Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 104

Epigenetic Determination of Sex in Reptiles
All reptiles (or at least reptile species that have no sex chromosomes) display tem-
perature-dependent sex determination (TSD), rather than genetic sex determination.
TSD is observed in many fish, all crocodilians, many turtles, and lizards. Sex in rep-
tiles is determined by the temperature of the egg halfway through embryogenesis
( Crews, 1993 ). When red-eared slider turtle ( Trachemys scripta elegans ) embryos are
incubated at low temperatures, they develop into all-male clutches, and the transi-
tion from an all-male to all-female sex ratio takes place abruptly if the temperature
increases by as little as 1°C ( Crews et al., 2001 ).
In reptiles, the temperature influences sex via the secretion of aromatase in the
brain, the enzyme responsible for the synthesis of estrogens via the aromatiza-
tion of androgens. Aromatase is detected in the gonads only after the temperature-
sensitive embryonic period, suggesting that the secretion of aromatase in the brain
may induce sexual differentiation of gonads in reptiles. The only organ that secretes
aromatase during the temperature-sensitive window in the embryos of the turtle
Malaclemys terrapin in the southern regions of the United States is the embryonic
brain ( Jeyasuria and Place, 1998 ). Larvae of the genotypically female Spanish ribbed
newt, Pleurodeles waltl (Michahelles, 1830), when reared at 32°C (before the forma-
tion of the genital ridge), developed into male newts ( Dournon and Houillon, 1985 ).
Obviously, the temperature cannot directly affect the expression of sex genes—
or any genes at all, for that matter. A correlation is observed to exist between sex
inversing temperatures and the expression of the aromatase, which is responsible
for the synthesis of estrogens and androgens. The fact that the temperature-sensitive
period in turtles occurs before the full development of gonads proves that sex deter-
mination in turtles is not determined by gonads.
Curiously, brain (as opposed to gonadal) determination of sex is also observed
in taxa that are known to have a clear genetically determined sex and sex chromo-
somes. For instance, sexual differentiation in chick embryos starts in the brain as
early as the fourth embryonic day ( Scholz et al., 2003 ) and the genetic sex of brain
influences the sexual differentiation of the animal phenotype ( Agate et al., 2003 ).
A South Korean team found recently that a set of genes are expressed in the brains
of male and female chick embryos on embryonic day 6, “before the influence of
gonadal hormones is felt” ( Lee et al., 2009 ).
A Single Genetic Toolkit, but Breathtaking Diversity of Forms
Looking at the diversity of forms, plants, animals, protozoa fungi, and bacteria, it
is easy to be amazed by the immense diversity of living forms on Earth. Discussing
the common origin of pigeon breeds in On the Origin of Species , Darwin wrote:
“The diversity of the breeds is something astonishing” ( Darwin, 1859 ), yet “all have
descended from the rock-pigeon ( Columba livia )” ( Darwin, 1859, p. 23 ). Since then,
the catalog of living forms has grown greatly. A recent study estimates the number of
extant species on Earth at 8.7 million (±1.3 million), including 7.77 million species