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Genetic and Developmental Origins of a Unique Foraging Adaptation in a Lake Malawi Cichlid Genus

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Genetic and Developmental Origins of a Unique Foraging Adaptation in a Lake Malawi Cichlid Genus Genetic and developmental origins of a unique foraging adaptation in a Lake Malawi cichlid genus Moira R. Conitha, Yinan Hub,c, Andrew J. Conithd, Maura A. Maginnisd, Jacqueline F. Webbb, and R. Craig Albertsond,1 aGraduate Program in Organismic and Evolutionary Biology, University of Massachusetts, Amherst, MA 01003; bDepartment of Biological Sciences, University of Rhode Island, Kingston, RI 02881; cBiology Department, Boston College, Chestnut Hill, MA 02467; and dDepartment of Biology, University of Massachusetts, Amherst, MA 01003 Edited by Hopi E. Hoekstra, HHMI and Harvard University, Cambridge, MA, and approved May 29, 2018 (received for review November 13, 2017) Phenotypic novelties are an important but poorly understood from model organisms, which are selected in part because of their category of morphological diversity. They can provide insights into conserved phenotypes (16, 17). Additionally, novel phenotypes of- the origins of phenotypic variation, but we know relatively little ten have ancient origins and/or occur in lineages that are not about their genetic origins. Cichlid fishes display remarkable diversity amenable to laboratory investigations. Here, we address the origin in craniofacial anatomy, including several novelties. One aspect of this of a novel phenotype by identifying the genetic mechanisms un- variation is a conspicuous, exaggerated snout that has evolved in derlying tissue-level changes of a unique craniofacial morphology in a single Malawi cichlid lineage and is associated with foraging a genus of cichlid from Lake Malawi, Africa. Cichlids are an iconic model system for the study of rapid and specialization and increased ecological success. We examined the – developmental and genetic origins for this phenotype and found that extensive craniofacial diversification (18 20). Within Lake Malawi, the snout is composed of two hypertrophied tissues: the intermax- the genus Labeotropheus exhibits extreme craniofacial anatomy that illary ligament (IML), which connects the right and left sides of the defines the boundary of cichlid morphospace (20). Among other extreme phenotypes, the two species in this genus possess an ex- upper jaw, and the overlying loose connective tissue. The IML is aggerated fleshy snout (e.g., Fig. 1A vs. Fig. 1B). We have previously present in all cichlids, but in its exaggerated form it interdigitates with shown that this elaborated snout folds in on itself, forming a flexible the more superficial connective tissue and anchors to the epithelium, flap that extends over the upper lip, sometimes reaching the distal forming a unique ligament–epithelial complex. We examined the β β → tip of the upper jaw dentition (21). It has been postulated that this Transforming growth factor (Tgf ) Scleraxis (Scx) candidate path- protuberance enhances foraging efficiency by acting as a fulcrum to way and confirmed a role for these factors in snout development. We crop algae off rocks by leverage as an alternative to the energetically β demonstrate further that experimental up-regulation of Tgf is suffi- costly bite-and-twist mode of benthic feeding common in other cient to produce an expansion of scx expression and concomitant cichlid species (22). In addition, prior genetic work suggests that this changes in snout morphology. Genetic and genomic mapping show trait is under directional selection (21). Labeotropheus is also one of that core members of canonical Tgfβ signaling segregate with quan- the most ecologically successful genera in the lake, as it has a cos- titative trait loci (QTL) for snout variation. These data also implicate a mopolitan distribution and dominates the near shore rocky habitat candidate for ligament development, adam12, which we confirm us- (23). Indeed, although this trait is rare relative to the number of ing the zebrafish model. Collectively, these data provide insights into species that possess it (i.e., 2 of over 800 cichlid species in Lake ligament morphogenesis, as well as how an ecologically relevant Malawi), it is well represented in the lake given the ubiquity of novelty can arise at the molecular level. Labeotropheus. Thus, as with novelties in general, the evolution of this putatively adaptive structure is coincident with a unique for- evo-devo | craniofacial | ligament | TGFβ | novelty aging mode and expanded ecological success. pecies with novel phenotypes, which define the extremes of a Significance Scollective morphospace, are a valuable resource for research across disciplines. In evolutionary biology, this phenotypic class can Biologists have long been captivated by novel traits because they provide insights into the origins and constraints on morphological provide insights into both the origin of and constraints on mor- evolution (1–4). Engineers have long used novel phenotypes as in- phological variation. The iconic adaptive radiations of cichlid fishes spiration for biology-based design and technology (5, 6). Such have led to incredible diversity of form, including some species phenotypes can also serve as “evolutionary mutant models” of with an exaggerated snout. This novelty is mechanically in- disease states, whereby the adaptive trait mimics a maladaptive tegrated with the upper jaw, appears to be under directional se- condition in humans, and potentially provides insights into the ge- lection, and is found in one of the most ecologically successful netic factors relevant for disease prediction and management (7). cichlid lineages. We used protein manipulation, gene expression, Finally, bizarre phenotypes serve to captivate the public and engage and genetic mapping to implicate the Tgfβ pathway in the de- them in the context of education (8). velopment of this unusual trait. Given the functions of Tgfβ sig- Novel phenotypes can arise as a dramatic reconfiguration of naling in tissue proliferation, migration, invasion, and organ the body plan (e.g., seahorses), from the elaboration of existing fibrosis, this represents an example of the cooption of existing structures (e.g., ref. 9) or as de novo structures with no obvious pathways in the evolution of novelty. precursor (e.g., ref. 10). Likewise, regulation at the molecular level can occur as the recruitment of existing genes and signaling Author contributions: M.R.C. and R.C.A. designed research; M.R.C., Y.H., A.J.C., M.A.M., EVOLUTION pathways (e.g., through changes in timing, location, or amount of J.F.W., and R.C.A. performed research; M.R.C., Y.H., A.J.C., J.F.W., and R.C.A. analyzed expression; refs. 9, 11, and 12) or by the evolution of new genes data; and M.R.C., A.J.C., and R.C.A. wrote the paper. (e.g., following gene/genome duplication events; refs. 13 and 14). The authors declare no conflict of interest. Unlike continuous phenotypic variation, novel morphologies This article is a PNAS Direct Submission. highlight the opportunistic and flexible nature of evolution to act This open access article is distributed under Creative Commons Attribution-NonCommercial- as both a tinkerer and an innovator (15). A major pursuit in NoDerivatives License 4.0 (CC BY-NC-ND). evolutionary developmental biology is to characterize the prox- 1To whom correspondence should be addressed. Email: [email protected]. imate genetic and developmental basis that can explain the ori- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. gin of novel phenotypes. However, our understanding remains 1073/pnas.1719798115/-/DCSupplemental. limited because most of our knowledge of development comes Published online June 18, 2018. www.pnas.org/cgi/doi/10.1073/pnas.1719798115 PNAS | July 3, 2018 | vol. 115 | no. 27 | 7063–7068 Downloaded by guest on September 27, 2021 We hypothesized that this unique structure develops as an ex- pansion of the intermaxillary ligament, which runs laterally across the upper jaw and connects the left and right maxillary heads (24). Given the dynamic movement in fish skulls (25, 26), this class of tissue (e.g., ligaments) is likely to play important roles in foraging adaptations. However, relative to the bony skeleton, the evolution of ligaments is grossly understudied. Here, we employ a combination of genetic and developmental approaches to test the hypothesis that elaboration of the intermaxillary ligament in Labeotropheus occurs via expansion of the Tgfβ → Scx candidate pathway (27–31). Through these analyses, we also identified a previously unknown candidate for ligament hypertrophy, adam12, and verified its roles in ligament development via functional analyses using the zebrafish model. Together these data support our main hypothesis and contribute to an understanding of the genetic regulation of complex soft tissue morphologies and, more generally, of how phenotypic novelties evolve. Results and Discussion Anatomy and Development of the Snout. The snout in Labeotropheus is a complex and dynamic soft tissue structure compared with Tropheops, a closely related near ecological competitor that lacks an exaggerated snout (Fig. 1 A and B). Histological data showed two distinct, but interdigitating, tissue types within the snout—aDirect Red-positive organized connective tissue high in collagen (pink, Fig. 1 C and D) and an Alcian Blue-positive loose connective tissue (blue, Fig. 1 C and D). In both species, this basic configuration is maintained across the medio-lateral expanse of the snout, but for consistency we focused our comparisons between species and treatments on midline sections of this structure. The anatomy and organization of the Direct
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