The origin and diversification of a novel protein family in venomous snakes.

The genetic origins of novelty are a central interest of evolutionary biology. Most new proteins evolve from preexisting proteins but the evolutionary path from ancestral gene to novel protein is challenging to trace, and therefore the requirements for and order of coding sequence changes, expression changes, or gene duplication are not clear. Snake venoms are important novel traits that are comprised of toxins derived from several distinct protein families, but the genomic and evolutionary origins of most venom components are not understood. Here, we have traced the origin and diversification of one prominent family, the snake venom metalloproteinases (SVMPs) that play key roles in subduing prey in many vipers. Genomic analyses of several rattlesnake (Crotalus) species revealed the SVMP family massively expanded from a single, deeply conserved adam28 disintegrin and metalloproteinase gene, to as many as 31 tandem genes in the Western Diamondback rattlesnake (Crotalus atrox) through a number of single gene and multigene duplication events. Furthermore, we identified a series of stepwise intragenic deletions that occurred at different times in the course of gene family expansion and gave rise to the three major classes of secreted SVMP toxins by sequential removal of a membrane-tethering domain, the cysteine-rich domain, and a disintegrin domain, respectively. Finally, we show that gene deletion has further shaped the SVMP complex within rattlesnakes, creating both fusion genes and substantially reduced gene complexes. These results indicate that gene duplication and intragenic deletion played essential roles in the origin and diversification of these novel biochemical weapons.

Gain of cis-regulatory activities underlies novel domains of wingless gene expression in Drosophila.

Changes in gene expression during animal development are largely responsible for the evolution of morphological diversity. However, the genetic and molecular mechanisms responsible for the origins of new gene-expression domains have been difficult to elucidate. Here, we sought to identify molecular events underlying the origins of three novel features of wingless (wg) gene expression that are associated with distinct pigmentation patterns in Drosophila guttifera. We compared the activity of cis-regulatory sequences (enhancers) across the wg locus in D. guttifera and Drosophila melanogaster and found strong functional conservation among the enhancers that control similar patterns of wg expression in larval imaginal discs that are essential for appendage development. For pupal tissues, however, we found three novel wg enhancer activities in D. guttifera associated with novel domains of wg expression, including two enhancers located surprisingly far away in an intron of the distant Wnt10 gene. Detailed analysis of one enhancer (the vein-tip enhancer) revealed that it overlapped with a region controlling wg expression in wing crossveins (crossvein enhancer) in D. guttifera and other species. Our results indicate that one novel domain of wg expression in D. guttifera wings evolved by co-opting pre-existing regulatory sequences governing gene activity in the developing wing. We suggest that the modification of existing enhancers is a common path to the evolution of new gene-expression domains and enhancers.

Body plan innovation in treehoppers through the evolution of an extra wing-like appendage.

Body plans, which characterize the anatomical organization of animal groups of high taxonomic rank, often evolve by the reduction or loss of appendages (limbs in vertebrates and legs and wings in insects, for example). In contrast, the addition of new features is extremely rare and is thought to be heavily constrained, although the nature of the constraints remains elusive. Here we show that the treehopper (Membracidae) 'helmet' is actually an appendage, a wing serial homologue on the first thoracic segment. This innovation in the insect body plan is an unprecedented situation in 250 Myr of insect evolution. We provide evidence suggesting that the helmet arose by escaping the ancestral repression of wing formation imparted by a member of the Hox gene family, which sculpts the number and pattern of appendages along the body axis. Moreover, we propose that the exceptional morphological diversification of the helmet was possible because, in contrast to the wings, it escaped the stringent functional requirements imposed by flight. This example illustrates how complex morphological structures can arise by the expression of ancestral developmental potentials and fuel the morphological diversification of an evolutionary lineage.

Evolutionary origin of a novel gene expression pattern through co-option of the latent activities of existing regulatory sequences.

Spatiotemporal changes in gene expression underlie many evolutionary novelties in nature. However, the evolutionary origins of novel expression patterns, and the transcriptional control elements ("enhancers") that govern them, remain unclear. Here, we sought to explore the molecular genetic mechanisms by which new enhancers arise. We undertook a survey of closely related Drosophila species to identify recently evolved novel gene expression patterns and traced their evolutionary history. Analyses of gene expression in a variety of developing tissues of the Drosophila melanogaster species subgroup revealed high rates of expression pattern divergence, including numerous evolutionary losses, heterochronic shifts, and expansions or contractions of expression domains. However, gains of novel expression patterns were much less frequent. One gain was observed for the Neprilysin-1 (Nep1) gene, which has evolved a unique expression pattern in optic lobe neuroblasts of Drosophila santomea. Dissection of the Nep1 cis-regulatory region localized a newly derived optic lobe enhancer activity to a region of an intron that has accumulated a small number of mutations. The Nep1 optic lobe enhancer overlaps with other enhancer activities, from which the novel activity was co-opted. We suggest that the novel optic lobe enhancer evolved by exploiting the cryptic activity of extant regulatory sequences, and this may reflect a general mechanism whereby new enhancers evolve.

Repeated morphological evolution through cis-regulatory changes in a pleiotropic gene.

The independent evolution of morphological similarities is widespread. For simple traits, such as overall body colour, repeated transitions by means of mutations in the same gene may be common. However, for more complex traits, the possible genetic paths may be more numerous; the molecular mechanisms underlying their independent origins and the extent to which they are constrained to follow certain genetic paths are largely unknown. Here we show that a male wing pigmentation pattern involved in courtship display has been gained and lost multiple times in a Drosophila clade. Each of the cases we have analysed (two gains and two losses) involved regulatory changes at the pleiotropic pigmentation gene yellow. Losses involved the parallel inactivation of the same cis-regulatory element (CRE), with changes at a few nucleotides sufficient to account for the functional divergence of one element between two sibling species. Surprisingly, two independent gains of wing spots resulted from the co-option of distinct ancestral CREs. These results demonstrate how the functional diversification of the modular CREs of pleiotropic genes contributes to evolutionary novelty and the independent evolution of morphological similarities.

Chance caught on the wing: cis-regulatory evolution and the origin of pigment patterns in Drosophila.

The gain, loss or modification of morphological traits is generally associated with changes in gene regulation during development. However, the molecular bases underlying these evolutionary changes have remained elusive. Here we identify one of the molecular mechanisms that contributes to the evolutionary gain of a male-specific wing pigmentation spot in Drosophila biarmipes, a species closely related to Drosophila melanogaster. We show that the evolution of this spot involved modifications of an ancestral cis-regulatory element of the yellow pigmentation gene. This element has gained multiple binding sites for transcription factors that are deeply conserved components of the regulatory landscape controlling wing development, including the selector protein Engrailed. The evolutionary stability of components of regulatory landscapes, which can be co-opted by chance mutations in cis-regulatory elements, might explain the repeated evolution of similar morphological patterns, such as wing pigmentation patterns in flies.

DrosoPhyla: Resources for Drosophilid Phylogeny and Systematics.

The vinegar fly Drosophila melanogaster is a pivotal model for invertebrate development, genetics, physiology, neuroscience, and disease. The whole family Drosophilidae, which contains over 4,400 species, offers a plethora of cases for comparative and evolutionary studies. Despite a long history of phylogenetic inference, many relationships remain unresolved among the genera, subgenera, and species groups in the Drosophilidae. To clarify these relationships, we first developed a set of new genomic markers and assembled a multilocus data set of 17 genes from 704 species of Drosophilidae. We then inferred a species tree with highly supported groups for this family. Additionally, we were able to determine the phylogenetic position of some previously unplaced species. These results establish a new framework for investigating the evolution of traits in fruit flies, as well as valuable resources for systematics.

Extremely Divergent Haplotypes in Two Toxin Gene Complexes Encode Alternative Venom Types within Rattlesnake Species.

Natural selection is generally expected to favor one form of a given trait within a population. The presence of multiple functional variants of traits involved in activities such as feeding, reproduction, or the defense against predators is relatively uncommon within animal species. The genetic architecture and evolutionary mechanisms underlying the origin and maintenance of such polymorphisms are of special interest. Among rattlesnakes, several instances of the production of biochemically distinct neurotoxic or hemorrhagic venom types within the same species are known. Here, we investigated the genetic basis of this phenomenon in three species and found that neurotoxic and hemorrhagic individuals of the same species possess markedly different haplotypes at two toxin gene complexes. For example, neurotoxic and hemorrhagic Crotalus scutulatus individuals differ by 5 genes at the phospholipase A2 (PLA2) toxin gene complex and by 11 genes at the metalloproteinase (MP) gene complex. A similar set of extremely divergent haplotypes also underlies alternate venom types within C. helleri and C. horridus. We further show that the MP and PLA2 haplotypes of neurotoxic C. helleri appear to have been acquired through hybridization with C. scutulatus-a rare example of the horizontal transfer of a potentially highly adaptive suite of genes. These large structural variants appear analogous to immunity gene complexes in host-pathogen arms races and may reflect the impact of balancing selection at the PLA2 and MP complexes for predation on different prey.

The Deep Origin and Recent Loss of Venom Toxin Genes in Rattlesnakes.

The genetic origin of novel traits is a central but challenging puzzle in evolutionary biology. Among snakes, phospholipase A2 (PLA2)-related toxins have evolved in different lineages to function as potent neurotoxins, myotoxins, or hemotoxins. Here, we traced the genomic origin and evolution of PLA2 toxins by examining PLA2 gene number, organization, and expression in both neurotoxic and non-neurotoxic rattlesnakes. We found that even though most North American rattlesnakes do not produce neurotoxins, the genes of a specialized heterodimeric neurotoxin predate the origin of rattlesnakes and were present in their last common ancestor (∼22 mya). The neurotoxin genes were then deleted independently in the lineages leading to the Western Diamondback (Crotalus atrox) and Eastern Diamondback (C. adamanteus) rattlesnakes (∼6 mya), while a PLA2 myotoxin gene retained in C. atrox was deleted from the neurotoxic Mojave rattlesnake (C. scutulatus; ∼4 mya). The rapid evolution of PLA2 gene number appears to be due to transposon invasion that provided a template for non-allelic homologous recombination.

Stepwise modification of a modular enhancer underlies adaptation in a Drosophila population.

The evolution of cis regulatory elements (enhancers) of developmentally regulated genes plays a large role in the evolution of animal morphology. However, the mutational path of enhancer evolution--the number, origin, effect, and order of mutations that alter enhancer function--has not been elucidated. Here, we localized a suite of substitutions in a modular enhancer of the ebony locus responsible for adaptive melanism in a Ugandan Drosophila population. We show that at least five mutations with varied effects arose recently from a combination of standing variation and new mutations and combined to create an allele of large phenotypic effect. We underscore how enhancers are distinct macromolecular entities, subject to fundamentally different, and generally more relaxed, functional constraints relative to protein sequences.