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metabolism of TH in teleosts, and the expression of TH-associated factors
before outlining what is known about other endocrine contributions to
metamorphosis.
3.1. Thyroid hormone
It became clear in the late 1980s that TH, which stimulates the metamor-
phosis of amphibians, also promotes metamorphosis of flatfish. Application
of exogenous TH accelerates metamorphic events, including fin
resorption, eye migration, and fin ray shortening (
Inui & Miwa, 1985;
Inui, Tagawa, Miwa, & Hirano, 1989; Klaren, Wunderink, Y¨ fera,
Mancera, & Flik, 2008; Miwa, Yamano, & Inui, 1992
). TH is also
necessary for flatfishmetamorphosis: treatment with TH-inhibiting goitro-
gens arrests metamorphosis, resulting in oversized pelagic larvae (
Inui &
Miwa, 1985
).
Further studies showed that spikes in thyroid activity or circulating TH
levels are associated with the larval metamorphosis of many teleost species,
including those with spectacular transformations (salmonids, eels, flatfish)
and those with more subtle metamorphoses (fatheads, greenlings, groupers,
minnow, sea breams, tilapia, tuna, zebrafish; see
Fig. 5.7
and hormone titer
citations in
Table 5.2
). Treatment with exogenous TH stimulates
metamorphic processes, while inhibition of TH synthesis with goitrogens
inhibits metamorphosis in numerous groups: eels, gobies, greenlings,
groupers, and salmon (morphological responses to TH manipulation
citations,
Table 5.2
). Metamorphic changes in fins, pigmentation, and body
size in zebrafish are inhibited by goitrogen treatment as well (
Brown, 1997
;
D.M. Parichy, unpublished data). Evolutionary modifications of TH signal-
ing effected by
cis
-regulatory changes at the thyroid-stimulating hormone
b
2
locus have also been implicated in the adaptive divergence of marine
and freshwater stickleback populations (
Kitano et al., 2010
). Together,
these studies suggest that TH involvement in metamorphosis is widespread
among teleosts, and that changes in this pathway can contribute to
evolutionary diversification. Interestingly, lampreys, relatives of early
teleosts, undergo a metamorphosis in which TH plays an inhibitory rather
than a stimulatory role (see
Youson, 1997
); the mechanistic bases for this
divergent TH effect, and whether other factors substitute for TH in promot-
ing metamorphosis, remain unknown.
TH synthesis and metabolism in teleosts are similar to that of tetrapods
(
Yen, 2001
). The genes that contribute to thyroid follicle patterning and
