Protochordates: The Evolutionary Roots Quiz

Protochordates: The Evolutionary Roots of the Chordate Family

Protochordates, also known as lowest chordates, are a group of aquatic organisms that share some but not all characteristics with their more advanced relatives, the vertebrates. They play a crucial role in our understanding of chordate evolution and the development of the nervous system.

Key Characteristics

Protochordates have a notochord for a period of their life cycle, which provides support to their bodies. They are coelomates, meaning they have a true coelom surrounded by mesoderm. This is a condition that occurs in more advanced chordates, such as vertebrates, where the coelom serves as a cavity that holds the organs of the body.

Subphyla and Habitats

The phylum Chordata, which includes protochordates, is divided into three subphyla based on the type of notochord they have:

• Hemichordata: These organisms have a dorsal notochord, which is not present in the tail.
• Urochordata: This subphylum includes tunicates and lancelets. Tunicates are filter-feeders that live in seawater, while lancelets are small fish-like creatures found in both salt and fresh water.
• Cephalochordata: This subphylum includes organisms like amphioxus, which have a notochord throughout their entire body.

Evolutionary Significance

Protochordates have been instrumental in understanding the evolution of the chordate family. They are believed to be the most primitive chordates, with some features that are not present in more advanced chordates, such as vertebrates. Their study has provided insights into the development of the nervous system, including the centralization of the nervous system.

Notable Species

One of the best-characterized tunicate model organisms is the solidary ascidian, Ciona intestinalis. This species has the ability to regenerate its neural complex (NC) and syphon following ablation of the NC.

Neural Plate Development in Protochordates

The development of the chordate neural plate is a crucial aspect of chordate evolution. The neural plate becomes confined to the neural tube over time, which eventually gives rise to the brain and spinal cord in more advanced chordates. However, in protochordates like amphioxus, the cells that express border region genes are not bona fide neural crest cells or placodes as they lack the capacity for invagination or as seen in the vertebrate head.

In summary, protochordates provide valuable insights into the evolutionary roots of the chordate family. Their study has helped us understand the development of the nervous system and the evolution of more advanced chordates, including vertebrates.