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Vol. 47 No. 4, 2008
Evolutionary Changes in a Y-Like Chromosome in Hybrids of Drosophila albomicans and D. nasuta
Hwei-yu Chang1,2,* and Ting-Yi Kung1
1Department of Entomology, National Taiwan University, Taipei 106, Taiwan
2Biodiversity Research Center, Academia Sinica, Nankang, Taipei 115, Taiwan
Hwei-yu Chang and Ting-Yi Kung (2008) The
initial steps of Y or even neo-Y chromosome evolution remain obscure,
because it is difficult to study using contemporary species. In
this report, we established 3 hybrid strains with Y-like chromosomes to
mimic the neo-Y chromosome of Drosophila albomicans by crossing a D. albomicans female to a D. nasuta male. In hybrid strains, a specific 3rd chromosome from D. nasuta
became a non-recombining, paternally inherited Y-like chromosome
associated but not fused with the Y chromosome. Through
backcrossing, we extracted Y-like chromosomes from hybridstrain males,
and then successfully established F3
offspring with a pair of homozygous Y-like chromosomes. The
presence of paired homozygous Y-like chromosomes in an organism is
essential to demonstrate the level of recessive degeneration of Y-like
chromosomes. Our results showed that higher recessive
degeneration was observed in old Y-like chromosomes (~280 generations)
compared to young ones (~20 generations). The small number of F1 offspring during chromosome extraction indicated incompatibility between hybrid-strain males and D. nasuta females. For F2
flies carrying a Y-like chromosome, two of the 8 extractions from the
old hybrid strain showed fewer males than females. This
observation supports the hypothetical dependence of this Y-like
chromosome in males on its intra-strain homologue. Furthermore,
one of 3 extractions from the young hybrid strains weakly supported the
presence of a sexually antagonistic effect of a Y-like chromosome
favoring males. Our study revealed that heterogeneous
incompatibility with parental species emerged in the hybrid
strains. Although the supporting data were not very strong, they
suggested the appearance of sexual antagonism at an early stage (around
the 20th generation) and the formation of inter-chromosome dependence
at a later stage (around the 280th generation) of neo-Y chromosome
evolution. Recessive degeneration on a Y-like chromosome could
also be observed within 280 generations, instead of tens of thousands
of years. This study indicates that Y-like chromosomes in hybrid
strains are a unique experimental model to investigate the evolution of
paternally inherited chromosomes without recombination.
Key words: Degeneration, Hybridization, Sex chromosome interaction, Sexual antagonism.
*Correspondence: E-mail:hwei@ntu.edu.tw
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