Question

One feature that unites Kingdom Animalia is embryonic development, which is not found elsewhere in the Unikonta. Explain how specific changes in embryogenesis resulted in the diversity in adult animal body plans across the major clades of Animals. Make sure to discuss how the stages of embryogenesis have evolved across the Animal phylogeny and how these changes have allowed for new adaptations to arise. Please include one paragraph (and no more) discussing the role of homeotic genes in embryonic development.

Answer 1

One feature of Kingdom Animalia is the embryonic development. According to Russell’s orthogenetic theory it has been suggested that the major diversity in adult animal body originated from evolutionary transformations that occured during early embryonic development. Since it is the structures which constitute this diversity between types first arise. Typal diversity is the structural differences between groups of organisms and can arise only if changes occurred early in development and continues so that it would normally develop. Russel had taken an example to prove this--that the difference between lobsters and crabs is due to typal diversity. The major structural features that make crabs differ from other decapods occurred early in the evolutionary history of the decapods. He also proved that the major structural differences between types are fundamentally nonadaptive. Therefore, the major animal diversity could be from directed transformations in early development.

The stages of embryogenesis and their changes have emphasized on the relation between development, ecology and broad patterns of animal biodiversity. If early development is an adaptable means by which animal species can exploit varied ecological niches, then there must be some animal groups to evolve developmental systems that can accommodate adaptive changes during their early stages. With studying the stages of embryonic development--the fertilization of the egg transforming it into a zygote, the formation of the primary axes of the embryo, the rapid nuclear or cellular divisions transforming the single cell into a multicellular organism, the activation of the zygotic genome during the maternal–zygotic transition, and the rearrangement of a single layer of cells into cell layers ofthe animal (gatrulation), all these steps are required to generate the raw materials needed for building a body plan, but it does not show conservation of the pathways that produce these steps. It is shown that early development can be characterized by a series of developmental events with divergent modes of development. Example- Drosophila in which the specification of all of the body segments from anterior to posterior occurred before gastrulation.

The major effect of long-germ development is to accelerate embryonic development of the larva with the shortening of the length of development. This shortening of embryonic development could be an adaptation to particular ecological differences. In parasitic wasps, an alteration of development evolved where several thousand embryos develop clonally from a single egg, which is called as polyembryony. This derived mode of development made a major changes in the axial patterning of the early embryo. Two larval castes are produced by the polyembryo-a soldier caste (defending) and a reproductive caste. These castes are identified by cellular asymmetries that appear at the four-cell stage. The rearrangement of cells results in the three primary cell layers that give rise to the structural topology and organ system of the animal. Sea urchins that developed into larvae have evolved divergent modes of gastrulation, axis formation and cell lineage-patterning relative to species that develop a feeding larval stages.

Homeotic genes are called regulator genes that make the development of particular body segments or structures. When these genes are overactivated or inactivated by mutations, body structures dev will be developed wrongly. It encodes transcription factor proteins that has a region called homeo domain.