These two molecules are covalently bound by a Schiff base linkage via a lysine residue (Wald 1968) to form a visual pigment. Molluscs possess a wealth of morphological diversity of eyes, including simple eye cups or pit eyes that are open to the environment, closed-lens eyes much like those seen in fish, compound eyes that superficially resemble the eyes of flies, pinhole eyes, and eyes with mirrors (Serb 2008; Serb and Eernisse 2008). This ecologically diverse clade includes the economically important scallop (Pectinidae), oyster (Ostreidae), and mussel (Mytilidae) families (von Salvini-Plawen 2008) and is characterized by an ancestral condition of protein byssal attachment.The amino acids surrounding the chromophore play an important role in light sensitivity as their interaction with the chromophore influences the portion of the light spectrum that will elicit a response from the visual pigment. Subsequently, these opsin subfamilies have expanded and diversified in a lineage-specific manner both in vertebrates (e.g., fishes [Hofmann et al. 1986]) and arthropod invertebrates, such as insects (Briscoe 2001; Frentiu et al. 2009), resulting in color perception within many of these lineages (e.g., Dulai et al. These types of eyes are the result of as many as 13 separate evolutionary origins (von Salvini-Plawen 2008). However, many lineages have lost the ability to byssally attach and are free-living species capable of swimming (some Limidae and Pectinidae) or, on the other end of the spectrum, are permanently attached to a substrate through cementation (e.g., Spondylidae and Ostreidae).Thus, changes in specific amino-acid sequences can alter (or tune) the spectral sensitivity of the visual pigment (Yokoyama 2000, 2002; Hunt et al. In this way, divergence of sequences after duplication of an opsin gene is an important mechanism that increases the coverage of the light spectrum perceived by the animal. In addition to the morphological diversity of eyes among molluscan species, a similar level of diversity in photoreceptive structures can occur within a single individual. An increase in mobility may be associated with the origin of eyes, but this association has not been formally tested.The opsin family has gone through multiple rounds of duplication early in metazoans and has generated three to five different opsin lineages that are defined, in part, by interaction with a specific trimeric G-protein (Terakita 2005; Plachetzki et al. For example, individuals at different life stages might utilize dissimilar photoreceptor organs (Hodgson and Burke 1988; Blumer 1999) or have different photoreceptive ability (Groeger et al. These qualities suggest the potential for a high level of diversity in visual pigments, making molluscs a rich system for studying the evolution of opsin. Within the Pteriomorphia is the large order Pectinoida (Waller 1978) (as ammended by Matsumoto 2003), which comprises deep-water Propeamussidae, swimming Limidae, cementing Spondylidae, and the most ecologically-diverse and species-rich group, the scallops (Pectinidae).2004; Saavedra and Peña 2006; Puslednik and Serb 2008) and for the evolution of life habits (Alejandrino et al.2011), providing a much-needed scaffold for examining the evolution of eyes and their molecular components within this group.Scallops (Pectinidae) are a promising molluscan model to study the evolution of opsin and its potential role in speciation.Recently, we discovered a second Gq-coupled, or r-, opsin gene expressed in the eyes of two scallop species.
Our results are congruent with a Gq-opsin gene duplication in an oyster-Pectinoida ancestor, approximately 470 Mya, and suggest the likelihood of retaining both genes is associated with either the presence of eyes and/or degree of mobility.As a consequence of these two phototransduction systems, each retina has a distinct physiological response to the presence (proximal cells depolarize) or absence (distal cells hyperpolarize) of light (reviewed in Wilkens 2006). This was a surprising result because previous workers identified a single Gq-opsin expressed in the scallop’s proximal retina (Kojima et al.It is generally accepted that scallops use their eyes to perceive movement (Land 1966), presumably for detection of predators (Wilkens 1981; Nilsson 1994; Morton 2000). 1997), whereas other molluscs, such as cephalopods, have only one copy of this type of opsin (e.g., Mathger et al. Using a gene-targeting approach, we examined how common these gene copies are across the scallop phylogeny and whether three closely related bivalve families within the order Pectinoida have multiple copies.This work thus expands on our knowledge of the evolution of opsin within bivalves, demonstrating that bivalves have a greater level of opsin diversity than previously thought.We examined eight species of scallop and a species from each of the three closely related families (Spondylidae, Limidae, and Propeamussidae) based on published work (Puslednik and Serb 2008; Alejandrino et al. Ethanol-preserved tissues from the Pectinoida were obtained from museum collections or provided by colleagues (Table 1).