The effect of thyroid hormone levels on development of the functional morphology of feeding in the Zebrafish (Danio rerio)

Demi Cruz Galindo

Cypriniformes and Acanthamorphs fishes have been highly successful and account for nearly one-third of all living vertebrates. Both lineages have independently evolved protrusile upper jaws and this innovation has been linked to their evolutionary success. The zebrafish (Danio rerio), a species of the Cypriniformes has a protrusive upper jaw that is not present until metamorphosis, a period marked by extensive bone remodeling and shifts in feeding biomechanics. One critical structure for jaw protrusion, the premaxilla, a bone found in the upper jaw, is not formed until metamorphosis. Metamorphosis is associated with large spikes in thyroid hormone levels, therefore alterations to TH levels could affect the functional morphology important for feeding. Employing our model organism, the zebrafish, we were able to test for morphological and functional effects of altered thyroid hormone levels. We tested the effects of reduced and elevated thyroid hormone using three euthyroid treatment groups, a transgenic line treated to induce hypothyroidism, and a congenitally hyperthyroid mutant line. We collected high-speed video recordings of fish feeding over a wide range of developmental stages (first feeding to adult) to measure the functional effects of TH manipulation and used shape analyses to compare the skull morphologies of the same specimens. Significant morphological differences were found between treatment groups at all developmental stages but accumulated throughout development. Upper jaw shape was altered in hypothyroid zebrafish and nearly eliminated jaw protrusion ability. Hyperthyroid zebrafish had elongated lower jaws. Since differing effects in the upper and lower jaws were observed from altering thyroid hormone levels, this suggests differential thyroid hormone signaling (THS) between the upper and lower jaws. Hypothyroid zebrafish exhibited reduced head shape variation, yet hyperthyroid fish had increased shape variation, which may be caused by the developmental delay and acceleration associated with each group, respectively. We also saw that altered TH levels affected patterns of morphological covariation between different cranial regions. Our results show that alterations to THS change the development of the functional morphology of feeding in the zebrafish and suggest that changes to TH signaling pathways may have played an important role in the evolution of fish jaw protrusion.

The effect of thyroid hormone levels on development of the functional morphology of feeding in the Zebrafish (Danio rerio)

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