Evolution of paired fins
Gill Arch Theory
According to Gillis, the fossil record doesn’t really tell much about how paired fins came about so together with his colleagues, they decided to discover whether the anatomical parallels that Gegenbaur found between the gill arches of cartilaginous fish may be like the molecular patterning mechanism in these system organs (Gillis, 2016). They performed several experiments to explore more about the branchial rays patterning in embryos of an oviparous cartilage fish, the little skate (Gillis, 2016). They paid more attention to the sonic hedgehog because according to Gills, decades of experimental embryological and molecular study have disclosed that sonic hedgehog plays a major role in both the anterior and posterior patterning of the limb (2016). They first explored if the shh signaling where revealed during the growth of the skate gill arch (Gillis, 2016). mRNA in situ hybridization disclosed that shh is in a pattern during the growth of gill arches and the skate hyoid (Gillis et al, 2016). They also performed a mapping experiment to analyze whether ptc2+ mesenchymal cells play a role in the branchial ray skeleton (Gillis, 2016). After the experiment, they found out that the mesenchymal cells play a role in the branchial ray skeleton. Which they proposed that the signaling pathway may have an impact on the behavior of the branchial ray progenitors (Gillis, 2016). They also tested to find out the role of the shh signaling during the growth of skate branchial ray (Gillis, 2016). They chose three stages for treatment. These stages were stage 22, which gills were formed and shh was present at the back of the epithelium arch, stage 27 which the arch was experiencing lateral expansion and stage 29 which was before the condensation of the gill arch endoskeleton (Gillis, 2016). After working on these stages, they noticed the branchial ray deficiency looked the same as the skeletal deficiency they noticed when they were using the shh signaling during the limb growth (Gillis, 2016). Upon completion, they proposed that it is attainable that limbs share a common pattern mechanism with gills arches (Gillis, 2016).
Fin fold theory
According to Tanaka, the embryos of the first cartilaginous fish was proposed to have lateral fin folds so he and his colleagues decided to examine how fin grow in the embryo of dogfish using scanning electron microscopy and explore the genes that are present in limb positioning, recognition and higher vertebrates (Tanaka at el, 2002). After performing the experiment, they didn’t find lateral fin fold in dog fish but engrailed 1 showed that the body of the dogfish is dorsal-ventral (Tanaka at el, 2002). As in higher vertebrates, the description of their limbs was through Tbx4 and Tbx5 genes. According to the authors, even though they did not find lateral fold in dogfish, they propose that there has been an early vertebrate which was dorsal-ventral with engrailed 1 expression and had lateral folds that showed a single Tbx4/5 gene (Tanaka et al, 2002). They concluded that lateral fold can possibly be primitive (Tanaka et al, 2002).
Personally, in my opinion the lateral fin fold is correct because it makes sense to me, but the arch gill theory doesn’t make sense to me because, some fishes have their pelvic fin backwards towards the tail which is not connected to the gill arch in anyway. I think It should be closer to the pelvic fin and be at the back of the gill arch in every fish. In that way if you look at the fish, it will make some sense because they are very close to the gill arch.
The strength of genetic evidence is very strong because most species have DNA which makes it easier for scientist to trace their ancestors and help them support their claims.
Gillis, Andrew. (2016, April 19). Gills, fins and the evolution of vertebrate paired appendages. Retrieved from http://thenode.biologists.com/gills-fins-evolution-vertebrate-paired-appendages/research/
Tanaka M, Munsterberg A, Anderson WG, Prescott AR, Hazon N, Tickle C. 2002. Fin development in a cartilaginous ?sh and the origin of vertebrate limbs. Retrieved from https://www.nature.com/articles/416527a.pdf