Ultrabithorax is required for wing identity in Tribolium – PART II
As we have seen in earlier post that in Tribolium , the second thoracic segment (T2 segment) bears sclerotized elytra and T3 bears more classical membranous wings for flight.The difference between dipterans and coleopetrans in developing membranous wings for flight( Dipterans have flight wings on T2 and Coleopetrans on T3) could be due to different expression pattern of Ubx hox protein in these two insects orders. However, detection of Ultrathorax (Utx), the protein product of the Tribolium Ubx orthologue, showed that Utx, like its Drosophila counterpart, is expressed in T3 but not in T2 imaginal discs. This suggests that the function of Ubx and/or other Hox genes could be different in beetles.
To analyze role of different hox proteins in Tribolium wing differentiation ,authors used mutants and performed larval RNAi for Hox proteins like Sex comb reduced (Scr), Antennapedia (Antp),Ultrabithorax (Ubx) and Abdominal-A (abdA). Results for each Hox proteins are summarized below:
1) Loss of Scr have additional elytron-like tissue in T1 segment,indicating that Scr function in repressing T1 wing development is conserved between fly and beetle.
2) Antp loss has no visible effect on hindwings or Elytra,indicating that the elytra represent a Hox-free state, despite their derived morphology.
3) Whereas loss of Ubx induced complete transformation of hindwing to elytron. If you remember in Drosophila Ubx, which normally modifies membranous wing development to produce halteres, Ubx in beetle seems to promote membranous hindwing development by repressing elytron identity.
The proof for role of Ubx in promoting membranous wing identity by repressing elytron comes from the authors study on expression patterns of more than 15 wing genes in Tribolium hindwing and elytron imaginal discs.Tribolium homologues of sal, iroquois (iro) complex genes and achaete-scute (ac-sc) are expressed differently in T2 versus T3 imaginal discs. The expression patterns of sal,iro and ac-sc in hindwings resemble exactly like that of elytron imaginal dics when Ubx is knocked down using larval RNAi technique. These results indicate that Ubx promotes membranous hindwing identity in part by controlling the expression of these genes in Tribolium.
Generally it is can be argued that the elytra are ancestral (on the basis of their location on T2) and that the beetle hindwing is derived but the expression data of important wing patterning gene in hindwings are more similar to those of fly forewings than are those of the elytra, indicating that the beetle hindwing represents the more ancestral state.
Reference:
1) Ultrabithorax is required for membranous wing identity in the beetle Tribolium castaneum.
Tomoyasu Y, Wheeler SR, Denell RE.
Nature. 2005 Feb 10;433(7026):643-7.
Link to Rob Dennel’s lab
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