**3. Inter-population and geographical variation in thyroid hormone physiology in animals**

Anatomical studies conducted in the 1960s and 1970s showed interspecies morphological variation for fishes and amphibians [49,50]. Since then, natural variation in thyroid hormone physiology has been extensively investigated in diverse taxa of vertebrate (Table 1). Some of the variation results from environmental factors. For example, environmental contaminants can cause goiter. In salmon populations introduced into the Great Lakes in the late 1960s, the frequency of thyroid goiter increased in the mid-1970s [51,52,53]. In addition, herring gulls *Larus argentatus* from the Great Lakes also suffered from goiter in the 1980s [54]. It was demonstrated that laboratory rats fed with the salmons caught in the lakes exhibited hypothyroidism and goiter, suggesting the presence of goitrogenic substances in the Great Lakes fishes [55].


**Table 1.** Variation in thyroid hormone physiology in natural animal populations

Goiters were also observed in hatchery fishes and possibly resulted from iodine deficiency, because iodine treatment was able to cure the goiter [56]. In the case of the threespine stickleback *Gasterosteus aculeatus*, interpopulation variation in susceptibility to goiter when reared in fresh water was observed [57,58], although whether the goiter in the sticklebacks was caused by iodine deficiency is unknown. Interestingly, a mammalian species, *Auliscomys boliviensis*, inhabiting an environment severely depleted of iodine did not show goiter [59], suggesting that genetic variation in the susceptibility to endemic goiter exists among populations and species.

40 Thyroid Hormone

divergence between humans and apes [47,48].

Spadefoot toad T4, T3, sensitivity Dry environment,

**Table 1.** Variation in thyroid hormone physiology in natural animal populations

**physiology in animals** 

Intraspecific variation

Interspecific variation

[42]. By comparing the synonymous and nonsynonymous substitutions in the human and chimp genomes, putatively positively selected genes were screened [46]. Genes expressed in the thyroid gland have an excess of rapidly evolving genes compared with other tissues, except testis, which has more putatively positively selected genes [46]. Changes in thyroid hormone physiology may contribute to some of the physiological and morphological

**3. Inter-population and geographical variation in thyroid hormone** 

goiter, suggesting the presence of goitrogenic substances in the Great Lakes fishes [55].

Species/Family Phenotypic variation Potential factors and functions Reference

 Coho salmon Goiter, T4, T3 Goitrogen [51,53,97] Chinook salmon Goiter Goitrogen [53] Herring gull Goiter Goitrogen [54] American alligator T4 Goitrogen [98] Japanese pond frog Morphology [49] Bottlenose dolphin T4 and T3 Temperature [62] Northern cardinal T4 and T3 [61] Alaskan husky T4 and T3 Temperature [60] Bonnethead shark T4 and T3 in yolk Temperature [70] Brook charr T4 and T3 Migration [87] Stickleback Goiter, TSHß, T4,T3 Migration, metabolism [57,58,86]

Poeciliidae Morphology, tumor [50,99,100]

 Big-eared mouse T4, T3, iodide Low iodide concentration [59] Rodent T4 Life span [101]

metamorphosis

[63]

Anatomical studies conducted in the 1960s and 1970s showed interspecies morphological variation for fishes and amphibians [49,50]. Since then, natural variation in thyroid hormone physiology has been extensively investigated in diverse taxa of vertebrate (Table 1). Some of the variation results from environmental factors. For example, environmental contaminants can cause goiter. In salmon populations introduced into the Great Lakes in the late 1960s, the frequency of thyroid goiter increased in the mid-1970s [51,52,53]. In addition, herring gulls *Larus argentatus* from the Great Lakes also suffered from goiter in the 1980s [54]. It was demonstrated that laboratory rats fed with the salmons caught in the lakes exhibited hypothyroidism and Latitudinal variation in plasma concentrations of thyroid hormone has been observed in both mammals and birds, and these variations might have evolved as adaptations to environments with divergent temperatures. Plasma total T4, free T4, and total T3 levels of sled dogs living in Alaska were higher than dogs in New York, especially in winter [60]. In addition, plasma T3 increased with increasing latitude in the northern cardinals *Cardinalis cardinalis*, whereas plasma T4 did not show a simple latitudinal cline: both southern and northern birds had higher T4 levels than birds living at an intermediate latitude [61]. In mammals, bottlenose dolphins *Tursiops truncatus* show variation in thyroid hormone concentrations between populations inhabiting different latitudes [62]: plasma total T3 and T4 were higher in dolphins from South Carolina with colder year-round temperatures than those from Florida with much warmer water temperatures. Since thyroid hormones play key roles in metabolism and heat generation, evolutionary adaptation to habitats with different temperatures may account for some of the latitudinal and geographical variation in thyroid hormone levels among natural populations. The genetic basis for the latitudinal variation is currently unknown.

Several studies have demonstrated that variation in thyroid hormone physiology correlates with other potentially adaptive traits. Interspecies variation in tissue thyroid hormone levels and tissue sensitivity to thyroid hormone may be correlated with variation in the duration of the larval period in spadefoot toads [63]. For example, the tadpole of the desert-dwelling toad *Scaphiopus couchii* has higher tail and liver levels of thyroid hormone, and the tail tip is more sensitive to thyroid hormone *in vitro* than tail tips of other closely related species [63]. Because frog metamorphosis is controlled by thyroid hormone, the higher thyroid hormone levels and the higher sensitivity may explain the short larval period in this species. Rapid metamorphosis (i.e., the short period of water-dwelling at the tadpole stage) observed in the desert toad is likely adaptive for survival in the deserts where water is scarce [64,65].

Thyroid hormones also play critical roles as yolk hormones in mammalian [66], bird [67], and teleost [68,69] development. In the bonnethead shark *Sphyrna tiburo*, the concentrations of T3 and T4 in the yolk from the Tampa Bay population were consistently higher than those in the yolk from the Florida Bay population [70]. The bonnethead shark in Tampa Bay develops faster and is larger at birth than that in Florida Bay [71]. Tampa Bay is located in a more northern region and is colder than Florida Bay. Because rapid growth is generally adaptive in colder environments [72,73], higher york thyroid hormone levels in the Tampa Bay population may be adaptive.

Thyroid hormone is also implicated in the regulation of longevity in animals [74,75]. Longlived species of squirrels, deer mice, bats and mole-rats maintain low levels of thyroid hormone [76,77,78,79]. Hypothyroid Wister rats live longer than hyperthyroid rats [80]. Furthermore, investigations in the Ames and Snell dwarf mice have demonstrated that mutation at the *Prop-1* and *Pit-1* genes lead to defects in the generation of pituitary cells including thyrotrope and the dwarf mice have extended longevity [81,82,83,84,85]. Thus, it is possible that changes in the thyroid hormone pathway are involved in variation of life span among wild animals, as is observed among human races (see above). Further research on the genetic basis for the low thyroid hormone levels observed in the long-lived animals should be conducted.

Ecological Genetics of Thyroid Hormone Physiology in Humans and Wild Animals 43

thyroid hormone physiology likely results from adaptation to environments with divergent ambient temperatures. Third, variation in thyroid hormone physiology may be associated with variation in longevity. Fourth, genomic scan of signatures of selection have revealed that some thyroid hormone-related genes experience selective pressure during evolution or

In humans, it is very difficult to experimentally test the adaptive significance of such variation. However, ecological experiments can be conducted using animals. For example, reciprocal transplant experiments on divergent populations or species with different thyroid hormone physiology can test whether wild animals have higher fitness in native habitats than in foreign habitats [95,96]. We can also investigate whether the fitness is correlated with the thyroid hormone levels. In addition, hormonal manipulation would be able to directly test whether the higher or lower thyroid hormone levels can change the fitness in a variety

Until recently, it has been difficult to study the genetic basis for physiological differences between natural animal populations. However, it is now becoming increasingly easier to conduct genomic studies because of the recent progress in genomic technologies. Therefore, we can test whether candidate loci involved in thyroid hormone signaling pathways are correlated with fitness in natural environments or laboratory conditions. Furthermore, ecological and genomic studies of wild animal populations will help answer fundamental evolutionary questions, such as whether the same environmental variables are strong agents of natural selection on the thyroid hormone pathways and whether genetic variation in the same genes caused the adaptive divergence in thyroid hormone physiology across diverse

*Ecological Genetics Laboratory and JST PRESTO, National Institute of Genetics, Mishima, Japan* 

This research is supported by JST PRESTO program, the Naito Foundation, Grant-in-Aid for Scientific Research on Innovative Areas (23113007 and 23113001) from the Ministry of Education, Science, Sports, and Culture to JK. AI is a Fellow of the Japan Society of

[1] Park SM, Chatterjee VKK (2005) Genetics of congenital hypothyroidism. J Med Genet 42:

domestication.

of environments.

taxa, including humans.

**Acknowledgement** 

Promotion of Science.

**5. References** 

379-389.

Corresponding Author

 \*

Asano Ishikawa and Jun Kitano\*

**Author details** 

Divergence in thyroid hormone physiology may also be important for adaptation of stickleback fishes to marine and freshwater environments [86]. Stream-resident populations of the threespine stickleback have repeatedly evolved from ancestral marine populations. First, Kitano et al. (2010) found plasma thyroid hormone levels and metabolic rate were lower in stream-resident populations than in ancestral marine populations [86]. Since thyroid hormones regulate metabolic rate in sticklebacks [86], it is likely that lower thyroid hormone in stream-resident sticklebacks is adaptive for permanent residency in small streams where oxygen and food are often scarce. In addition, the expression level of thyroid stimulating hormone *TSHß2* gene was significantly lower in the pituitary gland of streamresident fish than in that of marine fish. Allele-specific expression analysis with F1 hybrids revealed that some of the differences in *TSHß2* expression levels were caused by *cis*regulatory changes at the *TSHß2* locus. Importantly, a signature of natural selection was found at *TSHß2* locus: several SNPs within the *cis*-regulatory region exhibited marked differences in the allele frequency between marine and stream-resident populations. Thus, changes in the thyroid hormone pathways may play important roles in genetic adaptation to freshwater environments. In other fishes exhibiting alternate life history style, such as the brook charr *Salvelinus fontinalis* anadromous and resident forms show differences in thyroid hormone concentrations, although genetic factors seem to be of little importance in the interpopulation variation seen in the brook charr [87].

Other than the *TSH* loci in sticklebacks, there are few studies that have examined whether thyroid hormone-related genes are under selective pressure in wild animal populations. However, domestication seems to be a strong artificial selection on thyroid hormone-related genes. Very strong selective sweeps were found at the *TSHR* loci in chickens [88] and sheep [89]. Because TSH is found to regulate photoperiodic control of reproduction [90,91,92,93,94], artificial selection favoring continuous reproduction under domestication might act on the *TSH* locus.
