Abstract Skeletal forms in vertebrates have been regarded as good models of morphological diversification. Fish fins show great diversity in form, with their supporting skeletal structure being classified into soft rays and spiny rays. In fish evolution, spiny-ray morphologies are known to be sometimes extremely modified; however, it remains unknown how the developmental mechanisms of spiny rays have contributed to their morphological diversification. By using the rainbowfish Melanotaenia praecox for examination of the extracellular matrix (ECM) and cell dynamics of spiny-ray development, we demonstrate that spiny-ray development is independent of the actinotrichia (needle-shaped collagen polymers at the tip of fins), which are known as an important ECM in soft-ray morphogenesis. Furthermore, we found that in the thorny spiny ray of the filefish Stephanolepis cirrhifer , the lateral protrusions are associated with BMP-positive osteoblast condensation, as in the spiny-ray tips in M. praecox and S. cirrhifer . Taken together, our findings reveal that osteoblast distribution and signaling-molecule intensity would contribute to spiny-ray modification. In comparison to soft ray development, the independence from actinotrichia in spiny rays would facilitate growth direction change, leading to their morphological diversification. This suggests that variation in cell distribution and ECM usage may be important contributors to morphological diversification, not only in Acanthomorpha, but also in other animal taxa.

SUMMARY The orientation and distribution of fibrillar collagen are critical determinants of the shape and mechanical properties of bones and organs. ^1–3 However, how they are spatially organized within tissues is still poorly understood, ^4–7 as visualizing these collagen architectures remains challenging. Actinotrichia (AT), the spear-shaped fibrillar collagen structures located at the distal tips of fish fins, are easily observable due to their large size and distinctive morphology ^8–14 and have recently emerged as a model system for studying collagen fiber organization. ^15–19 In this study, we generated knockout lines for the fish-specific extracellular matrix (ECM) genes actinodin1 and actinodin2 ( and1/2 ), which are lost in tetrapods. ¹² Loss of these genes dramatically altered the orientation of collagen fibers, thereby inducing changes in fin morphology. In the wild-type fins, AT are orderly arranged beneath the epidermis, forming layers parallel to the fin surface, and their individual fibers radiate distally toward the fin tip. In contrast, double knockout (dKO) of and1/2 results in overall fin reduction accompanied by increased thickness. Examination of the collagen structure distribution revealed the presence of aberrant collagen fibers oriented perpendicular to the fin epidermis. Moreover, the vertically oriented fibers contributed to thickening of the mesenchymal region in which they were distributed. The number of abnormal fibers increased with the severity of and1/2 deficiency, suggesting that collagen fibers in fins inherently tend to align perpendicular to the epidermis when these genes are absent. Furthermore, in tetrapods lacking the and gene family—specifically amphibians, the tetrapod group most closely related to fish ²⁰ —examination of the developing limb, the organ homologous to paired fins in fish, ²¹ revealed collagen fibers oriented perpendicular to the epidermis. The distribution pattern also resembled that observed in the fin buds of and1/2 dKO fish. Together, these findings highlight collagen patterning alterations as a previously unrecognized factor contributing to the evolutionary divergence between thinned fins and thickened limbs. Moreover, the identification of mutants that dramatically alter collagen fiber orientation is unprecedented, suggesting that analysis of Actinodin (And) function unveil the mechanisms underlying collagen matrix formation. ^22–28

<jats:title>Abstract</jats:title><jats:p>The median fins of modern fish that show discrete forms (dorsal, anal, and caudal fins) are derived from a continuous fold-like structure, both in ontogeny and phylogeny. The median fin fold (MFF) hypothesis assumes that the median fins evolved by reducing some positions in the continuous fin fold of basal chordates, based on the classical morphological observation of developmental reduction in the larval fin folds of living fish. However, the developmental processes of median fins are still unclear at the cellular and molecular levels. Here, we describe the transition from the larval fin fold into the median fins in zebrafish at the cellular and molecular developmental level. We demonstrate that reduction does not play a role in the emergence of the dorsal fin primordium. Instead, the reduction occurs along with body growth after primordium formation, rather than through actively scrapping the non-fin forming region by inducing cell death. We also report that the emergence of specific mesenchymal cells and their proliferation promote dorsal fin primordium formation. Based on these results, we propose a revised hypothesis for median fin evolution in which the acquisition of de novo developmental mechanisms is a crucial evolutionary component of the discrete forms of median fins.</jats:p>