Evidence for neutral and selective processes in the recruitment of enzyme-crystallins in avian lenses.

Abstract

In apparent contrast to most other tissues, the ocular lenses in vertebrates show striking differences in protein composition between taxa, most notably in the recruitment of different enzymes as major structural proteins. This variability appears to be the result ofat least partially neutral evolutionary processes, although there is also evidence for selective modification in molecular structure. Here we describe a bird, the chimney swift (Chaetura pelagica), that lacks 8-crystallin/ argininosuccinate lyase, usually the major crystallin of avian lenses. Clearly, 6-crystallin is not specifically required for a functionally effective avian lens. Furthermore the lens composition of the swift is more similar to that of the related hummingbirds than to that of the barn swallow (Hirundo rustica), suggesting that phylogeny is more important than environmental selection in the recruitment of crystallins. However differences in e-crystallin/lactate dehydrogenase-B sequence between swift and hummingbird and other avian and reptilian species suggest that selective pressures may also be working at the molecular level. These differences also confirm the close relationship between swifts and hummingbirds.In many vertebrate species recruited enzymes comprise a large fraction of the crystallins, the soluble, structural proteins of the lens (1, 2). This phenomenon, a form of genesharing or protein multifunctionality, has wide implications for the processes of protein evolution and the mechanisms of differential gene expression. It also raises questions about the origins and roles of specific crystallins, why particular proteins were recruited as crystallins, and how important they are for correct lens development and function. Here we show that the expression of S-crystallin/argininosuccinate Iyase (ASL) (3) varies widely in avian lenses and appears to be more dependent on phylogeny than on any obvious functional or environmental selection. We have shown previously that two S-crystallin genes are abundantly expressed in the embryonic duck lens (refs. 4, 5; G.W. and J.P., unpublished data), whereas only one gene contributes significantly to 8-crystallin in the chicken lens (6, 7). Now we describe an avian species, the chimney swift (Chaetura pelagica), a fast-flying insectivore, that has no detectable lens expression of 8-crystallin at all, although its lens does contain abundant E-crystallin/lactate dehydrogenase B (LDH-B) (8). The hummingbirds are thought to be most closely related to the swifts in spite of marked superficial differences, most notably in feeding behavior. We have found that Anna's hummingbird (Calypte anna) does express detectable 8-crystallin but at a rather low level and also has extremely abundant e-crystallin. In contrast, the more distantly related barn swallow (Hirundo rustica), another fast-flying insectivore with similar feeding habits to the swift, has a quite different lens crystallin composition, with a more typical level of 6-crystallin and low or absent e-crystallin. Surprisingly, moreover...

Generation of evolutionary novelty by functional shift

“…1A, step 5) and that such conflict can be resolved by reversion of the co-option or by gene duplication. (27,28) Several metabolic enzymes have been recruited independently to work as crystallins in the lens of many vertebrate species without previous duplication of the gene. The protein generally retains both functions, and there are consequences to this retention.…”

“…(25) Raff (1) considers co-option as one of his principles of evolvability, defined as the elements that make it possible for ontogenies to evolve. Although the process of co-option also has been referred to as recruitment in the evolutionary developmental biology literature, (8,(26)(27)(28) we will retain the term co-option for historic reasons, for it was the first term used explicitly to designate an evolutionary process, (21) and it is used as a principle of evolvability by Raff. (1) …”

Evolution of antifreeze glycoprotein gene from a trypsinogen gene in Antarctic notothenioid fish

“…The antifreeze peptides (type II AFPs) from the sea raven, herring, and smelt share partial protein sequence identity with the carbohydrate recognition domain of C-type lectins or similar domains in lectin-like proteins from other organisms, suggesting evolutionary relatedness (26). Lectins in type II AFP-bearing fishes have not been characterized, and if they indeed gave rise to the type II AFPs in these fishes, it would represent an example of gene recruitment and expression of the same or a very similar protein to perform a different function, much like the recruitment of cellular enzyme genes such as lactate dehydrogenase and others, and express them at high levels to form the lens crystallins (27)(28)(29)(30). Evolution of notothenioid AFGP genes represents another evolutionary innovationrecruitment of segments of an existing protein gene plus de novo amplification of a short DNA sequence to spawn a novel protein with a new function.…”

The Recruitment of Crystallins: New Functions Precede Gene Duplication

“…This may lead to sequence modifications that enhance its function as a crystallin but that are not of benefit to the enzymatic role. This seems to have occurred in LDH-B/E-crystallin in some species (4,11). In some cases this could lead to an adaptive conflict that might be resolved either by reversion, with loss of crystallin expression, as illustrated by the loss of 6-crystallin in swifts (11) and the absence of e-crystallin in chickens and other birds (4), or by gene duplication and separation of function (Fig.…”