The Questions of Developmental Biology

It is not known whether the temperature sensitivity resides in the aromatase gene or
protein itself or in other proteins that regulate it. One hypothesis is that the temperature is sensed
by neurons in the central nervous system and transduced to the bipotential gonad by nerve fibers
(see Lance 1997). Another hypothesis is that aromatase activity may be regulated by Sox9.
This sex-determining gene is seen throughout the vertebrates, where its expression in gonads
correlates extremely well with the production of testes. When two species of turtles were raised at
female-promoting temperatures, Sox9 expression was down-regulated during the critical time for
sex determination. However, in the bipotential gonads of those turtles raised at male-promoting
temperatures, Sox9 expression was retained in the medullary sex cords destined to become Sertoli
cells (Spotila et al. 1998; Moreno-Mendoza et al. 1999).
The evolutionary advantages and disadvantages of temperature-dependent sex
determination are discussed in Chapter 21. Recent studies (Bergeron et al. 1994, 1999) have
shown that polychlorinated biphenyl compounds (PCBs), a class of widespread pollutants that
can act like estrogens, are able to reverse the sex of turtles raised at "male" temperatures. This
knowledge may have important consequences in environmental conservation efforts to protect
endangered turtle species.
Location-dependent sex determination in Bonellia and Crepidula
As mentioned in Chapter 3, the sex of the echiuroid worm Bonellia depends on where a
larva settles. If a Bonellia larva lands on the ocean floor, it develops into a 10-cm-long female.
If the larva is attracted to a female's proboscis, it travels along the tube until it enters the female's
body. Therein it differentiates into a minute (1 3-mm-long) male that is essentially a spermproducing
symbiont of the female (see Figure 3.1).
Another example in which sex determination is
affected by the location of the organism is the case of the
slipper snail Crepidula fornicata. In this species, individuals
pile up on top of one another to form a mound (Figure 17.21).
Young individuals are always male. This phase is followed by
the degeneration of the male reproductive system and a period
of lability. The next phase can be either male or female,
depending on the animal's position in the mound.
If the snail is attached to a female, it will become male.
If such a snail is removed from its attachment, it will become
female. Similarly, the presence of large numbers of males will
cause some of the males to become females. However, once an individual becomes female, it will
not revert to being male (Coe 1936). More examples of context-dependent sex determination will
be studied in Chapter 21.
Nature has provided many variations on her masterpiece. In some species, including most
mammals and insects, sex is determined solely by chromosomes; in other species, sex is a matter
of environmental conditions. We are finally beginning to understand the mechanisms by which
this masterpiece is created.