124
The Biology of Sea Turtles, Vol. II
4.4.2 ESTROGEN HYPOTHESIS
The physiological cascade underlying TSD is not well understood, but the leading
hypothesis is that female-producing incubation temperatures stimulate the production
of estrogen by the gonad, and the estrogen then stimulates the gonad to differentiate
into an ovary. 110 This hypothesis is supported by a number of findings. Numerous
studies of turtles have shown that exogenous estrogen injected into or applied topically
to eggs can cause the production of females from eggs incubated at male-producing
temperatures. 45 This includes a study of the olive ridley in which females were
produced by treating eggs with estrogen. 103 Furthermore, treatment of turtle eggs with
aromatase inhibitors (aromatase is the enzyme that produces estrogen from androgens) results in the masculinization of gonads. 113–116 In the European pond turtle,
Emys orbicularis, higher estrogen levels and higher aromatase enzyme activity occur
in the developing ovaries in comparison to the testes. 117,118 A study in the leatherback
sea turtle also revealed higher aromatase enzyme activity in developing ovaries. 119 In
a study of the diamondback terrapin, Malaclemys terrapin, higher levels of aromatase
mRNA were recorded at female-producing temperatures, in comparison to maleproducing temperatures, during the thermosensitive period of TSD. 120 Collectively,
the findings from these studies support the hypothesis that estrogen production may
play a pivotal role in the sex determination cascade of reptiles with TSD.
Although there is strong support for the involvement of estrogen in TSD, the
results from studies of several reptiles do not support this hypothesis. Studies of the
olive ridley sea turtle; 121 the red-eared slider turtle, Trachemys scripta; 122–124 the
saltwater crocodile, Crocodylus porosus; 125,126 and the American alligator, Alligator
mississippiensis 127,128 did not detect female-specific elevations in estrogen levels,
aromatase levels, and/or aromatase enzyme activity in the gonads during the thermosensitive period of TSD. However, some of these studies did detect elevations
after the thermosensitive period. Thus, although the estrogen hypothesis has substantial support, other studies suggest that elevated estrogen levels may be only a
downstream event that occurs after sex determination in the ovary.
Alternatively, some findings suggest that the brain (rather than the gonads) may
be a source of estrogen production during early development. 120,121 Consistent with
the hypothesis of brain involvement in TSD, a study of the olive ridley indicates
that the nervous system innervates the gonads prior to their sexual differentiation. 129
4.4.3 GENETICS OF TSD
In contrast to birds and mammals, heteromorphic sex chromosomes have not been
identified in reptiles with TSD, including the sea turtles. 84 In fact, it has been
hypothesized that the XX/XY sex-determining system in mammals and the ZZ/ZW
system in birds have evolved from autosomes of ancestral vertebrates with TSD. 130
A number of genes have been evaluated as potential factors in the sex determination–gonadal differentiation cascade of TSD in reptiles. The testis-determining gene
in mammals (i.e., sex-determining region Y [SRY]) has not been detected in reptiles
or birds, and is believed to have evolved well after the mammalian ancestors diverged
1123 book.book Page 124 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
4.4.2 ESTROGEN HYPOTHESIS
The physiological cascade underlying TSD is not well understood, but the leading
hypothesis is that female-producing incubation temperatures stimulate the production
of estrogen by the gonad, and the estrogen then stimulates the gonad to differentiate
into an ovary. 110 This hypothesis is supported by a number of findings. Numerous
studies of turtles have shown that exogenous estrogen injected into or applied topically
to eggs can cause the production of females from eggs incubated at male-producing
temperatures. 45 This includes a study of the olive ridley in which females were
produced by treating eggs with estrogen. 103 Furthermore, treatment of turtle eggs with
aromatase inhibitors (aromatase is the enzyme that produces estrogen from androgens) results in the masculinization of gonads. 113–116 In the European pond turtle,
Emys orbicularis, higher estrogen levels and higher aromatase enzyme activity occur
in the developing ovaries in comparison to the testes. 117,118 A study in the leatherback
sea turtle also revealed higher aromatase enzyme activity in developing ovaries. 119 In
a study of the diamondback terrapin, Malaclemys terrapin, higher levels of aromatase
mRNA were recorded at female-producing temperatures, in comparison to maleproducing temperatures, during the thermosensitive period of TSD. 120 Collectively,
the findings from these studies support the hypothesis that estrogen production may
play a pivotal role in the sex determination cascade of reptiles with TSD.
Although there is strong support for the involvement of estrogen in TSD, the
results from studies of several reptiles do not support this hypothesis. Studies of the
olive ridley sea turtle; 121 the red-eared slider turtle, Trachemys scripta; 122–124 the
saltwater crocodile, Crocodylus porosus; 125,126 and the American alligator, Alligator
mississippiensis 127,128 did not detect female-specific elevations in estrogen levels,
aromatase levels, and/or aromatase enzyme activity in the gonads during the thermosensitive period of TSD. However, some of these studies did detect elevations
after the thermosensitive period. Thus, although the estrogen hypothesis has substantial support, other studies suggest that elevated estrogen levels may be only a
downstream event that occurs after sex determination in the ovary.
Alternatively, some findings suggest that the brain (rather than the gonads) may
be a source of estrogen production during early development. 120,121 Consistent with
the hypothesis of brain involvement in TSD, a study of the olive ridley indicates
that the nervous system innervates the gonads prior to their sexual differentiation. 129
4.4.3 GENETICS OF TSD
In contrast to birds and mammals, heteromorphic sex chromosomes have not been
identified in reptiles with TSD, including the sea turtles. 84 In fact, it has been
hypothesized that the XX/XY sex-determining system in mammals and the ZZ/ZW
system in birds have evolved from autosomes of ancestral vertebrates with TSD. 130
A number of genes have been evaluated as potential factors in the sex determination–gonadal differentiation cascade of TSD in reptiles. The testis-determining gene
in mammals (i.e., sex-determining region Y [SRY]) has not been detected in reptiles
or birds, and is believed to have evolved well after the mammalian ancestors diverged
1123 book.book Page 124 Monday, November 11, 2002 11:11 AM