12

ORIGIN OF METAZOA

Most zoologists agree that metazoans share a common ances-try with sorne unicellular organism. The classical colonial theory, in which Metazoa is derived from a colony of flagel-lated protozoa, is the most widely accepted hypothesis among contemporary zoologists (Fig. 4-12). An alternative hypothe-sis, the syncytial theory, proposes that metazoans evolved from a multinucleate but unicellular plasmodium similar to a slime mold or perhaps a ciliate protozoan. Later, membranes evolved to produce a cell boundary around each of the nuclei. The syncytial theory receives sorne support from the development of organisms such as slime molds or insects (Drosophila), in which an early multinucleated stage is followed by cellularization to form a multicellular body. However, phylogenetic analysis based on morphology and gene sequences as weil as the developmental patterns of most animais contradict the syncytial theory and favor the colonial theory. For these reasons, we consider only the colonial theory here.

A modern version of the colonial theory states that the premetazoan (a protozoan) consisted of a small spherical colony bearing a surface layer of flagellated cells that was used for locomotion and feeding (Fig. 4-12C). The colony origi-nated from a cell that divided repeatedly by mitosis, but the daughter cells did not separate after cell division. Those daughter cells were surrounded and held together by a pro-teinaceous ECM in which they were deeply embedded. The gelatinous ECM also occupied much of the interior of the sphere. Similar to extant choanoflagellates, the cells each bore a single collared flagellum. A few nonflagellated cells, capable of giving rise to flagellated cells and gametes, were scattered in the subsurface ECM .

Although it is unrelated to metazoans, Volvox is an analog for an ancestral metazoan because it demonstrates how a mul-ticellular organism evolved from a unicellular ancestor, in this case a Chlamydomonas-like cell. Volvox is not the ancestor of Metazoa, but rather is an autotrophic organism with plantlike cells. rRNA sequence data indicate that multicellularity in Volvox evolved 50 to 75 million years ago, far too late for it to have been a progenitor of metazoans, which had their origin at !east 600 million years in the past. Thus, the volvocids evolved multicellularity in parallel with the metazoans (and with at !east four other groups: fungi, brown algae, red algae, and green plants).

The actual sister taxon of Metazoa is most likely Choanoflagellata. Among the. extant choanoflagellates, colonies of Proterospongia haeckeli closely resemble the hypothetical premetazoan. The colony consists of flagellated collar cells embedded in the surface of a gelatinous ECM. Cells undergoing division and lacking flagella occur deeper in the ECM . A surface sheetlike layer of cells and an ECM containing individual free cells foreshadow the two primary metazoan tissues, epiti).elial and connective, described earlier in this chapter. At the cellular leve!, choanoflagellate collar cells are virtually.identical to collar cells (choanocytes) found in the metazoan sponges. A choanoflagellate cell and sponge choanocyte both have a single flagellum surrounded by a collar of microvilli and a flagellar shaft with a bilateral ~nlike vane . In both, the flagellum is anchored

10 the cell by microtubules, which radiate from the flagellar basal body.

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The hypothetical first metazoan, the protometazoan may have differed from the premetazoan in severa! ways. First, the surface cells probably closly adjoined or were in contact with each other, thus

faCihtating intercellular communication and providing a gulatory barrier between the external environment and ECM from the cells, thus separating the ECM into external and Internal layers, each of which could then adopt indenent functions . Third, the body was polarizedized along the anterior-posterior axis. Fourth, the separation of layers and body polarity promoted cell specialization.

ORIGIN OF POLARITY AND

CELL SPECIALIZATION

Most motile protozoans are polarized cells that have leading (anterior) and trailing (posterior) ends or, if sessile and attached, they have oral free ends and aboral attached ends. Metazoans are similarly polarized, depending on whether they are motile or sessile, but how did the polarity of the multicellu-lar metazoan body evolve from the unicellular polarity of pro-tozoans? A clue to the answer is found in the eggs of severa! groups of metazoans. During oogenesis in these groups, the eggs express a rudimentary flagellum and a collar of microvilli at a site on the cell surface that corresponds to the animal pole ofthe zygote . In certain invertebrate taxa, the animal pole corresponds to the anterior end of the larva (although it is the posterior end in others). Additional research is needed, but the current evidence suggests a line of descent from the polarity of a choanocyte to the primary ante-rior-posterior polarity of the metazoan body .

The protometazoan probably was polarized along an anterior-posterior axis, but what environmental conditions might have selected for the evolution of such polarity? For an aquatic metazoan, the environment presents itself in gradients of light, temperature, oxygen, and food availability. If, for selection may have favored any organism capable of tracking a resource concentration graclient. Altematively, developmentai biologist Lewis Wolpert suggests that body polarity evolved from attachment to a substrat~. Attachment to a rock in water, for example, places an organism at an interface, a very steep gradi. ent. Once attached, variants would be favored that adhere well at the attached end and perform other tasks, such as feeding, at the opposite end. This again leads to a polarized body.

Once polarity was established, movement would create an environmental graclient along the locomotory axis that would favor differentiai expression of traits . For exam-ple, enhanced membrane sensitivity to environmental stimuli might be favored in cells at the anterior end of the body because they are the first to encounter changes in environ-mental conclitions. Similarly, enhanced flagellar growth, density, or activity might be favored in cells at the equator, or widest part of the body, since those locations best contribute to locomotion. Cells with a capacity for division, leading to growth, might be favored at the. posterlor end, because in that position they contribute to and interfere !east with locomo-tion. Thus, motility along a polar axis may itself promote cellular specialization because the cells occupy different fixed positions in an environmental graclient .

According to a hypothesis by developmental biologist Leo Buss, the origin of rnetazoan cell specialization may be related to a conflict between the demands for growth and locomotion. Volvox, Proterospongia, and planktonic blastula stages of metazoans require flagellated surface cells for loco-motion, but most flagellated cells cannat divide by mitosis be-cause the centrioles needed to form the mitotic spindles are already in use as the flagellar basal bodies. A metazoan flagel-lated cell can divide only after the flagellum regresses and its basal bodies are freed to form the mitotic apparatus. Thus, the options for growth in a premetazoan composed solely of fla-gellated cells (for example, the species of Proterospongia ) are: (1) enlargement of existing cells; (2) disassembly of flagella, cell clivision, and then flagellar reassembly; (3) clivi-sion of a few cells scattered throughout the embryo while ath-ers retain flagella; or ( 4) division of a few localized cells set aside for growth . Option 1, because of the restricted number of cells, limits ultimate body size, although Volvox daughter colonies and a few postembryonic micrometa-zoans grow by cell enlargement only. Option 2, by requiring the regression of flagella, compromises locomotion. Options 3 and 4 both permit growth and locomotion, but most inverte-brate metazoans have adopted option 4. The setting aside of mitotically active cells enables metazoan growth without compromising locomotion . (A related evolu-tionary event was the setting aside of germ cells capable of meiosis. Surface flagella are a functional necessity for locomotion in the protometazoan, but what is the optimal position for the set-aside growth cells? Buss suggests that placement at the surface might have resulted in overgrowth of locomotory cells or growth of a tumorlike appendage, either of which would negatively affect motility. Intemalizing the growth cells, however, would neither distort the body surface nor inteffere with the locomotory cells. Perhaps for these reasons the protometazoan did intemalize its growth cells and the process was preserved, as gastrulation, in its descendants. (Based on ideas in Buss, L. W 1987. The Evolution of lndividuality. Princeton University Press,Princeton, NJ 201 pp.

ORIGIN OF COMPLEXITY

Evidence supports the hypothesis that metazoans evolved from protozoan colonies in which initially similar cells became specialized for different functions. If so, the evolu-tion of Metazoa can be described as a replication of similar units (cells) followed by unit specialization and integration into an organism at a new, higher level of complexity. This replication-specialization-integration of units. sequence is a general pattern in the evolution of large body size and complexity. An example, as we have seen already, is the replication, specialization, and integration of cilia on the body of many ciliates, which are among the largest, most diverse, most active, and most complex protozoan cells. Among metazoans, one example is a body corriposed of a series of similar segments, as in earthworms or crustaceans. Later, these segments become specialized and integrated into regions, such as the head, thorax, and abdomen. Exam-ples at ali levels of biological complexity are illustrated nd described .

References :

Modern Text Book of Zoology: Invertebrates(Prof. R.L.Kotpal)

https://www.wikipedia.org

http://www.authorstream.com

http://ebooks.cambridge.org/chapter.jsf?bid=CBO9780511623547&cid=CBO9780511623547A043

CONCLUSION:

From the whole topic, on the basis of different theories proposed by the different scientists, we can say metazoan originated from different types of organisms of protozoan.