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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
2Institute 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
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