The molecular heterogeneity of hemocyanin: Its role in the adaptive plasticity of Crustacea F. Giomi, M. Beltramini ⁎ Department of Biology, University of Padova, Viale G. Colombo 3, I-35131 Padova, Italy Received 22 December 2006; received in revised form 14 February 2007; accepted 22 February 2007 Available online 25 April 2007 Abstract Crustacean hemocyanin (Hc) represents a unique case of molecular heterogeneity among oxygen-carrying proteins. The existence of different genes, encoding single polypeptide chains, constitutes the genetic basis for the inter- and intra-specific polymorphism. In addition, the large number of Hc subunits within crustacean species, together with their flexible expression, provides an efficient intrinsic mechanism of modulation of oxygen transport. This review presents a description and classification of the various aspects of crustacean Hc heterogeneity and defines its role in a perspective of crustacean adaptive physiology. © 2007 Elsevier B.V. All rights reserved. Keywords: Hemocyanin; Quaternary structure; Subunit; Crustacea; Oxygen binding; Molecular Heterogeneity; Phenotypic polymorphism 1. Introduction The evolution of Metazoa is realized with a progressive increase of complexity in structures and functions and a consequent development of physiological networks for their integrated regulatory processes. In this context, the evolution of reliable systems for efficient gas exchange and O 2 transport allowed for an increase of body size and tissue differentiation. Furthermore, the development of higher aerobic capacity and advanced metabolisms increased the power availability of the evolving species. The essential role of O 2 transport proteins is clearly underlined by the independent evolution of analogous carriers. Hemoglobins, hemerythrins and hemocyanins (Hcs), although extremely different in structure and origin, share convergent functions across phyla. Moreover, they contribute to the radiation of species diversity through several mechanisms of functional adaptations like regulation of expression, variation of synthesis and sensitivity to extrinsic modulators (Terwilliger, 1998). Among the O 2 binding proteins, the Hcs of Arthropoda, and those of Crustacea in particular, show the largest extent of heterogeneity, with a great variety of subunits expressed within the same oligomeric proteins and a wide degree of regulatory mechanisms. To describe the structural features and the quaternary architecture of Hc oligomers, to disclose the balance between functional constraints and adaptive modulation of the O 2 binding physiology, and to trace the evolutionary pathway that have led to the current complexity, several reviewers have differently explored the arthropod Hc heterogeneity. Detailed analyses of structural heterogeneity have led to the recognition of distinct subunit types which differ in immunogenicity and in their specific roles in complex oligomerization (Markl et al., 1979a,b, 1986; Markl, 1986; Markl and Decker, 1992). Hcs sequencing has provided a clear identification of distinct subunits in several species allowing for a precise definition of subunit families and of their phylogenetic relationships (Burmester, 2001). Thus, possible ambiguities on subunit identification by chromato- graphic or electrophoretic approaches are avoided. Numerous studies have investigated the origin and the architecture of the three protein domains that represent the characteristic fold of the subunits (Linzen et al., 1985; Salvato and Beltramini, 1990; van Holde et al., 2001; Decker and Terwilliger, 2000). The Gene 398 (2007) 192 – 201 www.elsevier.com/locate/gene ⁎ Corresponding author. Tel.: +39 049 827 6337; fax: +39 049 827 6300. E-mail address: mariano.beltramini@unipd.it (M. Beltramini). 0378-1119/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.gene.2007.02.039