Topic: Developmental genetic pathways to convergence
At first sight there is a fairly simple dichotomy between convergent features that have effectively the same genetic basis, and those where the same feature emerges but the underlying genetics are different. The former, however, is somewhat more complicated...
What is the relationship between developmental biology and convergence? Is convergence really just a superficial phenomenon because the underlying genes are the “same” and so pre-ordain what must develop? Or are the genes really peripheral to the story, little more than molecular switching-gear that can be assembled and re-assembled according to functional needs. Perhaps unsurprisingly the answer lies in the middle.
Stickleback and manatee pelvis reduction
At first sight there is a fairly simple dichotomy between convergent features that have effectively the same genetic basis, and those where the same feature emerges but the underlying genetics are different. The former, however, is somewhat more complicated than might at first appear. Take the example of pelvic reduction in the vertebrates. This is well documented in the stickle-back fish, and intriguingly the same genes are used for pelvic reduction in the manatees (sea-cows). Evidently the same genetic architecture is being used, and this is strikingly shown by the fact that in both fish and manatee there is a curious asymmetry whereby the left-hand side of the pelvis is the more reduced. In one sense this is a striking convergence, but is less impressive when we realize it is based on a genomically conserved pattern. Even here it remains to be established that the genetic cascade is identical, and certainly in other cases the basic genetic machinery is the same but the details leading to each convergence are not identical.
Gene co-option in Adenia
A further difficulty when discussing genetic convergence is that to speak of a developmental gene or genetic pathway being conserved begs interesting questions regarding function. What in particular was the original function? In the case of leaf development in plants, for example, it is evident that a genetic mechanism that was probably originally involved in the control of the apical region, its bifurcation and the growth of shoots, has been co-opted independently as and when leaf evolution occurs. Similarly in a particular plant known as Adenia, which is found mostly in East Africa and Madagascar, the hundred or so species show an extraordinary range of morphologies, ranging from trees to herbs and also including lianas. A key strategy in Adenia is water conservation (either by succulent stems or tubers), and both have evolved multiple times. In each case the same developmental mechanism is called upon, but it still counts as convergence because the actual morphology can flicker into existence as and when required. This process is evidently very rapid, and intriguingly may not result in intermediate forms. Of equal interest is the evidence that in some cases the parallelism seen in the morphology is a result of mutations of the cis-regulatory region of the genome (these are regions involved with the transcription of the relevant portions of the DNA) and that these too show parallelisms.
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Map of Life - "Developmental genetic pathways to convergence"
September 21, 2020