while the remainder develop varying degrees of 
ovotestes. This XY sex reversal has been postulated 
to be the result of incompatible interactions be- 
tween the domesticus Tdy and other autosomal 
and/or X-linked testis-determining genes. Results 
from these studies indicated that the Zfy genes are 
only expressed in the testes but not in ovotestes or 
ovaries from the same individuals, thus confirming 
the linkage of Zfy gene expression to spermatogen- 
esis. In addition, the data demonstrate that the Zfy 
genes are silent in adult XY ovarian environment. 
E. Possible involvement of the 2£x gene in sex de- 
termination. Sex reversal occurs naturally in the 
Scandinavian rodent, the wood lemming, due to 
mutations in a variant X* chromosome, such that 
X*Y individuals develop into fertile females. Chro- 
mosome mapping of homologous sequences {Zfy 
and Zfx) to the human ZFY gene has localized the 
Zfx sequence on bands pll-12 of both the normal 
X and variant X* chromosomes, at or proximal to a 
presumed breakpoint (pi 2) involved in the gen- 
eration of X* from X. Molecular differences be- 
tween the Zfx and Zfx* sequences are readily 
detected by Southern hybridization, suggesting 
that the Zfx gene might have been altered by 
such genetic rearrangements. Furthermore, at least 
15 copies of the Zfy sequences are distributed 
along the entire short arm of the Y chromosome. 
Multiple Zfy sequences have also been dem- 
onstrated on the Y chromosome of a South Ameri- 
can rodent, Akodon azarae, in which XY sex rever- 
sal also occurs naturally in both the wild and 
laboratory stocks. The multiplicity of the Zfy se- 
quences hence seems to be a common factor in 
these two species with XY females. If the Zfy 
gene(s) is indeed the testis-determining gene, the 
present observations suggest 1) the Zfx gene prob- 
ably interacts with the Zfy gene(s) in testis determi- 
nation, 2) the mutated Zfx* gene becomes incom- 
patible in such an interaction, and 3) the multiple 
Zfy sequences may potentiate the sex reversal in 
the X*Y animals. 
II. Characterization of the Male-Enhanced Antigen 
{MEA) and Related Genes. 
The MEA gene was initially isolated with a pool of 
specific antisera against the serological H-Y antigen. 
Molecular characterization established that the 
MEA gene is phylogenetically conserved and ex- 
pressed at high levels in adult testes, particularly in 
round spermatids. Two linked genes. Gene A and B, 
have been identified within 60 kb of human geno- 
mic DNA. The structural genes for both MEA and 
Gene A are present within a 40 kb human insert of 
a recombinant cosmid, CosMEA-A. The promoters 
of both MEA and Gene A are arranged in a head-to- 
head manner, such that their putative start sites are 
only separated by 450 bp of GC-rich sequence. This 
common promoter contains no consensus TATA or 
CCAAT boxes and is capable of mediating the 
expression of a reporter gene, chloramphenicol 
acetyltransferase (CAT), in a bidirectional manner. 
Preliminary analysis has identified a small seg- 
ment of —100 bp, defined by two putative DNase I- 
hypersensitive sites, within this promoter sequence 
that interacts specifically with testicular protein ex- 
tracts in a gel-retardation assay These results sug- 
gest the possible existence of tissue-specific fac- 
tor(s) regulating the expression of both MEA and 
Gene A in the testis. 
Five transgenic mice harboring the entire Cos- 
MEA-A cosmid were generated. The human MEA 
and Gene A in the offspring of these founders were 
expressed in a tissue-specific manner, similar to 
those of the endogenous mouse genes. These re- 
sults suggest that the 450 bp promoter segment 
may be important in mediating the tissue-specific 
expression of the human MEA gene and Gene A in 
these transgenic mice and support the hypothesis 
that the MEA and linked genes serve an important 
role(s) in mammalian spermatogenesis. 
Dr. Lau is also Assistant Professor in the Depart- 
ments of Physiology and Medicine at the University 
of California at San Francisco. 
PUBLICATIONS 
Books and Chapters of Books 
Deschepper, C.E, Mellon, S.H., Reudehuber, T.L., Gardner, D.G., Jen, J. , and Lau, Y-F 1988. In situ hybridiza- 
tion histochemistry on mRNA in endocrine tissues. In Endocrine Genes: Analytical Methods, Experimen- 
tal Approaches, and Selected Systems (Lau, Y-E, Ed.). New York: Oxford University Press, pp 31-41. 
Lau, Y-F. 1989- Are male-enhanced antigen and serological H-Y antigen the same? In Evolutionary Mecha- 
nisms in Sex Determination (Wachtel, S.S., Ed.). New York: CRC, pp 151-157. 
Continued 
244 
