Dipleurula concept and Echinoderm theory

 

Origin of Chordates- Dipleurula concept and Echinoderm theory

The term "dipleurula" was first introduced by Semon in 1888. However, a more accurate depiction of this hypothetical larval form was provided by Bather in 1900, which gained acceptance among most zoologists. Most echinoderms exhibit indirect development, featuring free-swimming, bilaterally symmetrical larval stages. These echinoderms produce small eggs that develop in seawater. The eggs undergo holoblastic, nearly equal, radial cleavage to form a hollow, one-layered, ciliated blastula. It is then transforms into a gastrula via invagination. The gastrula's cilia are arranged in two specific bands: (i) a large pre-oral band around the mouth on the ventral side and (ii) a smaller aboral band lining the mouth or stomodeum. This larval stage is referred to as the dipleurula larva, which is considered the hypothetical ancestral form of echinoderms.

Dipleurula as an Ancestral Form

The dipleurula larva represents an ancestral form for primitive deuterostomes. Various well-known larval forms of echinoderms, such as the bipinnaria and brachiolaria of sea stars, the auricularia of sea cucumbers and the plutei of sea urchins, are derived from this hypothetical dipleurula form. Bather's concept of the dipleurula, proposed in 1900, suggests that the common ancestors of echinoderms did not possess all the characteristics found in modern free-swimming bilateral larvae of different echinoderm groups. Instead, they likely exhibited some unique features not shared by any present-day larvae. Thus, the dipleurula concept does not perfectly illustrate the common ancestor of echinoderms but rather highlights shared features among current echinoderm larvae.

Echinoderm Theory of Chordate Origin

The origin of chordates from invertebrates is a widely accepted idea, although pinpointing the specific invertebrate group from which chordates evolved remains challenging. The soft-bodied nature of chordate ancestors has left little fossil evidence. Various theories propose that chordates evolved either directly from an invertebrate group or through protochordates. Although many invertebrate phyla, such as Coelenterata, Nemertea, Phoronida, Annelida, Arthropoda, and Echinodermata, have been suggested as potential ancestors, these theories lack convincing evidence and remain largely historical. Among these, the echinoderm theory has gained some acceptance.

Johannes Müller proposed the echinoderm theory in 1860, based on comparative studies of echinoderm and hemichordate larval stages. Garstang and de Beer later suggested that chordates evolved from echinoderm larvae through a process called neoteny.

Evidence Supporting Echinoderm Theory

1. Embryological Evidence: Echinoderms and chordates both exhibit enterocoelic coeloms, mesoderm formation and a deuterostomous mouth. The bipinnaria larva of echinoderms resembles the tornaria larva of hemichordates.

2. Serological Evidence: There is a close similarity between the proteins found in the body fluids of chordates and echinoderms, suggesting a closer evolutionary relationship.

 

The adult radial symmetry of echinoderms, however, appears to contradict their relationship with bilaterally symmetrical chordates. Nevertheless, the bilateria can be divided into two major groups based on embryonic and larval development: Protostomia (e.g., Annelida, Arthropoda) and Deuterostomia (e.g., Echinodermata, Pogonophora, Chordata).

Deuterostome Evolutionary Relationship

Several common features among deuterostomes (Echinodermata, Hemichordata, and Chordata) provide strong evidence of their close evolutionary relationship:

1. Early cleavage of the zygote is indeterminate.

2. The blastopore of the gastrula develops into the anus.

3. The coelom (enterocoelous, except in vertebrates) forms from the fusion of pockets in the embryonic archenteron.

4. Pelagic larvae of echinoderms and hemichordates resemble each other closely.

5. Deuterostomes use creatinine as a phosphogen, whereas invertebrates typically use arginine. Some hemichordates and echinoids use both.

Ancestry and Evolution of Chordates

Echinoderm Ancestry: The tornaria larva of hemichordates and the bipinnaria or dipleurula larva of echinoderms share significant similarities, such as being small, transparent, free-swimming, and bilaterally symmetrical, with similar ciliated bands, a dorsal pore, sensory cilia, and a complete digestive system. These similarities led Müller and Bateson to propose a common ancestry for echinoderms and hemichordates. However, the presence of an apical plate with eyespots in the tornaria larva raises doubts about this common ancestry. Garstang and de Beer suggested that the auricularia larva of echinoderms might have become sexually mature through neoteny, eventually giving rise to chordates. Fossil records from the Cambrian and Ordovician periods support the idea of a common ancestor, but the direct transformation of echinoderm larvae into chordates is no longer accepted. Instead, a common immediate ancestor is proposed.

Hemichordate Ancestry: Hemichordates exhibit features such as pharyngeal gill slits and a hollow dorsal nerve cord, suggesting a sedentary, sessile early evolutionary stage for deuterostomes. However, the absence of a true notochord and differences in adult body plans make it unlikely that hemichordates are direct ancestors of vertebrates.

Urochordate Ancestry: The ascidian theory, proposed by Garstang in 1928 and elaborated by Berrill in 1955, suggests that urochordates or ascidians are ancestral to vertebrates. While the adult tunicates reflect the primitive sessile, marine, filter-feeding condition, their body plans are too divergent to imagine a direct evolutionary transformation. However, the tadpole-like larva of ascidians, with its pharyngeal gill slits, notochord, dorsal hollow nerve cord, and muscular post-anal tail, resembles ancestral vertebrates. According to this theory, some ascidian larvae became neotenous and evolved into cephalochordates and vertebrates. Despite this, the theory's major drawback is that it considers the highly specialized sessile urochordates as ancestral to chordates, whereas sessility is generally a specialized condition in the animal kingdom.

Points to remember

1. The term "dipleurula" was coined by Semon in 1888, with a proper illustration by Bather in 1900 that gained acceptance among zoologists.

2. Echinoderms typically exhibit indirect development with free-swimming, bilaterally symmetrical larvae, starting from small eggs that undergo holoblastic, nearly equal, radial cleavage.

3. The cleavage results in a hollow, one-layered, ciliated blastula that transforms into a gastrula by invagination, forming two specific bands of cilia.

4. The dipleurula larva is considered the hypothetical ancestral form of echinoderms, exhibiting certain shared features among present-day echinoderm larvae.

5. The dipleurula larva represents an ancestral form for primitive deuterostomes, with various well-known echinoderm larvae (bipinnaria, brachiolaria, auricularia, plutei) derived from this form.

6. The origin of chordates from invertebrates remains widely accepted, with the echinoderm theory gaining some acceptance. Johannes Müller proposed it in 1860, suggesting chordates evolved from echinoderm larvae through neoteny.

7.  Echinoderms and chordates share features such as enterocoelic coeloms, mesoderm formation, and a deuterostomous mouth. The bipinnaria larva of echinoderms resembles the tornaria larva of hemichordates.

8. There is a close similarity between the proteins found in the body fluids of chordates and echinoderms, indicating a closer evolutionary relationship.

9. Deuterostomes (Echinodermata, Hemichordata, Chordata) share several common features, including indeterminate early cleavage, blastopore developing into the anus, enterocoelous coelom formation, similar pelagic larvae, and the use of creatinine as a phosphogen.

10. Similarities between hemichordate tornaria and echinoderm bipinnaria/dipleurula larvae support a common ancestry.

11. Hemichordates show some features but are unlikely direct ancestors of vertebrates due to the absence of a true notochord.

12. The ascidian theory suggests urochordate larvae evolved into cephalochordates and vertebrates through neoteny, though this theory has significant drawbacks considering sessility as a specialized condition.