Chromosome behavior and cell lineage in triploid and mosaic salivary glands of species hybrids in Sciara

Summary

A study of the salivary glands of triploid hybrids between females of Sciara ocellaris and males of Sciara reynoldsi throws light on several problems concerning chromosome behavior, species relationships and embryonic development. The specimens studied possess two sets of ocellaris chromosomes and one set of reynoldsi chromosomes. Three types of conditions are represented; 2X∶3A; 3X∶3A; and a mosaic possessing cells of both kinds.

Heterozygosity of chromosome band pattern in the hybrids shows that the ocellaris homologs are not true sisters and that the sperm probably united with a diploid fusion nucleus composed of two polar bodies and not with the oötid nucleus. This indicates strongly that in a previously described diploid hybrid mosaic some cells developed parthenogenetically from such a fusion nucleus, while others were biparental in origin. The evidence supports the view that the initial influences or reactions of the foreign reynoldsi sperm differ from those of the ocellaris sperm immediately after entry into the egg, but that then and later the differences are relatively slight, although numerous. This agrees with our earlier evidence from the hybrids in indicating that the two parental species are separated genetically, as well as in chromosome band pattern, by many small differences.

Species-specific characteristics of chromosome behavior are described which retain their specificity in the hybrids.

The 2X∶3A cells lack a reynoldsi X. The double ocellaris X in such cells (including those in the mosaic) exhibits the peculiar features manifest by the single X in ordinary diploid XO males in the pure species (and in many other Diptera). It is pale and diffuse in structural appearance, and throughout its entire length is uniformly much greater in diameter than it is in the 3X∶3A cells. The possible significance of this behavior is discussed in relation to problems of sex determination (e. g., Bridges' “ratio theory”) and those concerning time of gene action. The structural resemblance to the “puffs” and “bulbs” found in salivary gland chromosomes is striking and raises the question as to how far morphological characteristics of chromosomes can be used as criteria of gene activity. The double X in the 2X cells of the mosaic, surrounded by 3X cells, exhibits the same peculiarities as those in a pure 2X∶3A „male-type“ specimen, indicating that we are dealing with intracellular, not intercellular phenomena.

Conditions in the mosaic gland support the evidence from the diploid hybrids (some described here for the first time) in indicating that the salivary gland is formed by differentiation of cells which happen to lie in the proper region, regardless of their ancestry. The triploid mosaic specimen (and also a diploid mosaic) is presumed to be a female in which the normal male-producing type of chromosome elimination occurred in one or more mitotic divisions during cleavage or later.