ladybird early and ladybird late

Transcriptional Regulation

Expression of lbe and lbl depends on wingless. Previous studies have shown that ubiquitous expression of gooseberry ectopically activates the endogenous gsb gene in cells located anterior to the wild-type stripe. However, this ectopic induction is not observed in a wingless mutant background (Li, 1993). Heat shock gsb is also able to activate the formation of an ectopic strip of lbe. As for gsb, this phenomenon is wg-dependent and cannot be detected in wg mutants. Therefore, it is likely that wg function is required for both activation and maintenance of lbe and lbl expression, and for that matter, gsb as well (Jagla, 1997a).

In the dorsal epidermis, both wg and lbe are gsb-independent. It is concluded that whereas ventral epidermal wg expression may require gsb, in the dorsal epidermis, both wg and lbe are gsb-independent (Jagla, 1997a).

Ladybird is a component of a cardiogenic pathway required for diversification of heart precursors. Expression of lb genes in the subset of cardioblast and pericardial cell precursors is critically dependent on mesodermal tinman function, epidermal Wingless signaling and the coordinate action of neurogenic genes. lb-expressing heart progenitors contribute to the increased number of cardiac precursor cells in Notch, Delta, Enhancer of split, mastermind, big brain and neuralized mutants. Negative regulation by hedgehog is required to restrict ladybird expression to two out of six cardioblasts in each hemisegment. Overexpression of ladybird causes a hyperplasia of heart precursors and alters the identity of even-skipped-positive pericardial cells. Surprisingly, the number of eve-expressing pericardial cells is strongly reduced in overexpressors. These lb expressing cells are transformed into l-paracardial cells. Loss of ladybird function leads to the opposite transformation, suggesting that ladybird participates in the determination of heart lineages and is required to specify the identities of subpopulations of heart cells. Both early Wingless signaling and ladybird-dependent late Wingless signaling are required for proper heart formation. Thus, it is proposed that ladybird plays a dual role in cardiogenesis: (1) during the early phase, it is involved in specification of a segmental subset of heart precursors as a component of the cardiogenic tinman-cascade and (2) during the late phase, it is needed for maintaining wingless activity and thereby sustaining the heart pattern process. These events result in a diversification of heart cell identities within each segment. Since tinman, bagpipe, S59 and ladybird genes are all part of the same homeobox gene cluster, it is likely that their association has to do with the orchestrated diversification of mesoderm (Jagla, 1997b).

Targets of Activity

In the dorsal epidermis and the terminal regions of the body, expression of wingless is independent of gooseberry but requires a wingless-ladybird regulatory feedback loop. Loss of ladybird function reduces the number of wingless-expressing cells in dorsal epidermis and leads to complete inactivation of wingless in the anal plate. Consequently, mutant ladybird embryos fail to develop anal plates and ubiquitous embryonic expression of either one or both ladybird genes leads to severe defects of the dorsal cuticle. Lack of late wingless expression and anal plate formation can be rescued with the use of a heat-shock-ladybird transgene (Jagla, 1997a).

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