Modulates Abd-B Bithorax-Complex Functions in Drosophila melanogaster

Abdominal-B (Abd-B) mutants of the bithorax-complex (BX-C) were studied in trans with tuh-3 to evaluate their interactions with this homeotic mutant and the maternal effect locus (tuh-I) controlling tuh-3 expression. Head defects occur in tuh-3 offspring from tuh-lh homozygous mothers, while genital defects occur in homozygous tuh-3 offspring from mothers carrying the tuh-Ig allele. The influence exerted by the tuh-1 maternal effects on tuh-3 Abd-B transcript distribution was evaluated by whole mount in situ hybridization. Results demonstrated that: (1) of the 21 Abd-B mutants tested, head defects were produced by SGA62, Z127', Z127O and tuh-3, with Z127' and tuh-3 as the only mutants to require the head defect maternal effect for expression; (2) one specific cluster of regulatory ( r ) mutants, Uab', 65 and Z127', enhanced penetrance and expressivity of tuh-3 head defects; (3) the most profound suppression of head defect penetrance occurred when Abd-B mutants eliminated the morphogenetic (m) and r functions; (4) genital defects increased in frequency in tuh-3/Abd-B mutant trans-heterozygotes with loss of r function; (5) Abd-B transcription (class A, class B, class C) appears normal in tuh-3 embryos when their mothers pass on the tuh-Ih head defect maternal effect, whereas the regulatory transcripts (class B and class C) are reduced when tuh-3 mothers pass on the tuh-lg genital disc maternal effect; (5) tuh eye-antennal imaginal discs express ABD-B protein; and (6) tuh-3 misregulates both m and r function of Abd-B. T HE bithorax-complex (BX-C) is a cluster of homeotic genes controlling the identity of posterior thoracic segment 2 (pT2), T 3 and all abdominal segments (A1 through A9), which include the genitalia (LEWIS 1978; reviewed by DUNCAN 1987). The homeotics appear to function in parasegments (PS), with the organizational unit comprised of the posterior side of one segment with the anterior side of the following segment (MART~NEZ-ARIAS and LAWRENCE 1985). The entire BX-C has been cloned and the lesions for many mutants mapped at the DNA level (BENDER et al. 1983; KARCH et al. 1985). The complex has three lethal complementation groups, Ubx, abd-A and AbdB , each having its own homeobox (REGULSKI et al. 1985). The Ultrabithorax (Ubx) domain specifies identity of pT2 through anterior A1 (aAl) (PS5 and PS6). The abdominal-A (abd-A) domain gives identity to pAl through aA8 (PS7-PS13), while function of Abdominal-B (Abd-B) is required from pA4 through aA9 (PS 1 0-PS 14). Mutants disrupting function of an entire abdominal domain are designated abd-A or Abd-B, while mutants affecting subfunctions within a domain are referred to as infra-abdominal (iab) mutants. The iab designation identifies the anterior compartment of the segment most affected (LEWIS 1978; DUNCAN 1987). Abd-B has been further subdivided into a socalled morphogenetic function ( m ) and a regulatory function ( r ) . The r element is envisioned to suppress Genetics 133: 593-604 (March, 1993) function of m in PS14 (CASANOVA, SANCHEZ-HERRERO and MORATA 1986). Complementation analysis revealed three groups of Abd-B mutants. Class I mutants affected m function only (mr+), class I1 mutants affected r function only (m+ r-), while class I11 mutants did not complement either of the other two classes and are mr-. Functions of the Abd-B gene are encoded in four overlapping RNAs transcribed from separate promoters (ZAVORTINK and SAKONJU 1989; BOULET, LLOYD and SAKONJU 199 1). Whether or not the fourth transcript, Gamma, (KUZIORA and MCGINNIS 1988) has an Abd-B function remains an open question. The m function is provided by the class A transcript, while r function is provided by class B and the class C transcripts (BOULET, LLOYD and SAKONJU 1991). The molecular data indicate that r function is controlled by overlapping transcripts rather than a single regulatory element as originally envisioned by CASANOVA, SANCHEZ-HERRERO and MORATA (1 986). Although multiple Abd-B transcripts with different 5' ends and exon compositions have been found, they have been grouped into two types, encoding one of two homeoproteins (DELORENZI et al. 1988; CELNIKER, KEELAN and LEWIS 1989; DELORENZI and BIENZ 1990). The transcripts are expressed in patterns correlating with one of the functional domains. The level of the class A protein increases in parasegmental steps in the 594 D. T. Kuhn, J. A. Mack, C. Duan and G. Packert ectoderm from PSlO through PS13 during midembryonic development (DELORENZI and BIENZ 1990; BOULET, LLOYD and SAKONJU 199 1). Class B and class C transcript distribution is confined to PS 14 and the anterior side of PS 15 (BOULET, LLOYD and SAKONJU Tumorous-head-3 (tuh-3) has an insertion of the mobile element “Delta-88” at +200 kb on the BX-C DNA map (KARCH et al. 1985). Although the location of “Delta-88” is consistent with the BX-C functions disrupted, there has been no direct proof that the mobile element actually causes the mutant phenotype. The location provides circumstantial evidence. Thus, we assume here that “Delta-88” or the lesion created by the insertion is responsible for the homeotic defects