Critical Approaches to Sex Determination in Sea Turtles
125
from stem amniotic vertebrates. 130 In fact, SRY has not even been identified in
primitive mammals; i.e., monotreme mammals. 130
SOX9 is an SRY-related gene that appears to have a conserved role in testis
differentiation. 131,132 Studies of the olive ridley indicate that SOX9 is expressed in
gonads that are differentiating as testes, but its expression is downregulated in gonads
developing as ovaries. 133–135 Although these findings suggest that SOX9 may be
involved in testis differentiation, studies of TSD in the alligator indicate that SOX9
expression is a downstream event and is not the testis-determining gene. 136
DMRT-1 is another gene that exhibits elevated expression in the differentiating
testis of mammals 137 and birds, 138 and is required for testis differentiation in mammals. 139 DMRT-1 could potentially be involved in TSD because it has been shown
to be expressed in the testis of the red-eared slider, 140 but it has not been studied in
sea turtles.
Anti-müllerian hormone (AMH, also called müllerian inhibiting substance
[MIS]) is produced by the differentiating testis and causes the müllerian ducts to
degenerate in male vertebrates. 141 AMH has been identified in reptiles with TSD, 136,142
and it is expressed during the thermosensitive period in the alligator. 136 AMH has
not been investigated in sea turtles.
Steroidogenic factor-1 (SF-1) appears to be a master regulator of steroidogenic
genes, and it is required for the development of the gonads, the adrenal glands, and
the ventromedial hypothalamus. 143–147 SF-1 also regulates the AMH gene. 148 SF-1
has been shown to have a sex-specific pattern of expression in mammals 149,150 and
birds. 151 One of the steroidogenic genes regulated by SF-1 is the aromatase gene,
so SF-1 could have a role in TSD if estrogen is involved (see Section 4.4.2). SF-1
has been identified in reptiles with TSD, 111,152 and has been shown to have a sexspecific expression pattern in the alligator and in a freshwater turtle with TSD. 152,
153 SF-1 has not been examined in sea turtles.
The gene producing DAX1 has been implicated in mammalian sex determination
because overexpression of DAX1 is associated with male-to-female sex reversal. 154,
155 DAX1 has been identified in a reptile with TSD (the alligator), but no sex-specific
pattern of expression was detected. 152 DAX1 has not been investigated in sea turtles.
The gene producing Wilms tumor 1 (WT1) is necessary for the proper development
of the kidneys and gonads in mammals. 156 WT1 has also been hypothesized to act
synergistically with SF-1 in regulating AMH, whereas DAX1 antagonizes this synergy. 157 WT1 has been identified in the alligator, but no sex-specific pattern of expression has been detected during TSD. 152 WT1 has not been investigated in sea turtles.
In summary, the genetics of TSD is not well understood, but a number of
potential factors in the sex determination cascade have been identified. Although the
testis-determining gene in mammals (SRY) does not appear to be present in reptiles,
many other genes in the sex determination–sex differentiation cascade are conserved
in amniotic vertebrates, and some could potentially be involved in TSD. In addition,
as the specific functions of these genes are elucidated, the results will provide insight
into the putative involvement of estrogen in TSD. Although only a few studies have
addressed the genetics of sex determination in sea turtles, information from other
reptiles with TSD can act as a template for designing studies that can efficiently
evaluate the potential role of specific genes in the sex determination of sea turtles.
1123 book.book Page 125 Monday, November 11, 2002 11:11 AM
125
from stem amniotic vertebrates. 130 In fact, SRY has not even been identified in
primitive mammals; i.e., monotreme mammals. 130
SOX9 is an SRY-related gene that appears to have a conserved role in testis
differentiation. 131,132 Studies of the olive ridley indicate that SOX9 is expressed in
gonads that are differentiating as testes, but its expression is downregulated in gonads
developing as ovaries. 133–135 Although these findings suggest that SOX9 may be
involved in testis differentiation, studies of TSD in the alligator indicate that SOX9
expression is a downstream event and is not the testis-determining gene. 136
DMRT-1 is another gene that exhibits elevated expression in the differentiating
testis of mammals 137 and birds, 138 and is required for testis differentiation in mammals. 139 DMRT-1 could potentially be involved in TSD because it has been shown
to be expressed in the testis of the red-eared slider, 140 but it has not been studied in
sea turtles.
Anti-müllerian hormone (AMH, also called müllerian inhibiting substance
[MIS]) is produced by the differentiating testis and causes the müllerian ducts to
degenerate in male vertebrates. 141 AMH has been identified in reptiles with TSD, 136,142
and it is expressed during the thermosensitive period in the alligator. 136 AMH has
not been investigated in sea turtles.
Steroidogenic factor-1 (SF-1) appears to be a master regulator of steroidogenic
genes, and it is required for the development of the gonads, the adrenal glands, and
the ventromedial hypothalamus. 143–147 SF-1 also regulates the AMH gene. 148 SF-1
has been shown to have a sex-specific pattern of expression in mammals 149,150 and
birds. 151 One of the steroidogenic genes regulated by SF-1 is the aromatase gene,
so SF-1 could have a role in TSD if estrogen is involved (see Section 4.4.2). SF-1
has been identified in reptiles with TSD, 111,152 and has been shown to have a sexspecific expression pattern in the alligator and in a freshwater turtle with TSD. 152,
153 SF-1 has not been examined in sea turtles.
The gene producing DAX1 has been implicated in mammalian sex determination
because overexpression of DAX1 is associated with male-to-female sex reversal. 154,
155 DAX1 has been identified in a reptile with TSD (the alligator), but no sex-specific
pattern of expression was detected. 152 DAX1 has not been investigated in sea turtles.
The gene producing Wilms tumor 1 (WT1) is necessary for the proper development
of the kidneys and gonads in mammals. 156 WT1 has also been hypothesized to act
synergistically with SF-1 in regulating AMH, whereas DAX1 antagonizes this synergy. 157 WT1 has been identified in the alligator, but no sex-specific pattern of expression has been detected during TSD. 152 WT1 has not been investigated in sea turtles.
In summary, the genetics of TSD is not well understood, but a number of
potential factors in the sex determination cascade have been identified. Although the
testis-determining gene in mammals (SRY) does not appear to be present in reptiles,
many other genes in the sex determination–sex differentiation cascade are conserved
in amniotic vertebrates, and some could potentially be involved in TSD. In addition,
as the specific functions of these genes are elucidated, the results will provide insight
into the putative involvement of estrogen in TSD. Although only a few studies have
addressed the genetics of sex determination in sea turtles, information from other
reptiles with TSD can act as a template for designing studies that can efficiently
evaluate the potential role of specific genes in the sex determination of sea turtles.
1123 book.book Page 125 Monday, November 11, 2002 11:11 AM