ACQUISITION AND AMPLIFICATION OF A TESTIS-EXPRESSED AUTOSOMAL GENE, SSL, BY THE DROSOPHILA Y-CHROMOSOME

The acquisition of autosomal fertility genes has been proposed to be a n important process in human Y chromosome evolution, For example, the Y-linked fertility factor DAZ (Deleted in Azoospermia) appears to have arisen after the transposition and tandem amplification of the autoso mal DAZH gene, The Drosophila melanogaster Y chromosome contains tande mly repeated Su(Ste) units that are thought to affect male fertility a s suppressors of the homologous X-linked Stellate repeats, Here we rep ort the detection of a testis-expressed autosomal gene, SSL [Su(Ste)-l ike], that appears to be an ancestor of the Y-linked Su(Ste) units, SS L encodes a casein kinase 2 (CK2) beta-subunit-like protein, Its putat ive ORF shares extensive (45%) homology with the genuine beta-subunit of CK2 and retains the conserved C-terminal and Glu/Asp-rich domains t hat are essential for CK2 holoenzyme regulation, SSL maps within regio n 60D1-2 of D. melanogaster and D. simulans polytene chromosomes. We p resent evidence that SSL was derived from the genuine beta CK2 gene by reverse transcription, This event resulted in the loss of the first t hree introns in the coding region of the SSL ancestor gene. Evolutiona ry analysis indicates that,SSL has evolved under selective pressure at the translational level, Its sequence, especially in the 3' region, i s much closer to the Y-linked Su (Ste) tandem repeats than to the beta CK2 gene, These results suggest that the acquisition of testis-specif ic autosomal genes may be important for the evolution of Drosophila as well as human Y chromosomes.