelope. The estimated lengths of the bivalents ranged from 10.7 to 5.1 microns in one nucleus, from 11.5 to 4.2 microns in another, and from 8.5 to 4.4 microns in a third, with total haploid complement lengths of 45.5 microns, 47.3 microns, and 43.9 microns respectively. These values are considerably smaller than published light microscopical measurements.—The synaptonemal complex in N. crassa, as in other ascomycetes, has two banded ca. 400 Å wide lateral components held about 1200 Å apart by a central region containing the ca. 200 Å wide central component. With normal glutaraldehyde/OsO₄-phosphate buffered fixation the chromatin of the pachytene bivalents is poorly contrasted. Occasional local thickenings of the central component into electron dense nodes ca. 1000 × 500 Å in longitudinal section are characteristic of the complex.