Drosophila Regional differentiation

1 drosophila

1.1 anterior/posterior axis
1.2 segmentation , homeotic genes
1.3 dorsal/ventral axis

drosophila

anterior/posterior axis

the anterior/posterior patterning of drosophila come 3 maternal groups of genes. anterior group patterns head , thoracic segments. posterior group patterns abdominal segments , terminal group patterns anterior , posterior terminal regions called terminalia (the acron in anterior , telson in posterior).

the anterior group genes include bicoid. bicoid functions graded morphogen transcription factor localizes nucleus. head of embryo forms @ point of highest concentration of bicoid , anterior pattern depends upon concentration of bicoid. bicoid works transcriptional activator of gap genes hunchback (hb), buttonhead (btd), empty spiracles (ems), , orthodentical (otd) while acting repress translation of caudal. different affinity bicoid in promoters of genes activates allows concentration dependent activation. otd has low affinity bicoid, hb has higher affinity , activated @ lower bicoid concentration. 2 other anterior group genes, swallow , exuperantia play role in localizing bicoid anterior. bicoid directed anterior 3 untranslated region (3 utr). microtubule cytoskeleton plays role in localizing bicoid.

the posterior group genes include nanos. similar bicoid, nanos localized posterior pole graded morphogen. role of nanos repress maternally transcribed hunchback mrna in posterior. protein, pumilio, required nanos repress hunchback. other posterior proteins, oskar (which tethers nanos mrna), tudor, vasa, , valois, localize germ line determinants , nanos posterior.

in contrast anterior , posterior, positional information terminalia come follicle cells of ovary. terminalia specified through action of torso receptor tyrosine kinase. follicle cells secrete torso-like perivitelline space @ poles. torso-like cleaves pro-peptide trunk appears torso ligand. trunk activates torso , causes signal transduction cascade represses transcriptional repressor groucho in turn causes activation of terminal gap genes tailless , huckebein.

segmentation , homeotic genes

the patterning maternal genes work influence expression of segmentation genes. segmentation genes embryonically expressed genes specify numbers, size , polarity of segments. gap genes directly influenced maternal genes , expressed in local , overlapping regions along anterior/posterior axis. these genes influenced not maternal genes, epistatic interactions between other gap genes.

the gap genes work activate pair-rule genes. each pair-rule gene expressed in 7 stripes result of combined effect of gap genes , interactions between other pair-rule genes. pair-rule genes can divided 2 classes: primary pair-rule genes , secondary pair-rule genes. primary pair-rules genes able influence secondary pair-rule genes not vice versa. molecular mechanism between regulation of primary pair-rule genes understood through complex analysis of regulation of even-skipped. both positive , negative regulatory interactions both maternal , gap genes , unique combination of transcription factors work express even-skipped in different parts of embryo. same gap gene can act positively in 1 stripe negatively in another.

the expression of pair-rule genes translate expression of segment polarity genes in 14 stripes. role of segment polarity genes define boundaries , polarity of segments. means genes accomplish believed involve wingless , hedgehog graded distribution or cascade of signals initiated these proteins. unlike gap , pair-rule genes, segment polarity genes function within cells rather within syncytium. thus, segment polarity genes influence patterning though signaling rather autonomously. also, gap , pair-rule genes expressed transiently while segment polarity gene expression maintained throughout development. continued expression of segment polarity genes maintained feedback loop involving hedgehog , wingless.

while segmentation genes can specify number, size, , polarity of segments, homeotic genes can specify identity of segment. homeotic genes activated gap genes , pair-rule genes. antennapedia complex , bithorax complex on third chromosome contain major homeotic genes required specifying segmental identity (actually parasegmental identity). these genes transcription factors , expressed in overlapping regions correlate position along chromosome. these transcription factors regulate other transcription factors, cell surface molecules roles in cell adhesion, , other cell signals. later during development, homeotic genes expressed in nervous system in similar anterior/posterior pattern. homeotic genes maintained throughout development through modification of condensation state of chromatin. polycomb genes maintain chromatin in inactive conformation while trithorax genes maintain chromatin in active conformation.

all homeotic genes share segment of protein similar sequence , structure called homeodomain (the dna sequence called homeobox). region of homeotic proteins binds dna. domain found in other developmental regulatory proteins, such bicoid, in other animals including humans. molecular mapping revealed hox gene cluster has been inherited intact common ancestor of flies , mammals indicates fundamental developmental regulatory system.

dorsal/ventral axis

the maternal protein, dorsal, functions graded morphogen set ventral side of embryo (the name comes mutations led dorsalized phenotype). dorsal bicoid in nuclear protein; however, unlike bicoid, dorsal uniformly distributed throughout embryo. concentration difference arises differential nuclear transport. mechanism dorsal becomes differentially located nuclei occurs in 3 steps.

the first step happens in dorsal side of embryo. nucleus in oocyte moves along microtubule track 1 side of oocyte. side sends signal, gurken, torpedo receptors on follicle cells. torpedo receptor found in follicle cells; however, gurken signal found on anterior dorsal side of oocyte. follicle cells change shape , synthetic properties distinguish dorsal side ventral side. these dorsal follicle cells unable produce pipe protein required step two.

the second step signal ventral follicle cells oocyte. signal acts after egg has left follicle cells signal stored in perivitelline space. follicle cells secrete windbeutel, nudel, , pipe, create protease-activating complex. because dorsal follicle cells not express pipe, not able create complex. later, embryo secretes 3 inactive proteases (gastrulation defective, snake, , easter) , inactive ligand (spätzle) perivitelline space. these proteases activated complex , cleave spätzle active form. active protein distributed in ventral dorsal gradient. toll receptor tyrosine kinase spätzle , transduces graded spätzle signal through cytoplasm phosphorylate cactus. once phosphorylated, cactus no longer binds dorsal, leaving free enter nucleus. amount of released dorsal depends on amount of spätzle protein present.

the third step regional expression of zygotic genes decapentaplegic (dpp), zerknüllt, tolloid, twist, snail, , rhomboid due expression of dorsal in nucleus. high levels of dorsal required turn on transcription of twist , snail. low levels of dorsal can activate transcription of rhomboid. dorsal represses transcription of zerknüllt, tolloid, , dpp. zygotic genes interact each other restrict domains of expression.