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soon after introduction of TBT in antifouling paints. They described a “vestigial penis” in
some
Urosalpinx cinerea
females from the Bay of Chincoteague, USA.
To ascertain that TBT is the sole masculinizating compound, Bryan et al. (1988) tested a
collection of organotins on
N. lapillus
. Although TBT was shown not to be the only com-
pound capable of inducing imposex, all other organotins tested were far less potent. This,
together with the observation that TBT is the major organotin present in the environment,
forced the conclusion that imposex reflects principally TBT pollution.
The question remains whether factors could diminish female masculinization. An
inhibition of imposex development was obtained by Bettin et al. (1996) in animals
exposed to both TBT and a mix of estrone and 17β-estradiol (1 μg L
-1
). However, such
high concentrations of xenoestrogens are never observed in the environment. One bio-
logical factor is known to inhibit penis growth: parasitism by trematode flatworms. This
infection is visible to the naked eye and measurements of penis length in parasitized
individuals have to be discarded. It should be noted that, in this case,
vas deferens
devel-
opment is not affected.
Therefore, imposex measurement does seem to be a valid and specific biological indica-
tor of TBT pollution, even if it is not possible to exclude whether other factors could also
influence female masculinization.
9.1.3 Biological Mechanisms for Imposex Development
A summary of the studies published by Féral (1977, 1978, 1979, 1980a), Féral and Le Gall
(1982), Bettin et al. (1996), Oberdörster and McClellan-Green (2000), Heidrich et al. (2001),
Saitoh et al. (2001), Nishikawa et al. (2004), Nishikawa (2006), and Sternberg et al. (2010),
led to a hypothesis for the molecular mechanisms that could be implicated in imposex
development (Figure 9.2).
It should be noted that this simple diagram represents only a working hypothesis dis-
tilled from experiments where biases may be caused by nonphysiological doses, injection
TBT
+
RXR activation (cerebral ganglia)
PMF secretion (pedal ganglia)
+
-
Appearance and development
of male sexual organs
+
-
Secretion of androgens
Estrogens
-
TBT
FIGURE 9.2
Hypothetical molecular pathways implicated in imposex development.
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