Temporal expression profiles of zebrafish actinin genes as shown by semi-quantitative PCR. Each actinin gene shows a unique and dynamic regulation of expression through early development. cDNA samples used for each PCR reaction are indicated by column and range from adult ovary and oocytes to 8-dpf larval cDNA. The upper band of the actn1 doublet was sequenced to show it is a product from an unrelated locus.

Expression of the actinin gene family at 24-48 hpf. Expression of the actinins by whole mount in situ hybridization reveals distinct patterns for most genes. A-J: Lateral whole mount views with insets showing higher magnification dorsal or lateral views of somite gene expression; insets are magnified lateral (C,Gb,I,J), dorsal (A,B,D,E,Ga), or anterioventral (H) views); A′-J′: Transverse sections of axial gene expression taken at level indicated by blue lines in A-J. A,A′: Expression of slow-specific marker stnnc at 24 hpf. Expression was observed in slow muscle fibers including muscle pioneer cells (arrow) and in peripheral slow muscle sheath. B,B′: Expression of fast-specific marker myhz2 at 24 hpf. Expression is seen in most of the somite but no expression is seen in the muscle pioneer cells (arrow). C,C′: Expression of actn2 at 24 hpf. Expression is seen in the heart (arrowhead) and in the slow muscle pioneers (arrows). D,D′,E,E′: Expression of actn3a (D,D′) and actn3b (E,E′) at 24 hpf. Both actn3a and actn3b are widely expressed in the somite. Expression is also seen in the slow muscle pioneer cells (arrows). F,F′: Expression of actn1 at 36 hpf. Expression was observed in the skin (arrow), notochord (arrowhead), and generally expressed in the somite. Expression was not observed in the neural tube (nt) or other components of the CNS. G,G′: Expression of actn4 at 36 hpf. Expression is seen in the notochord (arrows) as well as the developing gut (arrowhead). H,H′: Expression of actn2 at 48 hpf. Expression is seen in the heart (arrowheads) but is no longer observed in the slow muscle fibers of the myotome (arrow). H″: Expression of stnnc at 48 hpf. Expression can be seen in the peripheral sheath of slow muscle fibers (arrow). I,I′,J,J′: Expression of actn3a (I,I′) and actn3b (J,J′) at 48 hpf. Both actn3a and actn3b are broadly expressed in most or all of the myotome muscle fibers.

(Overleaf) Muscle-specific actinins are differentially expressed in axial and craniofacial musculature at 96 hpf. A-C: Lateral whole-mount views. Inset in Aa: Transverse section at the posterior gut level; insets in Ab, B, and C are higher magnification lateral views of somite expression. A′-C′: Higher magnification lateral views of craniofacial expression from A-C. D: Camera lucida representation identifying muscles of zebrafish at 96 hpf (abbreviations listed in Table 1). E-I, F′-H′: Ventral views of craniofacial expression, with E-I having a ventral focal plane and F′-H′ a dorsal focal plane. Jaw structures and sternohyoideus muscles have been removed in G′ for visibility of more dorsal muscles. J-N: Transverse sections at the level of the heart. A,A′: Expression of actn2 at 96 hpf. Expression is seen in the heart (arrow) as well as the lateralis superficialis (white arrowhead) and inferior obliquus (black arrowhead). In the cranial musculature, strong actn2 expression was seen in lap muscles (green arrowhead). Inset AB shows broad expression of stnnc at 96 hpf in the slow fibers of the myotome. B,B′: Expression of actn3a at 96 hpf. Expression was seen in the axial musculature (white arrowhead, superficial expression). Expression was also seen in cranial and pharyngeal muscles and strong expression is seen in the sh (arrowhead). C,C′: Expression of actn3b at 96 hpf. Expression was seen in the axial musculature. Weak expression was seen in the sh (arrowhead). D: Craniofacial muscle groups are colored to demarcate pharyngeal arches of origin: red, mandibular arch (pharyngeal arch 1, P1); green, hyoid arch (P2); yellow, branchial arches 1-5 (P3-P7); purple, extraocular/cranial muscle; pink, somite-derived muscle; orange, cardiac muscle. E: stnnc was expressed uniformly in most cranial and pharyngeal muscle. Expression was light in sh muscles (white arrowhead). F-H: Expression of the muscle-specific actinins in sh muscles was variable between homologs. actn2 was weakly expressed in the sh (arrowhead, F), actn3a was very strongly expressed (arrowhead, G), and actn3b was moderately expressed (arrowhead, H). I: myhz2 was expressed strongly in sh muscles (arrowhead). J: actn2 expression was weak in tv and dpw muscles (white and green arrowheads, respectively) compared to its strong expression in lap muscles (black arrowhead). K,L: actn3a (K) was weakly expressed in tv (white arrowheads), dpw (green arrowheads), and lap (black arrowheads) muscle pairs. actn3b (L) expression was high in all pharyngeal muscles. M-Q: stnnc was expressed in the periphery of the sh (arrowhead, M). myhz2 was expressed in the center of the sh (arrowhead, N). actn2 was expressed in the periphery of the sh (arrowhead, O). actn3a was expressed in the center of the sh (arrowhead, P). actn3b was expressed in the center of the sh (arrowhead, Q).

Expression of the muscle actinins at 15 somites as shown by in situ hybridization. A, C, E: Dorsal views of 15 somite flatmounted embryos. B, D, F: Transverse sections taken at a level between somites 5 to 10. A-F: Expression of all three muscle actinins can be observed in the adaxial cells adjacent to the notocord (arrowheads). actn3a and actn3b are expressed more strongly and in a domain that has expanded slightly more dorsolaterally and ventrolaterally than actn2 at 15 somites (D,F).