The distribution of Ultrabithorax transcripts in Drosophila embryos

Abstract

We have used in situ hybridization to monitor the distribution of Ultrabithorax (Ubx) transcripts during the early stages of Drosophila embryogenesis. When first detectable, in the late syncytial blastoderm, Ubx transcripts are broadly distributed from 20 to 50% egg length (measured from the posterior pole). By the completion of cellular blastoderm formation, a precisely bounded zone one segment wide is defined by the accumulation of high levels of Ubx transcripts. This zone is probably the primordium for parasegment 6, that is, the posterior compartment of the third thoracic segment (T3) and the anterior compartment of the first abdominal segment (A1). Following gastrulation, seven metameric units of the germ band express Ubx transcripts in most or all cells of both the ectoderm and the mesoderm. This principal domain of Ubx expression is sharply bounded at superficial grooves in the embryo which probably mark the A/P compartment boundaries in T3 and A7, and so consists of parasegments 6‐12. Outside this region, Ubx transcripts can be detected in only a few cells of the ectoderm of parasegments 5 and 13, and in the amnioserosa. This distribution suggests that the major role of Ubx in the early embryo lies in the morphological domain generally ascribed to the control of the bithoraxoid gene. Later in embryogenesis, the pattern of Ubx transcript accumulation is different in each of the major germ layers, suggesting that different controls regulate Ubx in each. In the ectoderm, but not in the mesoderm, Ubx transcripts accumulate differentially in anterior and posterior compartments. Probes prepared from different regions of the Ubx transcription unit show related but different patterns of hybridization. These patterns suggest that the 3.2‐kb Ubx transcript is distributed throughout the wider domain of Ubx expression, but that the 4.7‐kb transcript accumulates preferentially in parasegment 6.

Temporal and spatial relationships between segmentation and homeotic gene expression in Drosophila embryos: distributions of the fushi tarazu, engrailed, Sex combs reduced, Antennapedia, and Ultrabithorax proteins.

“…We focus upon expression of the proteins in the simplest and most accessible two-dimensional cell layer, the early embryonic ectoderm, thus avoiding the complexities inherent in more complex arrays of cells (e.g., the nervous system at later stages). However, all of the genes studied are also expressed in the nervous system, and the relationships between gene expression patterns may be different in different tissues (Akam and Martinez-Arias 1985;Martinez-Arias et al 1987). The wild-type patterns of each individual protein have been described previously, though generally not in comparison with any other products.…”

The expression and regulation of Sex combs reduced protein in Drosophila embryos.

“…1987). 7he5e character15t1c5 0f U6x, Antp, and 5cr expre5510n have 6een de5cr16ed a5 ••pa1r-ru1e m0du1at10n •• {Akam 1985) and have 6een 5h0wn t0 6e dependent 0n ft2 funct10n (1n9ham and Mart1ne2-Ar1a5 1986). 1n ft2-em6ry05, tran5cr1pt10n 0f the three h0me0t1c 9ene515 reduced 0r e11m1nated 1n the even-num6ered para5e9ment5 at the ce11u1ar 61a5t0derm 5ta9e, 1.e., 1n the p1ace5 where ft2 15 n0rma11y act1ve.…”

“…N0rma11y, Antp (Mart1ne2-Ar1a5 1986) and U6x (Akam 1985) are expre55ed at a h19h 1eve1 1n P5 4 and 6, re5pect1ve1y. 1n w11d-type em6ry05, U6x RNA 15 a150 tran51ent1y accumu1ated 1n P5 8, 10, and 12 1n a ft2-11ke pattern (Akam 1985), wherea5 5cr RNA 15 detected f0r a 6r1ef t1me 1n a 5even-5tr1ped pattern very 51m11ar t0 the ft2 pattem (1n9ham and Mart1ne2-Ar1a5 1986; Mart1ne2-Ar1a5 et a1. 1987).…”

Segmental determination in Drosophila conferred by hunchback (hb), a repressor of the homeotic gene Ultrabithorax (Ubx).

“…The Ubx gene is activated in a midembryonic region of the blastoderm embryo (7), and its continuous activity in the descendants of these embryonic cells is needed until differentiation starts (8).…”

“…Boundaries of early Ubx expression depend on the segmentation gene hunchback (hb) (9)(10)(11), whose product carries anteroposterior positional information by virtue of its restricted distribution in the early embryo: hb protein (HB) is found at high levels anteriorly as well as posteriorly in the blastoderm embryo (12), in embryonic regions that are nearcomplementary to the early Ubx expression domain (7). HB binding sites were found within Ubx control elements, and these apparently mediate repression of the Ubx gene at the blastoderm stage (13,14).…”