Bringing the difference: Ultrabithorax in Lepidopterans and Dipterans hindwings
What makes Sean carroll’s articles and Books so unique and impressive? One answer to that would be some brilliant writing involved in them,which makes readers understand the content easily.Today we will look back into his article from Current Biology ,published on 20 January 1999 describing role of Ultrabithorax (Ubx) in Butterfly wings.
Hox genes control many features like segment morphology, appendage number(Arthropods) and pattern,axial morphology and limb pattern (vertebrates) ,which played significant role in modifications to their respective body plans during evolution.The morphological and functional evolution of appendages has played a critical role in animal evolution and as similar Hox gene regulate homologous appendage development ( Wing and Haltere in Drosophila), which can be attributed to attaining different target genes.In the present article role of Ubx has been studied in Butterfly wings and compared the results with target genes of Ubx in Drosophila in order to understand better,the role of Hox genes in morphological evolution.
Edward B. Lewis was the first person to link Hox genes to morphological evolution ,who proposed that the evolution of segmental diversity in the insect lineage involved the evolution of homeotic genes that were not present in primitive arthropods..But complete set of Hox genes are present in Onychophora (sister group of Arthropoda) proving that expansion of Hox genes preceded the origin and diversification of the entire arthropod clade.
Complete Hox complement was present in ancestors of Arthropods and Onychophors suggesting Hox genes might be functioning differently or regulating the genes in other fashion to bring in evolution among Arthropods.
Unlike primitive insects of order Odonata ,modern insects lack similar Fore and Hind wings.In Dipterans like Drosophilla Hind wings are modified by Hox gene Ubx into Haltere (by repressing wing patterning genes) and whereas in Lepidopetrans like Butterflies slightly different hind and fore wings are present in spite of Ubx being highly expressed in T3 segment ,similar to the situation like in Drosophila.
So now the question comes into mind is How can Ubx can form such different posterior flight appendages like tiny Haltere in Drosophila ,a elaborated patterned hind wings in Butterfly or in the case of Beetles like Tribolium ,where in differentially regulates Elytra (T2 appendage) and wing (T3)? The answer to this puzzling question lies in the differential regulation of target genes by Hox protein Ubx in the organisms of these orders.
Working on the Hindsight homeotic mutation of Butterfly ,in which portions of the ventral hindwing pattern are transformed to ventral forewing identity,Scott Weatherbee et al, show that Ubx gene expression is lost from patches of cells in developing Hindsight hindwings, correlating with changes in wing pigmentation,color pattern elements, and scale morphology.One thing which makes things clear here is that Ubx normally regulates regulates pigmentation, colour pattern elements and scale morphology in P. coenia hindwings and that loss of Ubx expression results in default patterns normally found in the forewing.
One marked difference between flight appendages ( fore and Hind wings) in Dipetrans like Drosophila is the size of the appendage.Haltere is relatively small when compared to fore wing ,but the situation is different in Lepidopterans where there is not much reduction of size in hind wings,suggesting some target genes of Ubx in Dipterans are not repressed by it in Lepidopterans.
Using this dominant homeotic hindsight mutant as tool the authors did comparison of the different targets of Ubx which are repressed in Drosophila haltere disc and found that DSRF-Drosophila serum response factor, AS-C (genes of Achaete scute complex)and wingless (wg ) ,which are crucial for wing patterning are expressed in fore wings in a similar manner like in Drosophila wing.This show that both Lepidopterans and Dipterans share same ground-plan for wing patterning.The exception lies in the hindwings of Butterflies , portions of the expression patterns of DSRF, AS-C and wg that are repressed by Ubx in Drosophila halteres are not repressed in P. coenia hindwings.Finally it can be concluded that changes in Hox regulated targets will most likely play a significant role in morphological differences in homologous structures in other animals.
Reference:
Scott D. Weatherbee, H. Frederik Nijhout, Laura W. Grunert,Georg Halder, Ron Galant, Jayne Selegue and Sean Carroll.
Ultrabithorax function in butterfly wings and the evolution of insect wing patterns.
Current Biology 1999, 9:109–115
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