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#gap genes

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Drosophila -ll

Giant gap gene activated posteriorly by:

Giant gap gene exhibits two expression domains: anterior domain activated by Bicoid and Hunchback, posterior domain regulated independently by Caudal and Tailless terminal factors. Posterior enhancer contains binding sites for Caudal, which itself forms posterior-to-anterior gradient opposite Bicoid via translational repression of bicoid. Nanos promotes Caudal translation posteriorly by repressing Hunchback. Loss of caudal or nanos reduces posterior giant stripe causing abdominal deletions. Bicoid alone insufficient posteriorly due to Hunchback repression; Hunchback represses anteriorly at high concentrations. Thus Caudal provides activating input posteriorly integrating terminal and posterior patterning systems ensuring giant defines abdominal region correctly during cellular blastoderm formation.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Giant posterior domain activated by Caudal gradient - regulatory logic.

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Drosophila -ll

Gap gene mutations typically result in:

Gap genes encode transcription factors expressed in non-periodic domains defining large embryonic territories. Loss removes adjacent segment anlage covered by domain, resulting larvae missing head-thorax for hunchback, thoracic segments for Kruppel, abdominal segments for knirps or giant, producing large gaps. Phenotype contrasts with pair-rule where every other segment missing repeats, and segment polarity where polarity reversed. Duplications characterize wingless defects. Large region deletion remains diagnostic of gap function as intermediaries interpreting maternal morphogen concentrations into broad fields that later regulate periodic gene expression and establish positional values for subsequent metameric patterning during development.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Gap mutants - removal of contiguous segment groups in larval cuticle.

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Fertilization in mammals-lll

Gap genes regulate:

Drosophila embryonic segmentation proceeds hierarchically: maternal gradients activate gap genes then pair-rule then segment polarity. Gap genes like hunchback, Krüppel, knirps, giant are transcription factors expressed in broad overlapping domains along anterior-posterior axis, each controlling large contiguous blocks of segments. Mutations delete several adjacent segments creating gaps in larval cuticle pattern, hence name. They regulate pair-rule gene stripe expression. Segment polarity genes control subsegments and polarity within each segment via Wg and En, homeotic genes confer identity not segment number. Therefore gap genes establish coarse regionalization specifying large segmental domains upstream of finer patterning cascades in early embryogenesis.

Ref: St Johnston & Nüsslein-Volhard, Cell 1992: Gap genes establishing broad segmental domains in Drosophila embryonic patterning.

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