Abstract
The three-spined stickleback Gasterosteus aculeatus is an important model for studying microevolution and parallel adaptation to freshwater environments. Marine and freshwater forms differ markedly in their phenotype, especially in the number of lateral plates, which are serially repeated elements of the exoskeleton. In fishes, thyroid hormones are involved in adaptation to salinity, as well as the developmental regulation of serially repeated elements. To study how thyroid hormones influence lateral plate development, we manipulated levels of triiodothyronine and thiourea during early ontogeny in a marine and freshwater population with complete and low plate phenotypes, respectively. The development of lateral plates along the body and keel was heterochronic among experimental groups. Fish with a low dosage of exogenous triiodothyronine and those treated with thiourea exhibited retarded development of bony plates compared to both control fish and those treated with higher a triiodothyronine dosage. Several triiodothyronine-treated individuals of the marine form expressed the partial lateral plate phenotype. Some individuals with delayed development of lateral plates manifested 1–2 extra bony plates located above the main row of lateral plates.
Highlights
The three-spined stickleback Gasterosteus aculeatus L. is a circumpolar species distributed across the Northern hemisphere, and it has emerged as an important model organism for studying many contemporary problems in ecology and evolution [1,2,3]
The concentrations were significantly higher in thyroid hormones (THs)-fish compared to the Thio-treated fish in both freshwater form (FF) and marine form (MF) crosses (Kruskal-Wallis tests, p < 0.05, Fig 1B)
There was no difference in T3 concentration between the Thio and control fish in the MF crosses, but the Thio-fish in the FF crosses had significantly lower T3 concentrations compared to the controls (Fig 1B)
Summary
The three-spined stickleback Gasterosteus aculeatus L. is a circumpolar species distributed across the Northern hemisphere, and it has emerged as an important model organism for studying many contemporary problems in ecology and evolution [1,2,3]. The species is phenotypically very diverse, but there are two basic forms recognized as marine and freshwater ecotypes [4]. The marine form (MF) is ancestral, and its morphology is thought to be conserved since the Miocene [5, 6]. Multiple colonizations of freshwater habitats have resulted in significant phenotypic changes [7, 8]. One of the most striking morphological differences between the two main forms is in the number of bony lateral plates protecting the body.
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