The Wnt genes encode a class of secreted glycoprotein ligands with approximately 350–400 amino acids in length, including 23–24 conserved cysteines with one or more N-terminal glycosylation sites and highly conserved sequence regions in different species [1]. Signaling pathways invoked by Wnt molecules are involved in the regulation of diverse biological processes that play crucial roles in the development of living organisms, including differentiation of cell morphology, maintenance of homeostasis as well as embryonic axis formation, regeneration of tissues, and regulation of immune responses [2–3]. Phylogenetic studies have suggested that 13 Wnt subfamilies may have existed in a common ancestor of cnidarians and bilaterian symmetrical animals, unique to metazoans but not yet found in unicellular eukaryotes and plants [4]. Interestingly, gene loss often occurs in the evolution of Wnt genes in bilateral animals [5], but significant Wnt gene expansion is found in mammals [6], implying that homologous Wnt genes may be involved in the process of selection and adaptation to environment. In molluscs, the number of Wnt genes was found to vary from 11–34, but varied between 11–13 in bivalves. Studies in Ruditapes philippinarum have suggested that Wnt gene expansions may play an important role in larval development and shell color pattern formation [7]. Studies in oysters suggest that Wnts may coordinate and participate in homeostasis and tissue morphogenesis, as well as in dealing with the challenging environment [8]. Twelve and 11 Wnt subfamilies have also been found in the Japanese scallop (Patinopecten yessoensis) and Zhikong scallop. However, no Wnt genes have been identified in Argopecten scallops up to now.
Bohai Red, a new Argopecten scallop strain selected from the hybrids between the Peruvian scallop, Argopecten purpuratus and the bay scallop northern subspecies, A. irradians irradians, is now one of the most cultured scallop strains in northern China. The bay scallops, including the northern subspecies, A. irradians irradians and the southern subspecies A. irradians concentricus that are native to the Atlantic coast of the United States, with a wide distribution from Cape Cod to New Jersey and North Carolina, were introduced from the United States to China in the 1980s and 1990s, respectively. They have developed rapidly into vast industries in northern and southern China mainly due to their fast growth, short culture period and wide temperature tolerance range. However, since both subspecies are small in adult size with relatively short life spans [9–11] and in addition, since long-term inbreeding since their original introduction had led to severe depression in genetic diversity, the bay scallop culture industry had suffered significant losses around the turning of the century. Peruvian scallop, also known as Chilean scallop, a medium-sized scallop naturally distributed along the Pacific coast of South America [12], was then introduced into China in 2007, in hope to replace the small-sized bay scallops [13]. Although they were successfully reproduced in the hatchery, most Peruvian scallops did not survive the high temperature (up to 28.6 oC) in the summer in Qingdao. To utilize the genetic resources in the Peruvian scallop and the bay scallop, hybrids were produced between them which exhibited great heterosis in growth [14]. As the resulted hybrids were mainly male sterile, to overcome the difficulty in seed production from these hybrids, three new hermaphroditic strain, Bohai Red [15], ‘QN orange’ [16] and ‘QN-2’ [17], were selected and soon became one of the most cultured scallop varieties in northern China. In order for better culture of these new strains, a series of studies were carried out on the biology of these new strains. Our aim of this study is to examine the expression profile of Wnt genes in the strain Bohai Red under different thermal stresses.
Global warming is becoming a global problem. Increased temperature greatly affected the marine ecosystem [18]. Temperature is a key variable in all biological systems, because it can directly affect all biological processes and physiological mechanisms such as metabolism and oxidative stress [19]. Oxidative stress is caused by an imbalance between the production of oxidants and their removal by antioxidants and antioxidant enzymes. The short-term increase of temperature will increase the oxidative stress of Antarctic vertebrates and invertebrates [20]. When exposed to an acutely increased temperature, Pagothenia borchgrevinki showed an increased antioxidant capacity, but showed significant oxidative damage to lipids when exposed to prolonged higher temperatures [21].
As Bohai Red scallops were originated from the hybrids between the bay scallop northern subspecies and the Peruvian scallop, whose genomes are recently published, we first performed genome-wide identification of the Wnt genes in the bay scallop northern subspecies and the Peruvian scallop, as well as the bay scallop southern subspecies, whose genome is also available recently. We then conducted an evolutionary analysis involving the newly identified Wnt genes in these Argopecten scallops. Finally, we analyzed the expression profiles of Wnt genes in Bohai Red scallops under different temperature stress conditions. This study may provide new basis for further understanding of the evolution of Wnt genes in bivalves and their possible roles in adaptation to thermal stress in scallops, which may eventually benefit Bohai Red culture industry.