How do fins and limbs develop and evolve?

Abstract

How do fish fins come into being and how do fins evolve into terrestrial tetrapod limbs? To answer this scientific question is substantially to understand how the genes that determine these traits are subject to natural and sexual selection, and how they thereby involve the innovation mechanics and evolution mechanisms with respect to development. Increasing evidence in recent 10 years suggests that fins might be as single fin-fold in early jawless fish, and then get makeable changes on the evolution roads from jawless fish to jaw fish and tetrapods, in which sequence divergence, expression pattern change, cooperative interaction, and even deletion of some required genes for fin development might play critical roles. The origin of fish fins and tetrapod limbs has been debated for more than three centuries from early natural philosophers and anatomists to modern developmental geneticists. In the recent 10 years, extensive and valuable data have been revealed from a lot of studies on paleontology, comparative anatomy and developmental genetics, and these complementary results have made us to understand the origin and underlying mechanisms behind the formation of the fish fin and tetrapod limb skeletons. It has been certified that fish has evolved to tetrapods and the multiple-basal bones of ancestral fish fins have been reduced to evolve into a single bone of tetrapod limb with fossil records. Similarly, during early embryogenesis, fish fins and tetrapod limbs share similar development mechanism, even though significant differences in histological, developmental and structural patterns exist between them. Many functional genes, such as Hox , And , Gli3 , Hand2 , play key roles in this transition or transformation. Natural selection and sex selection have led to sexual dimorphism of fish fins and morphological differences of tetrapod limbs. Along with the rapid development of genomics, genome editing and biotechnological manipulation techniques, the genetic basis of sexual dimorphism has been explored. Some gene families and signal pathways, such as Tbx gene family, Hox gene family, Ptch1 and its regulation elements, have been identified to involve in the morphological evolution. Significantly, it has been demonstrated that the differential expression of some sex-biased genes and its affected genes which are related with growth and gender differentiation may ultimately lead to sexual dimorphism of the fins. In this article, we have reviewed origin, evolutionary history and development patterns of fins, and elaborated molecule mechanisms on fin-to-limb evolution, sexual dimorphism of fish fins and morphological evolution of limbs. It is expected that a new era is coming for studies on developmental mechanisms and genetic basis of fin origin and fin-to-limb evolution, and some novel sights will be further provided.