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Center for Reproduction of Endangered Species, San Diego, California 92112
Mid-Pacific Genetics, Inc., Honolulu, Hawaii 96814
“Nothing in biology makes sense except in
the light of evolution.”
This statement was the title of a lecture presented to an audience of biology teachers in 1972,
urging a vigorous stand against the resurgent creationist movement (Dobzhansky, ’73)—a movement that unfortunately is still very much with
us today. Dobzhansky gave a number of wellknown examples in biology for which the only logical explanation was evolution. As biologists, we
too try to understand our research findings using
the logic of evolution. In 1990 the gene on the human and mouse Y chromosome that determines
sex (SRY in humans, Sry in mice) was discovered
(Gubbay et al., ’90; Sinclair et al., ’90), and this
gene is particularly difficult to understand in
evolutionary terms. It occurs only in placental
mammals, exhibits an extraordinary amount of
sequence variability among species (Whitfield
et al., ’93), and may not even be required for sex
determination, as some mammals undergo normal sex differentiation without any trace of SRY
sequence in their genome (Just et al., ’95). Despite an intense research effort in the seven years
since its discovery, its mode of action is still unknown. There is no evidence for an SRY gene in
egg-laying vertebrates, and SRY clearly evolved
from an SRY-like precursor (Graves and Foster,
’95; Graves, ’98), but why and how are still a mystery. Is the evolution of a placenta and viviparity
linked to the evolution of SRY? Why can some
mammals undergo sex differentiation in the absence of SRY? What does SRY do?
This special issue of The Journal of Experimental Zoology is a selection of papers from the First
International Symposium on the Biology of Vertebrate Sex Determination held in Honolulu, Hawaii, April 7–11, 1997. This meeting was organized
to address the process of sex determination from
an evolutionary standpoint. During the past five
years or so, interest in this area of research has
increased, as reflected in the increased number
of international meetings on the topic (Marshall
Graves, ’93; Short and Balaban, ’94). There have
been meetings restricted to sex determination in
reptiles (Lance, ’94) and others covering sex determination in some invertebrates and vertebrates
(Keystone Symposium, 1995). There has been
little interest in amphibians, however, and it has
been more than 20 years since a meeting on sex
determination in fishes has been held (Reinboth,
’70). No recent meeting has included all the vertebrate classes. This symposium was a small (a
total of 62 papers) but extraordinarily successful
meeting with registrants from 17 different countries in attendance. There were five plenary lectures and a series of invited papers that covered
sex determination from fish to humans. What was
soon apparent was that many of the genes now
known to be involved in gonadal development in
mammals (with the exception of SRY) are also involved in gonadal development in egg-laying vertebrates (Lance, ’97). What was also clear was that
the mechanism of sex determination in placental
mammals may be quite different from that in the
egg-laying vertebrates.
It is refreshing for a comparative endocrinologist to see the renewed interest in evolution among
cell and molecular biologists. Papers with such
titles as “Evolution of gene X: Comparison of
mouse and human” are now no longer considered
to address evolution. Today we see papers that
look at genes from invertebrates to humans in a
truly evolutionary manner (Kappen and Ruddle,
’93; Carroll, ’95). It is equally as refreshing to see
the study of sex determination in vertebrates
couched in an evolutionary context, as presented
at this meeting. Perhaps soon we will make sense
of the fascinating process of sex determination in
the “light of evolution.”
Grant sponsors: The March of Dimes Foundation; The Zoological
Society of San Diego; The Queens Medical Center, Honolulu, CIBAGEIGY, Basel, Switzerland; Mid-Pacific Genetics, Inc., Honolulu; and
Eli Lilly, Indianapolis, Indiana.
*Correspondence to: Valentine A. Lance, Center for Reproduction
of Endangered Species, P.O. Box 551, San Diego, CA 92112.
Received 17 February 1998; Accepted 18 February 1998
The organizers especially thank Lisa Morici and
Robbie Berkstresser for secretarial and organizational help.
Dobzhansky, T. (1973) Nothing in biology makes sense except
in the light of evolution. Am. Biol. Teach., 35:125–129.
Carroll, S.B. (1995) Homeotic genes and the evolution of
arthropods and chordates. Nature, 376:479–485.
Graves, J.A.M., and J.W. Foster (1994) Evolution of mammalian sex chromosomes and sex determining genes. Int. Rev.
Cytol., 154:191–259.
Gubbay, J., J. Collignon, P. Koopman, B. Capel, A. Economou,
A. Munsterberg, N. Vivian, P.N. Goodfellow, and R. LovellBadge (1990) A gene mapping to the sex-determining region
of the mouse Y-chromosome is a member of a novel family of
embryonically expressed genes. Nature, 346:245–250.
Just, W., W. Rau, W. Vogel, M. Akhverdian, K. Fredga, J.A.M.
Graves, and E. Lyapunova (1995) Absence of Sry in species
of the vole Elobius. Nature Genet., 11:117–118.
Kappen, C., and F.H. Ruddle (1993) Evolution of a regulatory gene family: HOM/HOX genes. Curr. Opin. Genet. Dev.,
Lance, V.A. (1994) Introduction: Environmental sex determination in reptiles: Patterns and processes. J. Exp. Zool.,
Lance, V.A. (1997) Sex determination in reptiles: An update.
Am. Zool., 37:504–513.
Reinboth, R. (1970) Intersexuality in fishes. Mem. Soc.
Endocrinol., 18:516–543.
Short, R.V. and E. Balaban, eds. (1994) The Difference Between the Sexes. Cambridge University Press, Cambridge.
Sinclair, A.H., P. Berta, M.S. Palmer, J.R. Hawkins, B.L.
Griffiths, M.J. Smith, J.W. Foster, A.M. Frischauf, R.
Lovell-Badge, and P.N. Goodfellow (1990) A gene from
the human sex-determining region encodes a protein
with homology to a conserved DNA-binding motif. Nature (Lond.), 346:240–244.
Whitfield, L.S., R. Lovell-Badge, and P.N. Goodfellow (1993)
Rapid sequence evolution of the mammalian sex-determining gene SRY. Nature, 364:713–715.
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