Height of conservation
Homology or conservation(sequence and function) forms crux of Evolution and today we look into two aspects where the conservation is really amazing.Firstly we talk about NK4 / tinman’s (NKx 2.5 Humans) association with Heart or Dorsal vessel and then move to Micro RNA’s in Hox complex of flies and Vertebrates.
Tinman/NK4 or NKx 2.5 :
Tinman is broadly expressed in mesoderm of flies and later restricted to cardiac mesoderm where it plays vital role is Heart Development. Nk genes were first discovered in flies and later found in almost all metazoans. Tinaman/NK4 or NKx 2.5, in which ever species it is known, performs a similar role of heart or dorsal tube patterning.Tinman homologs in Humans ,Mouse,Amphioxus, Ciona and Chicken are known to pattern heart, which is an fantastic exhibition of functional conservation.Recently The expression of the homologue of NK4/tinman in the pulsatile dorsal vessel of annelid Platynereis has been shown. This association between NK4 and Heart or heart like structure in metazoans always amazes me .
Micro RNA’s in Hox complex:
The degree of overall Hox homology and the conservation of regulatory complexity between vertebrate and insect Hox complexes is simply mind blowing.A microRNA, miR-196, is found in Hox complexes A, B, and C in vertebrates.In posterior regions of the embryo, miR-196 is transcribed by promoter elements of paralog 10, processed, and bound to the 3â€²-untranslated region of Hox-8 mRNAs. Hox-C8 mRNA and miR-196 have a single mismatch over the 22 nucleotide microRNA sequence, and binding triggers degradation by RNA interference (RNAi). In contrast anterior mRNAs in Hox-7 and Hox-6 paralogous groups also bind to miR-196 to but with more mismatches causing blockage of translation.This repression ensures anterior How genes dont express in posterior regions.Surprisingly infra abdominal 4 (iab -4) is found at equivalent position of miR-196 in bithorax complex of Drosophila.The iab-4 gene encodes a microRNA that binds to and inhibits Ultrabithorax mRNA translation.Ronshaugen et al has shown that over expression of iab-4 causes haltere to wing homeotic transformations.
Translation repression by Micro RNA’s could explain the long standing question of Posterior dominance in Hox field.