4 research outputs found
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Fin ray patterns at the fin-to-limb transition
The fin-to-limb transition was marked by the origin of digits and the loss of dermal fin rays. Paleontological research into this transformation has focused on the evolution of the endoskeleton, with little attention paid to fin ray structure and function. To address this knowledge gap, we study the dermal rays of the pectoral fins of 3 key tetrapodomorph taxa—Sauripterus taylori (Rhizodontida), Eusthenopteron foordi (Tristichopteridae), and Tiktaalik roseae (Elpistostegalia)—using computed tomography. These data show several trends in the lineage leading to digited forms, including the consolidation of fin rays (e.g., reduced segmentation and branching), reduction of the fin web, and unexpectedly, the evolution of asymmetry between dorsal and ventral hemitrichia. In Eusthenopteron, dorsal rays cover the preaxial endoskeleton slightly more than ventral rays. In Tiktaalik, dorsal rays fully cover the third and fourth mesomeres, while ventral rays are restricted distal to these elements, suggesting the presence of ventralized musculature at the fin tip analogous to a fleshy “palm.” Asymmetry is also observed in cross-sectional areas of dorsal and ventral rays. Eusthenopteron dorsal rays are slightly larger than ventral rays; by contrast, Tiktaalik dorsal rays can be several times larger than ventral rays, and degree of asymmetry appears to be greater at larger sizes. Analysis of extant osteichthyans suggests that cross-sectional asymmetry in the dermal rays of paired fins is plesiomorphic to crown group osteichthyans. The evolution of dermal rays in crownward stem tetrapods reflects adaptation for a fin-supported elevated posture and resistance to substrate-based loading prior to the origin of digits
Expression of Transposable Elements in Neural Tissues during Xenopus Development
Transposable elements comprise a large proportion of animal genomes. Transposons can have detrimental effects on genome stability but also offer positive roles for genome evolution and gene expression regulation. Proper balance of the positive and deleterious effects of transposons is crucial for cell homeostasis and requires a mechanism that tightly regulates their expression. Herein we describe the expression of DNA transposons of the Tc1/mariner superfamily during Xenopus development. Sense and antisense transcripts containing complete Tc1-2_Xt were detected in Xenopus embryos. Both transcripts were found in zygotic stages and were mainly localized in Spemann's organizer and neural tissues. In addition, the Tc1-like elements Eagle, Froggy, Jumpy, Maya, Xeminos and TXr were also expressed in zygotic stages but not oocytes in X. tropicalis. Interestingly, although Tc1-2_Xt transcripts were not detected in Xenopus laevis embryos, transcripts from other two Tc1-like elements (TXr and TXz) presented a similar temporal and spatial pattern during X. laevis development. Deep sequencing analysis of Xenopus tropicalis gastrulae showed that PIWI-interacting RNAs (piRNAs) are specifically derived from several Tc1-like elements. The localized expression of Tc1-like elements in neural tissues suggests that they could play a role during the development of the Xenopus nervous system
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Functional and comparative morphology of the pectoral fins among scorpaeniform fishes
Many benthic fishes use their pectoral fins for both swimming and substrate contact. Each behavior imposes different, potentially conflicting functional demands on the pectoral fins. It has been hypothesized that benthic fish decouple these demands through morphological and functional regionalization of the fins, but this hypothesis had not been experimentally tested. In this study, I demonstrated functional regionalization of the pectoral fin in the longhorn sculpin, Myoxocephalus octodecimspinosus, a benthic scorpaeniform fish. The ventral region of the pectoral fin is used during substrate contact; the dorsal region is used during swimming. I characterize a previously undescribed swimming behavior during which the pectoral fins are held in a steady, laterally extended posture while the dorsal, anal and caudal fins are used for propulsion. I also investigate the functional morphology of individual pectoral fin rays. I find that the curvature of the fin rays is higher in the ventral versus dorsal region of the fin, regardless of behavior. The fin rays of longhorn sculpin possess a previously undescribed morphology in cross-section. Specifically, the hemitrichia, or bony halves that make up the fin rays, are cylindrical in cross-section proximally and crescent-shaped distally. The cylindrical segments in the proximal regions of the fin rays of longhorn sculpin provide resistance to bending, which may confer strength and support for weight-bearing substrate-contact behaviors. Further, the ventral fin rays, which are used for substrate contact, have a higher proportion of their total length made up of cylindrical segments. Finally, I used a comparative approach to examine the distribution of morphological specializations and habitat (benthic or pelagic) of the pectoral fins among scorpaeniform fishes. I identified three discrete characters of the pectoral fins that are associated with a benthic habitat for analysis; (1) an asymmetrical fin membrane among ventral fin rays, (2) the presence of fin rays that are unbranched distally, and (3) the presence of ventral pectoral fin rays that are free of fin membrane. Each of these characters has evolved more than once among scorpaeniform fishes and is more common among benthic than pelagic fishes