Zoological Studies

Vol. 47 No. 3, 2008

Cooption of Neo-X and Neo-Y Chromosomes in Drosophila albomicans

Sung-Han Lin1, Yu-Yang Huang1, and Hwei-yu Chang1,2,*

1Department of Entomology, National Taiwan University, Taipei 106, Taiwan
2Research Center for Biodiversity, Academia Sinica, Nankang, Taipei 115, Taiwan

Sung-Han Lin, Yu-Yang Huang, and Hwei-yu Chang (2008) Drosophila nasuta has 8 chromosomes, whereas its sibling species D. albomicans has 6 because a pair of neo-sex chromosomes has evolved through fusion events in < 0.5 Ma.  It remains unclear how the newly joined sex chromosomes differentiated from their homologous 3rd autosome of D. nasuta.  The body size of F1 hybrid males from a cross between D. albomicans males and D. nasuta females was significantly smaller than those of the parental species, but an intermediate size was obtained from the reciprocal cross.  There are 2 possible explanations: (1) the ancestral mitochondria of D. nasuta are not compatible in the derived D. albomicans nuclear environment; and (2) the neo-Y chromosome cannot work well with the homologous ancestral 3rd autosome of D. nasuta.  In the present study, experiments were conducted to exclude the possible involvement of mitochondrial incompatibility.  We established 5 sets of coupled highly inbred D. albomicans strains and another 5 sets of D. nasuta strains, and subsequently examined their reproductive ability and the body size of their progeny.  Each set of coupled strains had nearly the same homogeneous nuclear genome but had different (self vs. non-self) mitochondria.  These coupled strains showed indistinguishable reproductive ability and body size between them, indicating that mitochondrial compatibility was not the major cause.  Alternatively, our cross experiments demonstrated that the body size of the offspring reverted to normal when the neo-X and neo-Y relationships were restored by backcrossing the small F1 males to D. albomicans females.  However, the body size remained small when F1 males were backcrossed to D. nasuta.  The backcross results support the 2nd explanation, thus implying that after coevolution of the neo-sex chromosomes, the neo-Y may depend on the presence of the neo-X chromosome in males, but not vice versa.

Key words: Coevolution, Drosophila nasuta, Hybrids, Sex chromosome.

*Correspondence: E-mail:hwei@ntu.edu.tw