Significance How body pattern evolves in nature remains largely unknown. Although recent progress has been made on the molecular basis of losing morphological features during adaptation to new environments (regressive evolution), there are few well worked out examples of how morphological features may be gained in natural species (constructive evolution). Here we use genetic crosses to study how threespine stickleback fish have increased their tooth number in a new freshwater environment. Genetic mapping and gene expression experiments suggest regulatory changes have occurred in the gene for a bone morphogenetic signaling molecule, leading to increased expression in the freshwater fish that have more teeth. Our studies suggest that changes in gene regulation may underlie both gain and loss traits during vertebrate evolution.
Teeth are a classic model system of organogenesis, as repeated and reciprocal epithelial and mesenchymal interactions pattern placode formation and outgrowth. Less is known about the developmental and genetic bases of tooth formation and replacement in polyphyodonts, which are vertebrates with continual tooth replacement. Here, we leverage natural variation in the threespine stickleback fish Gasterosteus aculeatus to investigate the genetic basis of tooth development and replacement. We find that two derived freshwater stickleback populations have both convergently evolved more ventral pharyngeal teeth through heritable genetic changes. In both populations, evolved tooth gain manifests late in development. Using pulse-chase vital dye labeling to mark newly forming teeth in adult fish, we find that both high-toothed freshwater populations have accelerated tooth replacement rates relative to lowtoothed ancestral marine fish. Despite the similar evolved phenotype of more teeth and an accelerated adult replacement rate, the timing of tooth number divergence and the spatial patterns of newly formed adult teeth are different in the two populations, suggesting distinct developmental mechanisms. Using genome-wide linkage mapping in marine-freshwater F2 genetic crosses, we find that the genetic basis of evolved tooth gain in the two freshwater populations is largely distinct. Together, our results support a model whereby increased tooth number and an accelerated tooth replacement rate have evolved convergently in two independently derived freshwater stickleback populations using largely distinct developmental and genetic mechanisms.
Prior research finds that racial and ethnic minority students are more likely than White students to receive school punishments, yet little prior research considers students' perceptions of the fairness of school rules and their enforcement. Using data from a nationally representative survey of students, the authors consider whether African American and Latino and Latina students, and particularly males, perceive school safety practices as less fair overall, less well communicated, and less evenly applied than White students do. The authors also consider whether particular security strategies (security guards, metal detectors, and locker checks) impact these perceptions of fairness. The results suggest that African American students perceive less fairness and consistency of school rules and their enforcement than do White students, though the perceptions of Latino/a students do not vary significantly from those of White students.
Synovial joints are the lubricated connections between the bones of our body that are commonly affected in arthritis. It is assumed that synovial joints first evolved as vertebrates came to land, with ray-finned fishes lacking lubricated joints. Here, we examine the expression and function of a critical lubricating protein of mammalian synovial joints, Prg4/Lubricin, in diverse ray-finned fishes. We find that Prg4 homologs are specifically enriched at the jaw and pectoral fin joints of zebrafish, stickleback, and gar, with genetic deletion of the zebrafish prg4b gene resulting in the same age-related degeneration of joints as seen in lubricin-deficient mice and humans. Our data support lubricated synovial joints evolving much earlier than currently accepted, at least in the common ancestor of all bony vertebrates. Establishment of the first arthritis model in the highly regenerative zebrafish will offer unique opportunities to understand the aetiology and possible treatment of synovial joint disease.DOI: http://dx.doi.org/10.7554/eLife.16415.001
The ligands of the Bone Morphogenetic Protein (BMP) family of developmental signaling molecules are often under the control of complex cis-regulatory modules and play diverse roles in vertebrate development and evolution. Here, we investigated the cis-regulatory control of stickleback Bmp6. We identified a 190 bp enhancer ~2.5 kilobases 5’ of the Bmp6 gene that recapitulates expression in developing teeth and fins, with a core 72 bp sequence that is sufficient for both domains. By testing orthologous enhancers with varying degrees of sequence conservation from outgroup teleosts in transgenic reporter gene assays in sticklebacks and zebrafish, we found that the function of this regulatory element appears to have been conserved for over 250 million years of teleost evolution. We show that a predicted binding site for the TGFβ effector Smad3 in this enhancer is required for enhancer function and that pharmacological inhibition of TGFβ signaling abolishes enhancer activity and severely reduces endogenous Bmp6 expression. Finally, we used TALENs to disrupt the enhancer in vivo and find that Bmp6 expression is dramatically reduced in teeth and fins, suggesting this enhancer is necessary for expression of the Bmp6 locus. This work identifies a relatively short regulatory sequence that is required for expression in multiple tissues and, combined with previous work, suggests that shared regulatory networks control limb and tooth development.
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