Zoological Studies

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