The D. nasuta subgroup of the immigrans group, subgenus Drosophila, is comprised of nine species. Six species of the D. nasuta subgroup are investigated in the present study. They are D. albomicans, D. kepulauana, D. kohkoa, D. nasuta, D. pulaua, and D. sulfurigaster albostrigata. Two other species of the D. immigrans group, D. paranasuta and D. rubra, are also included in the study.
Females of the D. nasuta species complex are indistinguishable from their external morphology. However the males can be differentiated into phenotypic groups based on markings on the frons and pleura. D. albomicans, D. nasuta and D. kepulauana are regarded as sibling species.
The D. nasuta subgroup has a wide geographic distribution which covers the east Ethiopian, Oriental (including the Pacific Ocean islands) and north Australian regions. The species are concentrated in Southeast Asia.
The salivary chromosomes of all species show four autosomal arms and one sex chromosome arm. The chromosome pattern of species of the nasuta complex can be easily homologised. Inversion polymorphism was detected in three species of the nasuta complex. Although the greatest number of different gene arrangements was detected in D.s. albostrigata, mean inversion heterozygosity calculated is highest for D. albomicans, closely followed by D. kohkoa. D. kohkoa showed non-random distribution of inversions. Most inversions detected in this species occurred on the proximal region of chromosome III. Two inversions in D. kohkoa showed non-random association. Three associated complex inversions on chromosome III in D. paranasuta are common and occur with high frequency. They appear to have high adaptive value. Flexible inversion polymorphism in D.s. albostrigata is illustrated by response to geographic variation for four common inversions and by response to seasonal or cyclic changes for three of these. Inversions unique to different populations and those detected in fairly constant frequencies are adaptive. Common inversions are useful in elucidating phylogenetic relationships between species.
Fourteen fixed inversions and a deletion have been involved in the genetic divergence of species of the nasuta complex from the polytenes of a common ancestor. Most of the fixed, inversions are on chromosomes I(X) and III. The phylogenies of individual chromosomes of the species are constructed.
The basic metaphase chromosome configuration of the D. nasuta subgroup consists of 2 pairs of rods, a pair of V's and a pair of dots. Most cases of karyotype variation encountered involved the addition of heterochromatin to one or more chromosomes. At least 2 types of Y-chromosome occur in D.s. albostrigata. D. kohkoa showed 3 types of Y chromosome and 2 types of chromosome IV. The chromosome number of D. albomicans is reduced to 3 pairs owing to centric fusion involving chromosomes III and the sex chromosomes. Extra centromeric heterochromatin was found in chromosomes of D. nasuta and D. kohkoa. Structural changes involved in metaphase karyotypes of the species complex are discussed.
Incipient speciation in D.s. albostrigata, D. albomicans and D. kohkoa is limited. The population of D.s. albostrigata from Chiang Mai shows signs of incipient speciation.
Sexual isolation between any two species of the nasuta complex is high. According to crossability, D. albomicans is closely related to D. nasuta and to D. kepulauana, whereas D.s. albostrigata is densely related to D. pulaua. Relationships are also based on extent of F1 sex-ratio distortion and sterility of hybrids. Excess of male hybrids over females from several crosses proved to be exceptions to Haldane's law. The causes of sterility of male hybrids are discussed. D. paranasuta and D. rubra show extremely high sexual isolation from one another and from members of the D. nasuta species complex.
Ethological isolation is very important in preventing gene exchange between species. Hybrid males are generally ineffective in courting mating partners. Evidence of hybridization in Nature between D. albomicans and D.s. albostrigata was found.