CHROMOSOME VARIATION IN DROSOPHILA SPECIES OF THE MULLERI COMPLEX.

Drosophila species in mulleri complex show five rod-shaped and one pair of dot-shaped chromosome. The sex-chromosomes represent the largest pair in the female, but are heteromorphic in the male, the Y-chromosome being shorter than the X-chromosome. The purpose of the research presented here is to determine whether chromosomes in the mulleri cluster species are longer than in the mojavensis cluster. The length of X-, Y-chromosomes and the rod-like autosomes were compared among the ten species studied. All the rod-like chromosomes were measured in 30 or more brain cells in each of the ten species. The length of the X-chromosomes were measured in hybrid female larvae. Also the metaphase chromosomes were stained for heterochromatin. The results of this study do not support the division of the mulleri complex into the two clusters. This study shows that the X-chromosome in mulleri complex can be divided into three categories; the long X-chromosome found in D.sp.A, D.aldrichi and D.wheeleri; the intermediate or medium X-chromosome found in D.mojavensis, D.mulleri, D.sp.S, and D.sp.S-5; and the short X-chromosome found in D.arizonensis, D.mayaguana, and D.sp.N. The intermediate and the short X-chromosome groups represent species from the two clusters. The Y-chromosome appears to be most variable of all. Based on overall chromosome lengths the ten species can be placed into three groups; (1) D.sp.A, D.aldrichi, D.wheeleri, D.sp.S, and D.sp.S-5 in the long chromosome group, (2) D.mojavensis and D.mulleri in the medium chromosome group and (3) D.arizonensis, D.sp.N and D.mayaguana in the short chromosome group. The differences in chromosome length seem to be due to heterochromatin. The results seem to suggest that the ancestral species had the mulleri-mojavensis chromosome length (i.e. medium) and the mulleri gene arrangement. The chromosomes of the species in the long chromosome group are a result of addition of heterochromatin. However the amount of heterochromatin gained varies from chromosome to chromosome within species and also from species to species among corresponding chromosomes. The sex chromosomes in the short chromosome group seem to have become shorter due to loss of heterochromatin while the autosomes generally remained unchanged. Heterochromatin seems to play a significant role in crosscompatibility among these species