The Questions of Developmental Biology

Environmental sex determination
Sex determination in an echiuroid worm: bonellia
When developmental mechanics was first
formulated, some of the obvious variables to manipulate
were the temperature and media in which embryos were
developing. These early studies initiated several
experimental programs on the effects of the environment
on development. For instance, Baltzer (1914) showed
that the sex of the echiuroid worm Bonellia viridis
depended on where the Bonellia larva settled. The female Bonellia worm is a marine, rockdwelling
animal, with a body about 10 cm long (Figure 3.1).
She has a proboscis that can extend over a meter in length. The male Bonellia, however,
is only 1 3 mm long and resides within the uterus of the female, fertilizing her eggs. Baltzer
showed that if a Bonellia larva settles on the seafloor, it becomes a female. However, should a
larva land on a female's proboscis (which apparently
emits chemical signals that attract the larva), it enters the
female's mouth, migrates into her uterus, and
differentiates into a male. Thus, if a larva lands on the
seafloor, it becomes female; if it settles on a proboscis, it
becomes male. Baltzer (1914) and Leutert (1974) were
able to duplicate this phenomenon in the laboratory,
incubating larvae in either the absence or presence of
adult females (Figure 3.2).
Sex determination in a vertebrate: alligator
Recent research has shown that the effects of the environment on development can have
important consequences. Such research has shown that the sex of the alligators, crocodiles, and
many other reptiles depends not on chromosomes, but on temperature. After studying the sex
determination of the Mississippi alligator both in the laboratory and in the field, Ferguson and
Joanen (1982) concluded that sex is determined by the temperature of the egg during the second
and third weeks of incubation. Eggs incubated at 30°C or below during this time period produce
female alligators, whereas those eggs incubated at 34°C or above produce males. (At 32°C, 87%
of the hatchlings were female.) Moreover, nests constructed on levees (close to 34°C) give rise to
males, whereas nests built in wet marshes (close to 30°C) produce females. These findings are
obviously important to wildlife managers and farmers who wish to breed this species; but they
also raise questions of environmental policy. The shade of buildings or the heat of thermal
effluents can have dramatic effects on the sex ratios of reptiles. We will discuss the mechanisms
of temperature-dependent sex determination further in Chapter 17.