Spermatogenesis in Trichosia pubescens (Diptera:Sciaridae).

This paper deals with the meiotic process in males of T. pubescens. The spermatocytes of this species are associated in clusters (rosettes) of some fifteen cells which are fairly synchronized during spermatogenesis. During meiosis, which is typically achiasmatic, 2 sets of chromosomes are eliminated into a narrow neck of cytoplasm (bud) which protrudes from the cell pole facing the centre of the rosette. The bud formation starts prior to the onset of the meiosis. The spindle that appears during the first meiotic division is unipolar with the fibres running from the pole at the end away from the bud up to the equatorial region of the cell. The chromosomes which move toward the spindle pole display a V-shape while the others, moving budward, do not show any consistent shape. We suggest that a special mechanism of chromosome motion is involved in the migration of these chromosomes. The second meiotic division is fairly similar to the ordinary type observed in other organisms except for the migration of both chromatids of one chromosome precociously toward the pole opposite the bud. At this division a typical bipolar spindle is present and all migrating chromosomes display a V-shape. The chromosomes eliminated during the first meiotic division clump together remaining in a pycnotic state at the distal portion of the bud until the end of spermatogenesis. The chromosomal set eliminated at the second meiotic division, as compared to the first, decondense simultaneously with the group located in the cell body forming a typical interphase nucleus. T. pubescens has 3 chromosomes limited to the germ-line cells. These chromosomes are typically heterochromatic, replicating their DNA out of phase with the S-chromosomes, probably at a later stage in interphase. Our results suggest that they are transcriptionally active in the interphase between the 2 meiotic divisions and just after the meiotic process.