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The Iroquois (Iro) family of genes are found in nematodes, insects and vertebrates. They usually occur in one or two genomic clusters of three genes each and encode transcriptional controllers that possess a characteristic homeodomain. The Iro genes function early in development to specify the identity of diverse territories of the body, such as the dorsal head and dorsal mesothorax of Drosophila and the neural plate of Xenopus. In some aspects they act in the same way as classical selector genes, but they display specific properties that place them into a category of their own. Later in development in both Drosophila and vertebrates, the Iro genes function again to subdivide those territories into smaller domains.
Fig. 2. Comparison of Iro early and late functions
in Drosophila imaginal discs and Xenopus
embryos. Top line, early functions. Iro genes are
expressed in the dorsal-most regions of the early
second instar eye and wing discs, respectively,
and in the neural ectoderm of stage 9-10 Xenopus
embryos (light pink). Absence of these early
functions prevents proper development of these
regions. The common functional ‘leitmotif’ is
apparently the specification of large territories.
Middle line, late functions. Iro genes are
expressed in more resolved patterns (dark pink).
Their absence removes pattern elements like
sensory bristles and wing veins and probably
causes transformation among different types of
neurons in the vertebrate CNS. Bottom line,
territories affected by early and late functions are
denoted by light and dark pink, respectively. nt,
section of neural tube. Iro genes are also
expressed in several regions of the Drosophila
embryo, other imaginal discs, and many other regions and tissues of the vertebrate embryos (Bellefroid et al., 1998; Bosse et al., 2000; Bosse et
al., 1997; Calleja et al., 2000; Christoffels et al., 2000; Cohen et al., 2000; Gómez-Skarmeta et al., 1998; Goriely et al., 1999; McNeill et al.,
1997; Tan et al., 1999), which suggests additional functions.
Fig. 5. Early Xiro function is required for neural plate formation.
(A) Drawing of a dorsal view of a Xenopus embryo at the neurula
stage. Xiro overexpression on the right side (injected with Xiro1
mRNA) expands the neural plate (np). This expansion is associated
with a reduction of the adjacent neural crest territory (nc). (B) Xenopus
embryo injected with Xiro1 and lacZ mRNAs. Compare the size of the
neural plate, as determined by expression of the Sox2 marker, in the
uninjected left side with the injected right side (black arrowhead;
green, X-gal-staining to reveal injected side). (C) Interference with
early Xiro function using a dominant negative construct suppresses
neural differentiation on the injected side (black arrowhead; brown,
Myc staining, which reveals localization of dominant negative protein).
(Data taken from Gómez-Skarmeta et al., 1998; Gómez-Skarmeta et
al., 2001; Bellefroid et al., 1998.) White arrowheads in B,C indicate
the position of the midline of the neural plate.
Fig. 6. Proneural genes are expressed within the larger domains of
Iro expression in both Drosophila imaginal disc and Xenopus
embryos. (A) Expression of caup and sc in late third instar wing
discs and a drawing showing the overlap between both patterns of
expression. ara and ac expressions are indistinguishable from those
of caup and sc, respectively. Proneural clusters whose presence is
known to depend on Iro-C are shown in brown. Other proneural
clusters are denoted in red. al, allula; DC, dorsocentral cluster; L3,
L3 proneural cluster; NP, notopleural clusters; WM, wing margin.
(B) Overlapping patterns of expression of Xiro1 and Xash-3 in the
neural plate of the Xenopus embryos at the neurula stage. Xiro2 is coexpressed
with Xiro1. Xiro3 is expressed in a similar pattern although
in slightly narrower bands. In addition, both Xiro1 and Xiro2 are
expressed in the prospective placode region (pc), whereas Xiro3 is
expressed in lateralmesoderm. Other Xenopus proneural genes such
as Xnrgr-1 are expressed in domains that partially overlap with those
of Xiro genes. (Data taken from Gómez-Skarmeta et al., 1996;
Gómez-Skarmeta et al., 1998; Bellefroid et al., 1998.
