Almost all extant animals have Hoxgenes that are involved in body patterning. Ho

Question

Almost all extant animals have Hoxgenes that are involved in body patterning. How might differential expression of and regulation by Hox genes contribute to mosaic evolution in which different segments of an animal body plan evolve different morphologies? What can we infer about the common ancestors of living animals based on the presence of these genes? Why do organisms that are incredibly distinct morphology, with complete different body plans, use so many of the same patterning genes? How can these same genes produce very different responses, ? in different tissues of one organism and (ii) in different organisms? a. b. c. d.

Explanation / Answer

a. The hox genes contributed to the mosiac evolution where different segments of an animal body plan evolve different morphologies such as some have long legs, some have a pair of legs, two pairs of legs, three pairs etc. This morphologies are due to the difference of expression of the hox genes. The hox gene Ultrabithorax(ubx) is responsible. With increased expression of ubx the leg growth increases. Ubx RNAi has opposing functions of T2 and T3 legs. Whereas ubx promotes growth of T2 legs then again it shortens T3 legs this is due to RNA interference gene(RNAi) of ubx. When ubx is depleted the embryos have longer T3 legs. Hence the expression of the ubx results in different morphologies.

b. The common ancestors all have the hox genes. But these animals did not have body plan with many morphological characters. The hox genes have changed their function during evolution such as ftz is a hox gene has changed its expression pattern and protein sequences and switched its function and expressed stripes. It also showed pair-rule segmentation in a striped pattern. We can conclude that in the ancestors the genes have not evolved and these did not show a complex body plan when compared to mammals and other other complex organisms.

c. The hox genes present at 3' anterior end of Hox complex affect the body structures present on anterior parts whereas genes present at 5' end affects posterior region of the animal. The hox genes are present as hox clusters or hox complexes. Hox complexes have the same patterning genes in animals with complete different body plan. But the presence of the genes at the 3' end and 5' end affects the regions of their body parts. So they use many of same patterning genes.

d. The Hox genes are same in all the living organsims but in different tissues it acts in a different way. For example ubx is responisble for leg formation and this same gene helps to form the wings in the same organism but at different sites or tissues. ii) The genes such as ftz has evolved to form pair rule segmentation in Drosophila but this hox gene retains its original function in other lower organisms. These genes produce responses depending on the use of a particular organism. Ex: mammals do not need wings whereas insects do but same genes are found in both the organism but their function is different.

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