Expression of Brachyury during development of the dendrobatid frog Colostethus machalilla

Abstract

The expression of Brachyury (Bra) during development of Colostethus machalilla was analyzed with a polyclonal antibody. The observed molecular mass of Bra was of 48 kDa, as in Xenopus laevis. During cleavage, low levels of Bra were expressed. In contrast, in the blastula Bra became up-regulated, and Bra protein was present in a wide ring of surface cells. The surface expression of Bra disappeared in the gastrula, and a new ring of Bra-positive nuclei was detected in deep cells around the closing blastopore. The C. machalilla external and internal rings of Bra-positive nuclei apparently mark the prospective mesoderm in the blastula and gastrula, respectively. The two Bra expression rings were dissociated in time in the fairly slow developing embryos of this frog. Brachyury expression in the notochord became visible only after the blastopore closed, in contrast with X. laevis. In addition, Bra expression in the notochord indicated that dorsal convergence and extension occurred after blastopore closure. The C. machalilla Bra-positive notochord was originally exposed on the gastrocoel roof, in agreement with a superficial component of the prospective mesoderm.

A comparative analysis of frog early development

“…In fact, a deep ring of Bra-positive cells was first detected in the late gastrula of these frogs, whereas, in X. laevis, the Xbra-positive ring is visible from the onset of gastrulation (4,20,39,40). In contrast to Bra, the expression of Lim1 was detectable at the onset of gastrulation in G. riobambae (Fig.…”

“…In contrast, in embryos of G. riobambae and C. machalilla, notochord elongation and CE were retarded until after closure of the blastopore (Fig. 5 E and I), as detected by Bra expression (4,7,20).…”

“…In these frogs, elongation of the archenteron and notochord is retarded. Notochord elongation and, consequently, CE begin in the post gastrula (7,20,26). Elongation of the archenteron, however, begins earlier in species with larger eggs, such as E. coqui (Fig.…”

“…The secondary Abs were sheep anti-rabbit IgG or sheep anti-mouse conjugated to alkaline phosphatase (Roche Molecular Biochemicals, Mannheim, Germany). Immunostaining was done as described (20,49). Embryos were analyzed and photographed with a Stemi SV 6 and with an Axiophot (Zeiss, Oberkochen, Germany).…”

“…Moreover, CE occurs after blastopore closure, as in G. riobambae but without formation of an embryonic disk. In contrast, elongation of the archenteron begins in the midgastrula (7,19,20). It is of interest to know whether the gastrulation pattern is altered in dendrobatid frogs with much larger eggs.…”