Zoological Studies

Vol. 36 No. 3, 1997

Karyotype Polymorphism in Hybrid Populations of Drosophila nasuta and D. albomicans

Yuh-Chyn Yu1, Fei-Jann Lin2 and Hwei-yu Chang1,2,*

1Department of Plant Pathology and Entomology, National Taiwan University, Taipei, Taiwan 106, R.O.C.
2lnstitute of Zoology, Academia Sinica, Taipei, Taiwan 115, R.O.C.

Yuh-Chyn Yu, Fei-Jann Lin  and Hwei-yu Chang (1997) Karyotype polymorphism in hybrid populations  of Drosophila nasuta and D. albomicans. Zoological Studies 36(3): 251-259. Drosophila albomicans (2n = 6) and D. nasuta (2n = 8) are 2 sibling species with indistinguishable morphology; but distinct karyotypes.  The former is distributed from Japan through Taiwan to Thailand, and the latter from India to the east coast of Africa. Although they are completely crossable in the laboratory, no polymorphic karyotype has ever been found in any one population of these two species in nature. In this report, 2 questions are addressed. First, does karyotype polymorphism exist in hybrid populations of these 2 species after long-term cultivation? Second, do the chromosomes originating from the same species interact with each other during their trans­ mission from generation to generation in hybrid offspring? To answer these questions, 22 interspecific hybrid strains were established. Each strain was initiated by crossing one D. nasuta male and one D. albomicans female. Their hybrid offsprings were maintained with the non-overlapping generation method for 2.5 y (i.e., 45 generations) and then subjected to karyotype analysis. The origins of the 3rd, 4th, and sex chromosomes in the hybrid offspring were identified by larval karyotyping, while an esterase electrophoretic technique was applied for distinguishing the origin of the 2nd chromosome. Among these hybrid offsprings, 92% of the 4th chromosomes and 38% of the 2nd chromosomes are derived from D. albomicans. The frequencies of the 4th and the 2nd chromosomes are significantly different from their initial values (50%), therefore, we may conclude that chromosome 4 is leaning to D. albomicans, and chromosome 2 to D. nasuta in hybrid strains. On the contrary, 72.2% of the 3rd and X chromosomes (70.3% in male and 72.8% in female) are derived from D. albomicans, which is close to the initial value (66.7%). Our data clearly reveal that karyotype poly­ morphism exists in hybrid offspring after long-term cultivation, and the evolution of the 3rd and X chromo­ somes is apparently not associated with other chromosomes.

Key words: Hybridization, Karyotype selection.

*Correspondence: Department of Plant Pathology and Entomology, National Taiwan University, Taipei, Taiwan 106, R.O.C.