In Drosophila melanogaster, another K50HD protein, Bicoid (Bcd), has evolved to replace Otd’s ancestral function in embryo patterning.
The evolution of body plans is driven by alterations to these networks, including changes to cis-regulatory elements and neofunctionalization of transcription factors (TFs) after gene duplication (Thornton et al. 2003; Carroll 2008; Peter and Davidson 2011; Tautz and Domazet-Loso 2011; Abascal et al. 2013).
Despite its critical functions in Drosophila, Bcd is not well conserved even within insects. Rather, Bcd arose after a recent gene duplication event and rapidly evolved to play an important role in embryonic patterning (Stauber et al. 1999; Casillas et al. 2006).
In Drosophila, otd has evolved to become a zygotic target gene of Bcd-dependent activation (Finkelstein and Perrimon 1990).
This suggests that an important step in Bcd’s evolution was the conversion of Q50 in the ancestor to K50, which changed its DNA-binding preference and allowed it to usurp some of the anterior patterning roles played by Otd in ancestral insects.
Our results define specific roles for Bcd and Otd in embryonic patterning in Drosophila and shed light on the molecular mechanisms that alter regulatory networks during evolution.
This result is not surprising in view of the fact that the evolved Bcd- and Otd-coding regions show very little sequence conservation (38% sequence identity within their HDs) (Fig. 2A) and no detectable homology outside the HD.
In this study, we compared the in vivo functions of two K50HD proteins (Bcd and Otd) within a framework of evolution.
We showed that Otd and Bcd have evolved independent functions in vivo, and HD swaps between the two proteins indicated that the major structural differences mediating their distinct in vivo activities can be traced to their HDs.
Bcd has evolved rapidly to become a powerful morphogen in Drosophila but is not well conserved even within Diptera (Stauber et al. 1999).
In Drosophila, otd has evolved to become a zygotic target gene of Bcd (Finkelstein and Perrimon 1990).
Convergent evolution and a robust core anterior patterning network
some functional convergence of these proteins must have occurred during insect evolution.
The evolution of Bcd likely involved the retention of the maternal promoter and the acquisition of three characteristics required for anterior embryonic patterning: (1) UTR sequences that control anterior mRNA localization, (2) protein motifs that mediate translational repression, and (3) amino acid substitutions that alter its DNA-binding preferences; namely, a Q50-to-K50 mutation in its HD.
Such dramatic changes in these two genes may be attributed to reduced selective pressure on maternal genes (Barker et al. 2005; Demuth and Wade 2007), which permits the exploration of the evolutionary landscape and the acquisition of new functional roles.
Perhaps this set of target genes represents an ancestral core network that is well conserved in evolution.
it is possible that the cis-regulatory motifs (and consequently the enhancers) are functionally robust in the evolution of anterior embryo patterning, while trans-acting factors can accumulate mutations.
This allows for a conserved set of targets that makes up a canalized anterior patterning network that allows the regulating TFs to evolve.
Emphasis added
“evol” 18 times?
OK, let’s see how those “evolution”-related terms are technically supported in details. I don’t see it, but let’s investigate it carefully.
Common Design 