Evolution of median fin modules in the axial skeleton of fishes

Abstract

Detailed examples of how hierarchical assemblages of modules change over time are few. We found broadly conserved phylogenetic patterns in the directions of development within the median fins of fishes. From these, we identify four modules involved in their positioning and patterning. The evolutionary sequence of their hierarchical assembly and secondary dissociation is described. The changes in these modules during the evolution of fishes appear to be produced through dissociation, duplication and divergence, and co-option. Although the relationship between identified median fin modules and underlying mechanisms is unclear, Hox addresses may be correlated. Comparing homologous gene expression and function in various fishes may test these predictions.The earliest actinopterygians likely had dorsal and anal fins that were symmetrically positioned via a positioning module. The common patterning (differentiation) of skeletal elements within the dorsal and anal fins may have been set into motion by linkage to this positioning module. Frequent evolutionary changes in dorsal and anal fin position indicate a high level of dissociability of the positioning module from the patterning module. In contrast, the patterning of the dorsal and anal fins remains linked: In nearly all fishes, the endo- and exoskeletal elements of the two fins co-differentiate. In all fishes, the exoskeletal fin rays differentiate in the same directions as the endoskeletal supports, indicating complete developmental integration. In acanthopterygians, a new first dorsal fin module evolved via duplication and divergence. The median fins provide an example of how basic modularity is maintained over 400 million years of evolution.

Postembryonic staging of wild‐type goldfish, with brief reference to skeletal systems

“…18E). With regard to the nomenclature problem of the “spine,” as indicated by Mabee et al (2002), these serrated and segmented “spine‐like” fin rays in goldfish differ from the true bone spines observed in Acanthopterygii (spiny finned) species; true spines are not segmented, in contrast to goldfish “spines”. Thus, to avoid confusion, we have designated these goldfish spine‐like fin rays as “segmented fin spines” (sfs in Fig.…”

“…Of the internal skeletal structures of the pectoral girdle, the cleithrum can be observed at early stages (Kimmel et al, 1995; Grandel and Schulte‐Merker, 1998; Du et al, 2001; Mabee et al, 2002). By the prot stage at the latest, this skeleton can be observed as calcified tissues (Fig.…”

“…Furthermore, segmented fin spines are absent in zebrafish (Mabee et al, 2002). Thus, we first consider the ontogenetic appearance of the scales, anal and dorsal fins, and the segmented fin spines, to highlight the dissimilarities between zebrafish and goldfish at postembryonic stages.…”

“…Although the details of the divergence of their ontogenetic sequence remain unknown, it is assumed that the ontogenetic sequences may reflect the differences in morphology and relative size of the median fins between goldfish and zebrafish (Fig. 2; Mabee et al, 2002; Parichy et al, 2009). …”

“…Comparative studies of ontogenetic process require not only embryonic, but also postembryonic staging tables, as several important morphogenetic processes which form characteristic features of each species (for example, internal and external skeletal structures and scale patterns) are known to proceed during postembryonic stages (Arratia et al, 1990, 1991; Cubbage and Mabee, 1996; Schilling and Kimmel, 1997; Van der Heyden and Huyssune, 2000; Van der Heyden et al, 2000; Witten et al, 2001; Yelick and Schilling, 2002; Mabee et al, 2002; Bird and Mabee, 2003; Sire and Huysseune, 2003; Webb and Shirey, 2003; Elizondo et al, 2005; Sire and Akimenko, 2004; Thorsen and Hale, 2005; Patricia et al, 2007; Patterson et al, 2008; Parichy et al, 2009; Kimmel et al, 2010; Budi et al, 2011; Bensimon‐Brito et al, 2012). However, at present there is no widely available postembryonic developmental staging table for goldfish, in practice.…”