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We have isolated a Xenopus homeodomain sequence, XNkx-2.5, which shows significant similarity to mouse Nkx-2.5 and to the Drosophila tinman gene product. In Drosophila, tinman is required for formation of the heart and visceral mesoderm structures. In situ hybridization studies show that XNkx-2.5 is expressed in the heart region during early Xenopus development and later is also expressed in guttissue. The observed similarity of sequences and expression patterns suggests that the regulatory mechanisms underlying heart formation may be conserved between distant species.
FIG. 2. Expression of XNkx-2.5 during embryogenesis detected by whole-mount in situ. hybridization. The dark-stained product of the alkaline
phosphatase reaction marks the position of XNkx-2.5 expression. (A) Face-on view of a cleared stage 15 embryo showing the hi-lobed expression
of XNkx-2.5 (arrowed) in front of the c(osing anterior neural folds. The nonspecific dark stain on the dorsal side (up) is due to focusing through
the archenteron. (B) Face-on view of an embryo that has just closed its neural folds (stage 18). Staining in the region in front of the neural folds
indicates that XNkx-2.5 expression is in the presumptive heart region. (C) A cleared, side view of the embryo in (B) showing the staining at the front of the expanding archenteron (a). (D) Expression of XNk.'C-2.5 in a tailbudembryo (stage 26/27). Anterior is left and dorsal is up.
Expression is concentrated in the heart, which is beginning to differentiate at this stage, but the staining extends further dorsal than the heart
boundaries. (E) Ventral view of a tail bud embryo (stage 26/ 27) showing that expression in the heart region still appears hi-lobed. (F) Expression
in other tissues can be detected later in development (stage 38/40). Expression continues in the heart (h), but is also visible in the anteriorgut
(g). The staining in the cement gland (cg) is nonspecific. (G) Ventral view of the embryo showing that the gut expression appears to be in a
region that is forming a tubular structure. Anterior is up. (H) View of an embryo from above, showing thin parallel stripes of expression in the
pharyngeal region (p).
