The evolutionary genetic studies of more than ten species or subspecies belonging to the D. nasuta subgroup lead the following conclusions :Morphological differentiation has been observed qualitatively as well as quantitatively, but closely related species could not be identified by means of the techniques of external morphology.By Judging from the allozyme analyses, the local populations of D. nasuta were genetically similar in two Esterase loci. The Chiangmai population was differentiated from other allopatric populations of D. sulfurigaster albostrigata. The pattern of the speciation process in the D. nasuta subgroup was discussed.

Metaphase chromosome configurations of 23 species and subspecies belonging to five subgroups of the immigrans species group of Drosophila were examined by the Giemsa staining method, from the point of view of the phylogenetic relationships in this species group.New to science are the descriptions of the karyotypes of five species, Taxon-C, Taxon-F, D, nivei f rons,, D. quadrilineata and D, neohypocausta which are 2n=8, 2R+1V+1D; 2n=8, 2R+1V+1D; 2n=8, 2R+1V+1D; 2n =12, 4R+ 1V+ 1D; and 2n = 6, 2R+ 1V, respectively.The karyotypes of the other species were also examined and compared with earlier descriptions. Some of our observations agreed with earlier ones but the remainder showed some major or minor differences from the previous reports.The basic karyotype of the species belonging to the immigrans species group was 2n=8, 2R+1V+1D (or short rod). The karyotypes of D. annul ipes and D. quadrilineata of the D. quadrilineata subgroup, of D, argentostriata and D, silvistriata of the D, lineosa subgroup and of D, neohypocausta of the D. hypocausta subgroup differed fundamentally from the basic type of the immigrans species group. These five species might be somewhat remote from the other species of the immigrans group.Some species exhibited intraspecific variations among the forms of the Y chromosome and Chromosome 4, with additional heterochromatin.In the D. nasuta subgroup, it seemed that fusions were important factors in their evolution. D, albomicans is the most advanced species, and from considerations of morphological similarities, results of hybridization tests, karyotypes, and geographical distribution, D, nasuta is the most plausible species from which D, albomicans originated. D, albomicans collected from the Chiangmai population in Thailand had supernumerary chromosomes, the most peculiar phenomenon among the karyotype variations of the immigrans species group. These chromosomes may be maintained consistently in this population.

Phylogenetic relationships within the Drosophila sulfurigaster species-complex, which belongs to the D. nasuta subgroup, were investigated on the basis of chromosomal constitution and morphology. D. pulaua is thought to be the most ancestral species, from which D. s. sulfurigaster and D. s. bilimbata derived in one branch and D. s. albostrigata and D. s. neonasuta in another branch.