Volvox

Volvox is a genus of microscopic chlorophyceous algae that usually forms spherical and hollow-shaped colonies or coenobia, surrounded by biflagellate surface cells and linked together by cytoplasmic connections. Inside the colony there are multiple oospores. This primitive organism lives in oxygen-rich waters.

Features

Cell colonies show some degree of cellular specialization, with numerous small vegetative or somatic cells.

Embryogenesis and Colonial Development

Introduction

The embryogenesis of Volvox has been studied mainly in the species Volvox carteri and presents multiple simple division patterns that lead the organism to demonstrate certain complexity at the peak of its development. Volvox carteri has 2 cell types in its mature state. Reproductive cells -or gonidia- of the germinal type which do not present patterns of senescence (or programmed cell death) known to date. The second type are the somatic cells that confer motility to the colony, they have a higher division rate, so in number (from 2000~4000) they are many more than gonidia, but they eventually enter a cell death process afterwards. to fulfill its function of support and mobility.

Volvox Structure Scheme

The differentiation of cells in Volvox is dichotomous (a cell can only be differentiated as a gonidia or as a somatic cell) and is dependent on the expression of different proteins -mainly membrane proteins- that signal the differentiation of cells in one or another type of cell. This means that it has been proposed as a model to explain multicellularity in phylogenetically primitive organisms and that it is fairly easy to control it genetically to evaluate its resulting phenotypes. In addition, some works have tried to show evolutionary relationships between Volvox species and close clades by studying the duration of this differentiation and how the morphology and presence of characters throughout ontogeny change between groups. Volvox embryogenesis has been studied mainly in the species Volvox carteri and presents multiple simple patterns of divisions that lead the organism to evidence of a certain complexity at the peak of its development. Volvox carteri has 2 cell types in its mature state. Reproductive cells -or gonidia- of the germinal type which do not present patterns of senescence1 (or programmed cell death) known to date. The second type are the somatic cells that confer motility to the colony, they have a higher division rate, so in number (2000~4000) they are many more than gonidia, but they eventually enter a process of cell death later. to fulfill its function of support and mobility.1

Volvox Structure Diagram The differentiation of cells in Volvox is dichotomous (a cell can only be differentiated as a gonidia or as a somatic cell) and is dependent on the expression of different proteins -mainly membrane proteins- that signal the differentiation of cells into one type or another. of cell. This means that it has been proposed as a model to explain multicellularity in phylogenetically primitive organisms and that it is fairly easy to control it genetically to evaluate its resulting phenotypes.1 In addition, some works have tried to show evolutionary relationships between Volvox species and close clades by studying the duration of this differentiation and how the morphology and presence of characters throughout ontogeny change between groups.

Cell Differentiation

Cell differentiation in Volvox describes a pattern of a multicellular organism –similar to gastrulation-. Initially, the asexual cell presents unequal and asymmetric cleavage to generate the embryo; however, during the sixth division, where there are 32 cells, half of the cells are determined to be reproductive cells (gonidia) so they follow an unequal cleavage pattern, while the other half have a somatic cell fate and present a pattern of cleavage. symmetrical cleavage. This pattern is explained because after this cleavage, this set of 16 cells is made up of quadrants of 8 cells that overlap one another and divide in the anterior-posterior plane, resulting in a multicellular organism with 16 reproductive-type cells. and approximately 2,000 to 4,000 flagellated somatic cells.

Levels of gonidia in embryo

In this process described in Volvox carteri it is important to know how the gonidia are organized during cleavage. This process is mediated by 4 major proteins that are expressed chronologically and hierarchically. The model presented by Sumper(1982) is based on the fact that these membrane proteins are produced by cells determined to be gonidia, have contact points through them and signal the different cleavage steps. There are 5 levels of division and in each of these a new protein enters to express itself. At level 0 there is only one mother cell in which protein A is expressed on its membrane. At level 1, the embryo expresses protein A and there is an A-a contact point that will give rise to the first division septum, resulting in two daughter cells. At the second level, membrane protein B begins to be expressed, which has contact points between cells that only express protein B and cells that express proteins A and B. The result is two new cells (A, B, AB, AB) At the third level, protein C begins to be expressed. An embryo with 8 cells is produced (ABC, ABC, BC, BC, C, C, C, C) In the fourth level there is production of protein D. At the end, obtaining 16 mature gonidias, each one of which has the information of the mother cell. Finally, each mature gonidia generates an embryo with the same cell content as the mother, following the same cleavage pattern, establishing again colonies of asexual Volvox carteri. After the sequencing of the Volvox carteri genome in 2007, doors have been opened for the identification of genes strictly and deeply involved in development. One of them is the glsA gene that codes for a chaperone protein that has a J domain similar to that of E. coli(J-like) Apparently this protein is responsible for the location of the mitotic spindle in the blastomere after undergoing phosphorylation. It is striking that gonidia have a functional copy of this gene. However, somatic cells present a mutation and that is why their cleavage is affected, showing asymmetric divisions, possibly due to the wrong location of the microtubules during mitosis. Likewise, other important gene families have been shown in Volvox ontogeny.. The Gls or Gonodialess (Without gonidia) genes show a phenotype without gonodias, with only somatic cells, or the Lag genes (Late gonidia) develop smaller gonodias with morphologies similar to somatic cells. In the Volvox genome sequencing report it is mentioned that the complexity in the differentiation and development of multicellularity in Volvox apparently originates from small species-specific sequence changes and not from the invention of large and novel protein domains. This is why Volvox continues with a great breakthrough for the study of differentiation, multicellularity and cell segmentation. Future studies should look for new comparisons between species and delve into gene expression and compare these with exomes and proteomes.

Playback

Volvox reproduce sexually and asexually:

• Asexual reproduction: the mother colony can be divided and formed daughter colonies. When the mother colony dies and breaks down, the daughter colonies are free.
• Sexual reproduction: colonies form male sperm gametes and female egghead gametes that are released to water. When a sperm fevers an ovocell, a zygote is formed that gives rise to a new colony.

A study has used Volvox carteri as a model to explain the genetic changes that lead to anisogamy. The transcriptomes of Chlamydomonas reinhardtii and V. carteri, finding a great expansion of the loci that determine the sex of the gamete. In particular, the homologous gene for the retinoblastoma tumor suppressor, MAT3, processes its transcript in V. carteri in a gender-differential manner.