On the chromosomes of the geometrid moth genus Cidaria

Abstract

1. The oogenesis of 44 (48) species of the Geometrid moth genus Cidaria has been studied. Spermatogenesis has been studied in two species only. 2. The most common (haploid) number among the Cidaria species so far investigated is 30, which is likewise the number second in commonness among the Lepidoptera as a whole. Most (33, or 75 percent) of the species have a chromosome number between 29 and 32, like most of the other groups of Lepidoptera. 3. No species has more than 32 chromosomes, whereas eleven have less than 29. The smaller chromosome numbers found are 28, 27, 25, 20, 19, 17, 13 and 12. 4. The great differences in chromosome numbers between closely related species are of interest. Such discrepancies are found in the subgenera Thera (variata and obeliscata 13, firmata 19, cognata 20 and juniperata 30), Lampropteryx (minna 17 and suffumata 32), and Hydrelia (testaceata 13 and flammeolaria 30). 5. The chromosomes are clearly bigger in the species with a low chromosome number than in those with a high one. In the chromosome sets of most Cidaria species studied the chromosomes are approximately equal in size. 6. Photometric measurements revealed that the DNA-content of closely related species is almost equal, in spite of great differences in chromosome numbers. This is also true of the subgenera investigated. This indicates that one chromosome in a species with a low number of chromosomes corresponds to two or more chromosomes of another one with a high chromosome number. 7. The discrepancies in the chromosome numbers among Lepidoptera have not arisen from polyploidy or differences in the degree of polyteny. They are due to “fragmentations” or “fusions”, which are rendered easier by the diffuse kinetochore. 8. It is obvious that in animals with a diffuse kinetochore some mechanism, possibly the location of sex-determining genes, reduces the role of chromosomal rearrangements in chromosomal evolution from what the diffuse kinetochore otherwise would allow. 9. Contrary to earlier assumptions of the author, chiasmata are not formed in the bivalents during oogenesis in the Lepidoptera. This is especially evident in preparations stained with Feulgen, when the elimination chromatin contained by the bivalents in the female remains unstained.