Mercury delays cerebellar development in a model songbird species, the zebra finch
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Mercury delays cerebellar development in a model songbird species, the zebra finch Sheila A. Scoville1 Claire W. Varian-Ramos2,3 G. Alden Adkins2,4 John P. Swaddle2 Margaret S. Saha2 Daniel A. Cristol2 ●
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Accepted: 11 August 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Mercury exposure can disrupt development of the cerebellum, part of the brain essential for coordination of movement through a complex environment, including flight. In precocial birds, such as fowl, the cerebellum develops embryonically, and the chick is capable of leaving the nest within hours of hatching. However, most birds, including all songbirds, are altricial, and spend weeks in the nest between hatching and fledging. The objective of this study was to describe the normal development of the cerebellum in a model altricial songbird so as to determine the effect of exposure to mercury on cerebellar maturation. Adult zebra finch (Taeniopygia guttata) pairs were fed either a control diet, or a diet augmented with one of four treatment-levels of methylmercury (0.3–2.4 μg/g wet weight), and their offspring, the subjects of this study, were fed the same diet by parents. We documented, for the first time, the schedule of cerebellar development in an altricial bird, and compared stages of development among methylmercury-exposed groups. For all treatments of methylmercury, the age of completion of cellular migration was later than for control zebra finches, indicating a delay in cerebellar maturation. Displaced (heterotopic) Purkinje neurons, a pathology typical of methylmercury exposure in developing vertebrate brains, were more numerous in methylmercury-exposed birds, and persisted at least until the age of independence. Delays in maturation of the cerebellum could delay fledging in altricial bird species, with potential serious implications for the fitness of exposed individuals, as predation rates in the nest are often very high. Keywords Cerebellum Development Mercury Methylmercury Songbird Zebra finch ●
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Introduction Anthropogenic emissions of mercury have resulted in elevated levels of methylmercury (MeHg) in the diets and tissues of wild birds, even in the remotest locations (Bond and Robertson 2015), and including the most numerous avian taxon, terrestrial songbirds (Cristol et al. 2008).
* Daniel A. Cristol [email protected] 1
Department of Pathology and Anatomy, Eastern Virginia Medical School, Norfolk, VA 23507, USA
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Department of Biology, William & Mary, Williamsburg, VA 23185, USA
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Present address: Biology Department, Colorado State University – Pueblo, Pueblo, CO 81001, USA
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Present address: University of Virginia School of Medicine, Charlottesville, VA 22908, USA
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Songbirds populations have declined precipitously in the last 50 years (Rosenberg et al. 2019), and pollutants such as mercury may play a role through disruption of reproduction, critical behaviors, or demanding life-cycle stages such as migration (Seewagen 2020). Much is known of th
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