Biology Reference
In-Depth Information
both male and female reproductive structures, and possibly infiltration of testicular tis-
sues with oocytes.
It is interesting to note at this point, that although much of the data published in recent
years have sought to compare frogs from agricultural zones with those from nonagri-
cultural zones, a recent study by Skelly et al. (2010) documented a much higher preva-
lence of TOs among male frogs collected from urban and suburban environments in the
Connecticut River Valley. The authors sampled green frogs (
R. clamitans
), the same species
for which there had been no difference in TO prevalence when comparing agricultural
and nonagricultural zones in Ontario (McDaniel et al. 2008), and reported 0%, 6%, 16%,
and 21% incidence rates of TOs among green frogs from undeveloped landscapes, agricul-
tural landscapes, urban landscapes, and suburban landscapes, respectively.
The occurrence of gonadal abnormalities is not necessarily indicative of an absence of
function, and histology offers other biomarkers of testicular functionality. The various
phases of spermatozoa production can be quantified in histological sections, although
only a few studies have exploited this biomarker. Hayes et al. (2010) described reduced
sperm production in
X. laevis
exposed to 2.5 µg L
-1
of atrazine and reduced fertility in sub-
sequent mating trials. Orton et al. (2006) also described significantly fewer spermatids and
significantly more spermatocytes among
R. pipiens
metamorphs after larval exposure to
10 μg L
-1
of atrazine, and Hecker et al. (2005a) described a slight but nonsignificant reduc-
tion in spermatozoa among adult male
X. laevis
exposed for 49 days to 10 and 100 μg L
-1
of atrazine. Similar diminution of testis function was not found in leopard frogs collected
from agricultural areas where atrazine was in use (McDaniel et al. 2008).
9.3.3 Looking beyond the Gonad
The specter that has overshadowed the various atrazine studies in the past 10 years is that
of the unexplained toxicity reported by Hayes et al. (2002a, 2003). The demonstrable occur-
rence of TOs among field-collected frogs and in laboratory studies—inconsistent though
they may be—has kept the focus on gonad morphology as the primary biomarker of endo-
crine disruption in amphibians. Gonad histology provides a direct biomarker of repro-
ductive status. However, as with any organism or chemical exposure scenario, a suite of
different biomarkers is required in order to establish cause and effect relationships. Hayes
et al. (2002a, 2003) also adopted other biomarkers of endocrine disruption, which fall into
two categories: those that are indicative of effects on secondary sex characteristics and
those that probe the mechanisms of endocrine disruption.
9.3.4 Biomarkers of Secondary Sex Characteristics
Among most sexually dimorphic species, male frogs advertise to female frogs using dis-
tinct calls. Anatomically, this ability is conferred upon male frogs by a developmental
elaboration of the larynx. Hayes et al. (2002a) reported a decrease in the size of larynges
of postmetamorphic
X. laevis
after exposure of larvae to >1 μg L
-1
of atrazine. This effect
on this secondary sex characteristic was confirmed in a later study (Hayes et al. 2010) fol-
lowing larval exposure to 2.5 μg L
-1
atrazine. The latter study differed from the earlier
studies in that all larvae used in the exposure were produced from the mating of sex-
reversed adult females (i.e., ZZ females; see Hayes 1998; Wallace et al. 1999), which on a
genetic basis are expected to be 100% male. Other studies have not been able to demon-
strate similar effects in laryngeal dimensions (Carr et al. 2003; Coady et al. 2005; Smith et
al. 2005). Hayes et al. (2010) also described diminution of other secondary sex characteristic
Search WWH ::
Custom Search