Observations on spermatogenesis in Drosophila

Summary

• 1. Chromosome behavior during spermatogenesis is considered in several species ofDrosophila. The material is not favorable for detailed study. Perhaps the most favorable species isD. funebris; but since the genetic behavior of this species is not well known, others are used for the main account, supplemented by observations on funebris.

• 2. Homologous chromosomes undergo synapsis in the telophase of the last spermatogonial division, as in other Diptera, and apparently remain in intimate association during the growth stages.

• 3. The sex chromosomes remain relatively condensed and attached to or incorporated in the nucleolus, which shows wide divergence in size and appearance in different species.

• 4. The autosomes become diffuse at an early growth stage and are so indistinct that no account can be given of their detailed behavior. InD. funebris they form a granulated canopy or net-like cap at one side of the nucleus and arise in prophase by the condensation of this and coalescence of the granules.

• 5. No indications of definite threads such as seen during the leptotene and early diplotene stages of many insects and other animals are found here.

• 6. It is difficult to attach any positive significance to the chromosome behavior so far as the question of genetic crossing over is concerned.

• 7. The enormous growth of the spermatocytes brings into prominence many interesting cytoplasmic structures which have been noted here but not really studied, although they merit careful observation.

• 8. The nuclear wall remains intact at both divisions, permitting a distinction between cytoplasmic and nuclear components.

• 9. In some species numerous non-chromosomal bodies (fragments of nucleolus, cytoplasmic granules, etc.) stain deeply and sometimes bear a slight superficial resemblance to chromosomes, at the first division.

• 10. Both maturation divisions are normal as far as chromosome distribution is concerned — the first division being reductional so far as known (e. g. where dimorphic sex chromosomes are found).

• 11. At the first division the chromosomes tend to be drawn out and irregular in contour, especially when long and V-shaped. And they do not, as a rule, form a flat metaphase plate, save inD. obscura and its relatives. Figures are consequently best analysed in side view.

• 12. The second division is more nearly of the ordinary type and presents no special peculiarities.

• 13. The behavior of the sex-chromosomes in the first spermatocyte prophase is such as to permit their identification on this basis alone.

• 14. The evidence indicates that Y remains relatively dense and compact while X behaves more like the autosomes. This leads to the view that Y may remain relatively inert or passive while X is more active during the growth period.

• 15. X and Y appear to be connected only for a short region, or regions; from which it is inferred that they many be similar in constitution in these regions only.