Taxonomic Problems in the Drosophila melanica Species Group (Diptera: Drosophilidae) from Southern China, with Special Reference to Karyotypes and Reproductive Isolation

Abstract

Karyotypes and reproductive isolation were studied in two allopatric populations of Drosophila tsigana, one from Guizhou Province in southern China and the other from Hokkaido in northern Japan, and in one population of a closely related species, D. longiserrata, from Guizhou. In metaphase plates of larval brain cells, both geographic strains of Drosophila tsigana showed 2n=10 chromosomes, with 2 pairs of metacentric (V-shape), 2 pairs of acrocentric (R-shape), and 1 pair of dot-liked (D-shape) chromosomes. Drosophila longiserrata showed the same number, 10 chromosomes, comprising 2V, 1J (sub-metacentric chromosome), 1R, and 1D. X chromosomes of both species were acrocentric, the presumed ancestral form. Premating isolation was complete between D. tsigana and D. longiserrata, and successful mating was also limited in crosses between the two geographic populations of D. tsigana, especially in crosses between Japanese (JP) females and Guizhou (GZ) males. F1 hybrids were obtained only from crosses between GZ females and JP males, and fertilities of both F1 females and males were quite incomplete. The results of morphological observations, karyotypic analyses, and crossing experiments clearly showed that the GZ and JP populations of "D. tsigana" were highly divergent from each other and that each population should be recognized as a biologically valid species. The present morphological observations and chromosomal analyses, together with the original descriptions, strongly suggest that "Guizhou D. tsigana" might be conspecific with D. bisetata Toda, 1988 from Myanmar, and that D. longiserrata might be conspecific with D. afer Tan, Hsu, and Sheng, 1949 from Meitan, Guizhou.

Independent Origins of New Sex-Linked Chromosomes in the melanica and robusta Species Groups of Drosophila

“…A more direct test of the plausibility of chromosomal fission in these groups would require a phylogenetic analysis including additional taxa contained in the melanica-robusta clade and having unfused sex chromosomes–coupled with better resolution of the relationships among D. micromelanica , D. nigromelanica , and the other species in the melanica group. By inference from other studies, the ancestral arrangement of the X in D. longiserata and D. tsigana [59], coupled with the placement of this species pair as a single clade arising from a basal node of the melanica group [49], further suggests fission as an unlikely mechanism for generating the observed karyotypic variability and implies the independent origin of the rearranged sex chromosomes in the robusta and melanica species groups. Preliminary phylogenetic analyses utilizing available sequences of D. moriwakii indicate it is a sister of D. colorata , so unfortunately, its placement does not provide an additional node to inform the history of the chromosomal forms, but it could help resolve the position of these species within the melanica - robusta clade (see below).…”

Revision of Asian species of the Drosophila melanica species group (Diptera: Drosophilidae), with a description of a new species from Vietnam

“…In addition, reproductive isolation between the Guizhou and the Hokkaido populations of D. tsigana is very strong, if not complete (Wang et al . 2006a). Few fertile F 1 progeny were produced only by crosses between Guizhou females and Hokkaido males.…”

“…; Wang et al . 2006a). The Japanese population of this species has been reported under different species names.…”

“…However, Wang et al . (2006a) suggested, based on the results of crossing experiments and comparison of morphology and karyotype with the Hokkaido (Sapporo) population, that the Guizhou population is of a biologically good species distinct from D. tsigana , probably being conspecific with D. bisetata . In addition, Wang et al .…”

“…Complete reproductive isolation between D. longiserrata and both Guizhou and Hokkaido populations of D. tsigana was proved by Wang et al . (2006a). Thus, the species status has not yet been determined for some Asian populations currently called “ D. tsigana ”.…”