Zoological Studies

Vol. 50 No. 3, 2011

Early-Stage Evolution of the Neo-Y Chromosome in Drosophila albomicans

Chia-Hao Cheng1, Ching-Ho Chang2, and Hwei-yu Chang1,3,*

1Department of Entomology, National Taiwan Univ., Taipei 106, Taiwan
2
Institute of Ecology and Evolutionary Biology, National Taiwan Univ., Taipei 106, Taiwan
3Biodiversity Research Center, Academia Sinica, Nankang, Taipei 115, Taiwan

Chia-Hao Cheng, Ching-Ho Chang, and Hwei-yu Chang (2011) Numerous theories have specified that an originally autosomal neo-Y chromosome arm is expected to undergo degenerative evolution. Neosex chromosomes of Drosophila albomicans originated from 2 Robertsonian translocation events, one for X and the other for Y, between ancestral Drosophila sex chromosomes and a pair of autosomes homologous to the 3rd chromosomes of its sibling species D. nasuta. Since the neo-sex chromosome in D. albomicans is still evolutionarily young, we used genetic approaches to reveal changes in the entire neo-Y chromosome. Non-disjunction is an indicator used to investigate differences between homologous chromosomes. In this study, we first confirmed that no male recombination had occurred in hybrid males of these 2 sibling species. With the aid of molecular marker genotyping and direct karyotyping of aneuploid offspring produced through specially designed crosses and backcrosses of fertile hybrids, we found that the non-disjunction rate was significantly higher in hybrid males with the neo-Y chromosome than in hybrids without it. The high nondisjunction rate made it possible to generate 3,X,X/neo-Y F2 females and X,neo-Y/neo-Y F3 male offspring which can reveal recessive effects of the homozygous 3rd chromosome arm. Results of this aneuploid study revealed severe recessive inviability of the neo-Y chromosome. Our results further suggested that increased non-disjunction in hybrid males with the neo-Y chromosome is likely due to changes that occurred on the Y arm, whereas recessive deleterious alleles might be located on the 3rd arm of the neo-Y chromosome. Taken together, the elevated non-disjunction rate and severe recessive inviability revealed significant changes in the neo-Y chromosome at this early stage of chromosome evolution in D. albomicans.

Key words: Meiosis, Neo-sex chromosome, Non-disjunction, Recessive deleterious allele.

*Correspondence: Chia-Hao Cheng and Ching-Ho Chang contribute equally to this work. E-mail:hwei@ntu.edu.tw