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First published online November 17, 2006
Journal of Experimental Biology 209, 4663-4675 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02581
Lipid remodeling in wild and selectively bred hard clams at low temperatures in relation to genetic and physiological parameters
1 Institut de Recherche sur les Zones Côtières, 232B rue de
l'Église, Shippagan, Nouveau-Brunswick, E8S 1J2, Canada
2 Institut des Sciences de la Mer, 310 allée des Ursulines, Rimouski,
Québec, G5L 3A1, Canada
3 Department of Fisheries and Oceans, Science Branch, Gulf Fisheries Centre,
PO Box 5030, Moncton, New Brunswick, E1C 9B6, Canada
* Author for correspondence (e-mail: fpernet{at}umcs.ca)
Accepted 4 October 2006
A temperature decrease usually induces an ordering effect in membrane phospholipids, which can lead to membrane dysfunction. Poikilotherms inhabiting eurythermal environments typically counteract this temperature effect by remodeling membrane lipids as stipulated in the homeoviscous adaptation theory (HVA). Hard clams, Mercenaria mercenaria, can suffer high overwintering mortalities in the Gulf of St Lawrence, Canada. The selectively bred M. mercenaria var. notata can have higher overwintering mortalities than the wild species, thus suggesting that the two varieties have different degrees of adaptation to low temperatures. The objective of this study was to investigate the changes in lipid composition of soft tissues in wild and selected hard clams in relation to their metabolic and genetic characteristics. Clams were placed at the northern limit of their distribution from August 2003 to May 2004; they were exposed to a gradual temperature decrease and then maintained at <0°C for 3.5 months. This study is the first to report a major remodeling of lipids in this species as predicted by HVA; this remodeling involved a sequential response of the phospholipid to sterol ratio as well as in levels of 22:6n-3 and non-methylene interrupted dienoic fatty acids. Hard clams showed an increase in 20:5n-3 as temperature decreased, but this was not maintained during overwintering, which suggests that 20:5n-3 may have been used for eicosanoid biosynthesis as a stress response to environmental conditions. Selectively bred hard clams were characterized by a higher metabolic demand and a deviation from Hardy-Weinberg equilibrium at several genetic loci due to a deficit in heterozygote frequency compared with wild clams, which is believed to impose additional stress and render these animals more vulnerable to overwintering mortality. Finally, an intriguing finding is that the lower metabolic requirements of wild animals coincide with a lower unsaturation index of their lipids, as predicted by Hulbert's theory of membranes as pacemakers of metabolism.
Key words: lipid composition, sterol, fatty acid, homeoviscous adaptation, bivalve, winter acclimatization
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