Heterochronic development of lateral plates in the three-spined stickleback induced by thyroid hormone level alterations

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.

Fig 1. T₃ concentrations in the three-spined stickleback.

(A) Plasma T₃ concentrations of wild caught three-spined sticklebacks. (B) Whole-body T₃ concentrations in laboratory-raised three-spined sticklebacks (61 dpf). Middle point is median, box and whisker are quartiles and range respectively. Samples are combined and each contains between 4 and 13 individuals; the sample size for each group is indicated above the bar. Different lowercase letters above the bars indicate significant differences between forms (p < 0.05, Mann-Whitney U-test for wild fish) or between experimental groups (p < 0.05, Kruskal-Wallis test with Dunn’s post hoc test for laboratory-raised fish). Differences between groups were tested independently within each panel and each cross.

Fig 2

(A) Alizarin red stained control fish of the freshwater (FF, 130 dpf age) and marine (MF, 101 dpf) forms. (B) Sites of sampling of parental fish at the White Sea basin. Maps were obtained from http://www.sasgis.org.

Fig 3. Four of the TH-0.5 treated individuals (101 dpf) of the MF with the partial lateral plate phenotype.

Ellipses delineate areas with no lateral plates.

Fig 4

(A) Heterochronies in development (appearance) of lateral (left y-axis, four upper lines) and keel plate (right y-axis, four lower lines) in different experimental groups of the MF fish. Development of keel plates is given until forming a single continuous row with anterior plates. Plotted values are treatment-specific means. (B) Differences in degree of development of the lateral plates in MF cross at 42 dpf. Representative specimens of three sizes (largest, intermediate, and smallest) are shown from each experimental group.

Fig 5. Additional two small bony plates (arrows).

Additional plates are located above the main row of lateral plates, behind the head as exemplified from a TH-0.5 fish of marine form (fully plated phenotype, right side), age 101 dpf, TL = 34.7 mm (alizarin red S stained). Scale bar = 1 mm.

Fig 6. Meristic characters of Gasterosteus aculeatus.

A–number of anal fin rays, C–caudal fin rays, D–dorsal fin rays, P–pectoral fin rays (clipped fin shown), LP–total number of lateral plates, GR–number of gill rakers on the lower and upper branches of the first branchial arch, Vt–number of total vertebrae, Va–abdominal vertebrae, and Vc–caudal vertebrae.