Analysis of the phylogenetically ancient NK-2 class of homeobox genes
has opened up an entirely new approach to molecular, genetic, and bioc
hemical analysis of early heart development. The Drosophila NK-2 homeo
box gene tinman plays an essential role in segregating cardiac and vis
ceral muscle potentiality, as well as that of some somatic muscles, in
nascent mesoderm of the fly embryo. In its absence, precursor cells f
or these muscles do not form. tinman homologues have now been isolated
from vertebrate genomes and at least one of them, Nkx2-5, is expresse
d in heart progenitor cells and is essential for myogenic and morphoge
netic differentiation of the mammalian heart. Signaling pathways that
establish the tin expression domain also appear to be conserved in ver
tebrates. These findings suggest that heart development in flies and v
ertebrates utilize similar genetic pathways and engender optimism that
the dissection of mammalian heart development will profoundly profit
from the rich genetics of Drosophila. The findings also prompt the que
stions: are the hearts of vertebrates and invertebrates actually homol
ogous, and how much can we learn from the comparative approach? In the
sections below, the structure, regulation, function, and evolution of
NK class homeobox genes will be reviewed, emphasizing and contrasting
the roles of tinman and Nkx2-5 in heart development. (C) 1996 Academi
c Press, Inc.