Fin to limb within our grasp

Trends and variation in vertebrate patterns as outcomes of self-organization

In extant vertebrates, natural motifs such as coat markings, spongy bone structures, neuronal arborescence or collective swarms correspond to diverse pattern types, from fractals to periodic stripes or tessellations, and so on. In this subphylum, evolution produced an apparent paradox: a given pattern may vary tremendously in its extent, periodicity or regularity, but follows general geometrical trends and is produced with meticulous precision. In this review, we discuss the role of self-organization, a patterning strategy in which spontaneous order arises through local interactions without gradient formation, in shaping both natural pattern differences and common themes. Mathematical models evidenced a wide high adaptability of self-organizing dynamics, long-advocating for their contribution to natural pattern diversity. Recent empirical and theoretical approaches taking into account network topologies and natural variation also replaced outcomes of self-organization in more constrained biological contexts, shedding light on mechanisms ensuring pattern fidelity.

Development of zebrafish paired and median fin musculature: basis for comparative, developmental, and macroevolutionary studies

The model organism Dario rerio (zebrafish) is widely used in evo-devo and comparative studies. Nevertheless, little is known about the development and differentiation of the appendicular musculature in this fish. In this study, we examined the development of the muscles of all five zebrafish fin types (pectoral, pelvic, anal, dorsal and caudal). We describe the development of the muscles of these fins, including some muscles that were never mentioned in the literature, such as the interhypurales of the caudal fin. Interestingly, these caudal muscles are present in early stages but absent in adult zebrafishes. We also compare various stages of zebrafish fin muscle development with the configuration found in other extant fishes, including non-teleostean actinopterygians as well as cartilaginous fishes. The present work thus provides a basis for future developmental, comparative, evolutionary and evo-devo studies and emphasizes the importance of developmental works on muscles for a more comprehensive understanding of the origin, development and evolution of the appendicular appendages of vertebrate animals.

Comparative anatomy of zebrafish paired and median fin muscles: basis for functional, developmental, and macroevolutionary studies

In the last decades, Danio rerio became one of the most used model organisms in various evo-devo studies devoted to the fin skeletal anatomy and fin-limb transition. Surprisingly, there is not even a single paper about the detailed anatomy of the adult muscles of the five fin types of this species. To facilitate more integrative developmental, functional, genetic, and evolutionary studies of the appendicular musculoskeletal system of the zebrafish and to provide a basis for further comparisons with other fishes and tetrapods, we describe here the identity, overall configuration, and attachments of appendicular muscles in a way that can be easily understood and implemented by non-anatomist researchers. We show that the muscle pattern of the caudal fin is very different from patterns seen in other fins but is very consistent within teleosts. Our observations support the idea of the developmental and evolutionary distinction of the caudal fin and point out that the musculature of the adult zebrafish pectoral and pelvic fins is in general very similar. Both paired fins have superficial and deep layers of abductors and adductors going to all/most rays plus the dorsal and ventral arrectors going only to the first ray. Nevertheless, we noted three major differences between the pelvic and pectoral fins of adult zebrafishes: (i) the pectoral girdle lacks a retractor muscle, which is present in the pelvic girdle - the retractor ischii; (ii) the protractor of the pelvic girdle is an appendicular/trunk muscle, while that of the pectoral girdle is a branchiomeric muscle; (iii) the first ray of the pectoral fin is moved by an additional arrector-3. The anal and dorsal fins consist of serially repeated units, each of which comprises one half-ray and three appendicular muscles (one erector, depressor, and inclinator) on each side of the body. The outermost rays are attachment points for the longitudinal protractor and retractor. Based on our results, we discuss whether the pectoral appendage might evolutionarily be closer to the head than to the pelvic appendage and whether the pelvic appendage might have been derived from the trunk/median fins. We discuss a hypothesis of paired fin origin that is a hybrid of the fin-fold and Gegenbaur's theories. Lastly, our data indicate that D. rerio is indeed an appropriate model organism for the appendicular musculature of teleosts in particular and, at least in the case of the paired fins, also of actinopterygians as a whole.