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First published online June 29, 2007
Journal of Experimental Biology 210, 2419-2429 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.002568

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Rhesus glycoprotein gene expression in the mangrove killifish Kryptolebias marmoratus exposed to elevated environmental ammonia levels and air

C. Y. C. Hung^1,2, K. N. T. Tsui², J. M. Wilson³, C. M. Nawata², C. M. Wood² and P. A. Wright^1,*

¹ Department of Integrative Biology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
² Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario, L8S 4K1, Canada
³ Ecofisiologia CIMAR Rua dos Bragas 289, 4050-123, Porto, Portugal

^* Author for correspondence (e-mail: patwrigh{at}uoguelph.ca)

Accepted 25 April 2007

The mechanism(s) of ammonia excretion in the presence of elevated external ammonia are not well understood in fish. Recent studies in other organisms have revealed a new class of ammonia transporters, Rhesus glycoprotein genes (Rh genes), which may also play a role in ammonia excretion in fish. The first objective of this study was to clone and characterize Rh genes in a fish species with a relatively high tolerance to environmental ammonia, the mangrove killifish Kryptolebias marmoratus (formerly Rivulus marmoratus). We obtained full-length cDNAs of three Rh genes in K. marmoratus: RhBG (1736 bp), RhCG1 (1920 bp) and RhCG2 (2021 bp), which are highly homologous with other known Rh gene sequences. Hydropathy analysis revealed that all three Rh genes encode membrane proteins with 10–12 predicted transmembrane domains. RhBG, RhCG1 and RhCG2 are highly expressed in gill tissue, with RhBG also present in skin of K. marmoratus. Exposure to elevated environmental ammonia (2 mmol l^–1 NH₄HCO₃) for 5 days resulted in a modest (+37%) increase in whole-body ammonia levels, whereas gill RhCG2 and skin RhCG1 mRNA levels were upregulated by 5.8- and 7.7-fold, respectively. RhBG mRNA levels were also increased in various tissues, with 3- to 7-fold increases in the liver and skeletal muscle. In a separate group of killifish exposed to air for 24 h, RhCG1 and RhCG2 mRNA levels were elevated by 4- to 6-fold in the skin. Thus, the multifold induction of Rh mRNA levels in excretory tissues (gills and skin) and internal tissues in response to conditions that perturb normal ammonia excretion suggests that RhBG, RhCG1 and RhCG2 may be involved in facilitating ammonia transport in this species. Furthermore, the findings support earlier studies demonstrating that the skin is an important site of ammonia excretion in K. marmoratus.

Key words: Kryptolebias marmoratus, ammonia excretion, gills, skin, ammonia transporter

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