Fish fins are one of the most diversified organs in the vertebrates. These structures acquire various functions and facilitate adaptations to various habitats. The free parts of teleost fins are commonly supported by bone structures called “fin rays,” which are further divided into “soft rays” and “spiny rays”. Soft rays are flexible, segmented and mostly branching, whereas spiny rays are stiff, unsegmented, fused of two half-segments (hemitrichia), and which terminate in an acute point. They have evolved independently in several teleost lineages. Spiny rays of acanthomorph fishes are called a “true spine,” and these structures extremely modified in several clades. For example, the sucking disc of remoras has “pectinated lamellae,” formed by bilateral extensions of the base of spiny rays. Anglerfish have an “illicium,” in which spiny rays are modified into and serve as a fishing apparatus. The diversity of spiny ray morphology has attracted considerable morphological, and evolutionary interest. Thus, it is important to understand the underlying mechanism(s) by which these spiny rays form and enable their diversification in some Acanthomorpha lineages.
Developmental mechanism of spiny ray is unknown. To elucidate the mechanism, we should examine the morphogenesis using laboratory animals at the levels of cellular and molecular developmental biology. However, popular model fish, zebrafish and medaka, lack spiny rays and do not suitable for studies of spiny rays. Thus, we used the dwarf neon rainbowfish Melanotaenia praecox, a small freshwater fish within the order atheriniformes native to northern New Guinea, as a new model fish to study the spiny ray ontogeny. This species has spiny rays in their dorsal, anal, pectoral, and pelvic fins. In our previous studies, the post embryonic staging of this species and microinjection procedure for genetic engineering were established.
In this report, we examined cell and extracellular matrix dynamics in the process of the spiny rays development of M. praecox. Surprisingly, we found that actinotrichia, which is known as an important ECM structure in zebrafish fin ray growth, are not play a role in spiny ray development. Furthermore, we describe the distribution of the mesenchymal cells, ECM structures and gene expressions of spiny rays in detail. We found that there are the aggregation of the mesenchymal cells and thickened ECM, such as Laminin, layer surrounding the tip of the spiny ray. From our results, we conclude that soft and spiny rays have greatly different morphogenetic processes. Taken together, we speculate that this unique morphogenesis of the spiny rays may facilitate their evolvability and diversification.