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The Development of Volvox Carteri

Volvox carteri is a multi-cellular chlorophyte alga belonging to the volvicene group of algae. Morphologically, V. carteri is composed of two cell types: a mortal somatic cell and an immortal germ cell called gonidia. The mortal somatic cells are small biflagellate with a fixed number of chlorophyll and incapable of reproduction. They number at approximately 2000, line the external sphere of the organism and differentiated specifically for movement. The gonidia, on the other hand, number around 16 and are much larger compared to the somatic cells.

They are incapable of movement and are primarily designed for growth reproductive purposes. V. carteri has a 48-hour life cycle that allows for easy laboratory culture and includes an embryogenesis program that features many similarities with animal and plant development. These features include embryonic axis formation, asymmetric cell division, a gastrulation-like inversion, and differentiation of germ and somatic cells which make the organism a frequent object of study. It is closely related to another member of the volvocine algal group named Chlamydomonas reinhardtii, a unicellular member of the taxa.

Due to this relation and its simple structure, V. carteri is often the subject or specimen of choice in studies focused on germ-differentiation and evolution of multi-cellularity. One such study is the research conducted by Kirk et al. which focuses on a specific Volvox gene (labeled as regA) long accepted to have a central role in the germ-soma differentiation process in V. carteri. Findings from their research reveal that RegA – the product of regA transcription – functions as a transcriptional repressor of reproductive functions.

This conclusion was made due to the fact that, though the BLAST search for a transcriptional repressor protein sequence of the deduced peptide yielded no results, a number of protein peptides with a similar sequence were identified. This finding was further reinforced by the PSORT program which assigned regA a nuclear location. Additionally, these peptide sequences are generally abundant in amino acids A, Q, and P, which are the key unifying “feature[s] of a group of transcription factors that often have little discernible structural similarity otherwise, but that all function as ‘active’ repressors” (Kirk 645).

Most importantly, the authors found that the genes being regulated by the RegA repressor were responsible for encoding key chloroplast proteins which led the authors to hypothesize that RegA “prevents somatic cells from entering the reproductive pathway […] by preventing transcription of genes that are required for chloroplast biogenesis and (indirectly) cell growth” (Kirk 646). A second study further explores the aspect of repression in V. carteri somatic cells through the study of two enhancers and one silencer in the introns of regA.

Introns, also known as intragenic regions or intervening sequences, are DNA regions that are not translated into proteins. Introns generally play a role in mRNA splicing, transcription, and, as of recent discovery, miRNA (micro RNA) transcription. In this particular study, researchers focused on introns 3, 5, and 7 found in the regA gene. Introns 3 and 5 are both required to express regA in somatic cells while intron 7 is essential in silencing regA in gonidia. The study shows that intron 7-lacking regA genes can be used to “rescue” normal phenotype mutant somatic cells.

However, this resulted in the development of a new phenotype the researchers labeled as fruitless since the resulting gonidia were only able to reproduce weakly and soon died out. Apparently, intron 7 is only active in gonidial cells wherein it interrupts the binding of trans-acting-factors usually encouraged by introns 3 and 5 in order to express or activate the regA gene. As a result, the researchers concluded that the true function of intron 7 in gonidia is that it is a classical cell-type-specific, promoter specific enhancer of the inhibitory type commonly referred to as silencers (Stark, Kirk, and Schmitt 1449, 1458).

With a more thorough understanding of how multi-cellularity is genetically determined and maintained in the simplest multi-cellular organism known as V. carteri, a more recent study – by researchers Duncan et al. – decided to focus on genetic similarities between the two often studied species in Volvocaceae: Chlamydomonas reinhardtii and Volvox carteri. The researchers expanded on the knowledge that the gene regA found in V. carteri shares a conserved region with several predicted V. carteri and C. reinhardtii proteins called the VARL domain.

An analysis of the genome sequences of both species was then conducted to further identify additional genes with the potential to encode VARL domain proteins. The analysis reveals that there is a complex evolutionary history between the 12 C. reinhardtii and 14 V. carteri VARL gene families and that both have lost and gained VARL genes over time. It was also found that there are still possible orthologous genes identifiable and that regA is part of four VARL genes in V. carteri. Most importantly, the researchers found that “proto-regA gene was present in a common unicellular ancestor of V.

carteri and C. reinhardtii” but was lost in the latter lineage (1). As all three researches imply, the key to understanding the evolution of multi-cellularity in all eukaryotes lie in the complex study of genetics. The studies conducted on Volvox carteri, one of the simplest multi-cellular organisms, reveal that the gene regA plays a vital role in cell differentiation. Furthermore, analysis of this specific gene indicates that DNA is not the only factor responsible for cell development and reproduction.

If these three studies were to imply one thing is that transcription – the process by which proteins are made – plays a vital and complex role in maintaining and differentiating cellular make-up in multi-cellular eukaryotes. As the third research asserts, genetic understanding of unicellular and multi-cellular organisms belonging to the same taxa is a key to understanding evolution.

Works Cited

Duncan, Leonard, Ichiro Nishii, Alexandra Harryman, Stephanie Buckley, Alicia Howard, Nicholas R. Freidman, and Stephen M. Miller. “The VARL Gene Family and the Evolutionary Origins of the Master Cell-Type Regulatory Gene, regA, in Volvox carteri.” Journal of Molecular Evolution 65. 1 (2007): 1-11. Kirk, Marilyn M. , Klaus Stark, Stephen M. Miller, Waltraud Muller, Bruce E. Taillon, Heribert Gruber, Rudiger Schmitt and David L. Kirk. “RegA, a Volvox Gene that Plays a Central Role in Germ-soma Differentiation, Encodes a Novel Regulatory Protein. ” Development 126. 4 (1999): 639-647. Stark, Klaus, David l. Kirk, and Rudiger Schmitt. “Two Enhancers and One Silencer Located in the Introns of RegA Control Somatic Cell Differentiation in Volvox carteri. ” Genes Development 15 (2001): 1449-1460.

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