How does multicellular algae reproduce

Our results demonstrate a surprisingly extensive compartmentalization of the transcriptome between cell types: More than half of all genes show a clear difference in expression between somatic and reproductive cells. This high degree of differential expression indicates a strong differentiation of cell types in spite of the fact that V.

The analysis of cell-type specific gene expression also made it possible to provide new information about the expression pattern of previously investigated Volvox genes roughly four hundred genes in number.

In the long term, the study of molecular processes in simple organisms should lead us to a better understanding of the developmental history and key functions of far more complex life forms.

View the latest posts on the On Biology homepage. The multicellular green algae Volvox carteri. Rough approximation of the evolution of volvocine green algae. About Latest Posts. Armin Hallmann. Algae are small, non-vascular, non-flowering, aquatic plants, containing chlorophyll for photosynthesis. The plant body of algae is a thallus, and it is not differentiated into a true stem, root, leaves.

Two types of algae can be identified as unicellular and multicellular algae. Unicellular algae are called microalgae while multicellular algae are called macroalgae.

Cyanobacteria blue-green algae , green, red, and brown algae are microalgae. Seaweeds such as kelp are macroalgae. This enhances mobility by allowing for dispersal by wind.

However, any spore benefits by storing material that will be utilized in establishing the new growth following spore germination. The more material saved the heavier the spore is and the more limited its dispersal. These same considerations are significant to seeds, which are multicellular propagules considered in a Chapter Because reproduction utilizes material and energy that might otherwise be used to perpetuate the life of the parent organism, r eproduction generally diminishes the likelihood that the parent will be perpetuated through time.

The magnitude of this detrimental effect varies from highly significant, when reproduction insures the death of the parent salmon, wheat plants to extremely trivial, when reproduction has virtually no effect on the survival of the parent. Because of this impact on the reproducing organisms, reproductive effort is evolutionarily modified and often controlled by specific environmental cues Chapter Th is effect is strongly dependent upon on other conditions.

Most students equate sex and reproduction, but they really are two separate processes that happen to be combined in the organisms that we are most familiar with. Reproduction is about making new organisms; sex is about mixing the genetic information of two organisms.

Bacteria and Archaea exchange genetic information by several different processes conjugation, transformation, transduction but none of these are considered to be sex. Sex is defined as a particular type of genetic exchange that can only happen in organisms with chromosomes eukaryotic organisms. Sex requires the fusion of two cells syngamy , producing a cell with twice the number of chromosomes as either of the parent cells.

Generally, both of the fusing cells have one copy of each chromosome and are described as being haploid, while the fused cell has two copies of each chromosome and is described as being diploid. Sex also requires a mechanism that can produce haploid cells from diploid cells. If one looks at the chromosomes of a diploid cell, one sees ten chromosomes. But closer examination of the chromosomes reveals that there are actually five distinct types of chromosomes present, and their are two chromosomes of each type.

A chromosome can be recognized by its size and shape. And genetic analysis reveals that they are also distinct in the genes that they possess. In Arabidopsis, meiosis produces cells with five chromosomes not ten, moreover, it produces cells that have one of each type of chromosome, i.

Sex is a process that allows genetic material genes from two different organisms to be mixed. It almost always involves producing new individuals reproduction. However, the unicellular organism Paramecium demonstrates that sex can happen with no reproduction: two cells, each with a diploid nucleus join temporarily. The diploid nucleus of each cell undergoes meiosis to form four haploid nuclei, three of these disintegrate and the remaining one divides mitotically to produce two haploid nuclei in each of the joined cells.

Finally, in each cell, the two nuclei fuse to form a diploid nucleus, the original condition. Thus, the cells have undergone the sexual cycle but have not reproduced: there were two cells at the beginning and there are two cells at the end. Although sex and reproduction are different process, they often especially in familiar organisms occur simultaneously. Specifically, sex requires:. Most students consider sex to be related to the fusion process, but it is important to appreciate that meiosis is also an essential part.

The essential components of s ex syngamy and meiosis are sometimes distantly separated in time and may be separated between organisms. We will study a number of situations where two types of organisms are produced, both associated with the same species; one organism develops from a diploid cell zygote produced by syngamy and is diploid; the other results from a haploid cell spore and is haploid.

However, there are other organisms where the cell created by syngamy immediately undergoes meiosis , i.

As we will see there are lots of variations on the basic sexual cycle. Sex is not universal. Many organisms, including some very successful groups Archaea and Bacteria , the endomycorhizal forming Glomeromycota, most dinoflagellates, many fungi have no sexual process.

While sex is generally considered to be significant to the process of evolution because it promotes the variation that natural selection can act upon, it is important to realize that variation and evolution can occur without sex and that the success of a group of organisms at one point in time , and through time , is possible even if that group has no sex. In the familiar case of humans, that cell is the zygote. But in many organisms it is a haploid product of meiosis, generally called a spore, that has the developmental potential to proliferate and form a multicellular organism.

Such organisms will alternate between a haploid stage derived from the spore and a diploid stage derived from the zygote. The rest of this chapter will illustrate several examples of sex and reproduction, showing a diversity of patterns from several different groups.

It is important to realize that for many of the larger groups generally phyla that we study, in particular for the macroalgae green, red and brown algae , sex and reproduction are NOT consistent across the group, i.

In separate chapters we will consider groups that do show some consistency: with several of the fungal phyla Chapter 12 , with non-seed plants Chapter 13 , with seed plants Chapters 14 and Chlamydomonas is a unicellular green alga that primarily reproduces asexually left side of diagram below , i. It is important to keep in mind here that large-scale comparative genomic studies typically uncover only big differences in gene families, or differences in well-known genes and gene families.

Such studies might not uncover subtle differences, such as small changes in the sizes of gene families that occur when a gene is duplicated or lost in one species but not the other. Evolutionary biologists think that gene duplication events could be extremely important for the evolution of new traits, because the new genes are free to change over time and subsequently function somewhat differently from the genes they were copied from.

These mutants are then used to clone the affected genes. After that, researchers analyze the unicellular species genome to determine whether the same orthologous genes exists and, if so, whether or how they differ from the multicellular versions. These types of investigations using current, living organisms are very powerful. On the whole, sorting out the differences between multicellular and unicellular organisms lends clues to how multicellularity may have evolved.

What have the volvocine algae taught us about how multicellularity evolves? Recently researchers sequenced and compared the Chlamydomonas and Volvox genomes and found them to be remarkably similar Prochnik et al. By almost every measure — overall genome size , number of protein-coding genes, number of different kinds of protein domains encoded, and distribution of gene family sizes — the two organisms are very much the same.

When these investigators looked carefully at certain families of genes, especially those known to be involved in regulating the sorts of developmental processes that occur in Volvox but not Chlamydomonas , they again found only similarities, for the most part. Here it is important to point out that the cell wall surrounding Chlamydomonas has two parts: an inner layer and an outer one.

Volvox has versions of both, but the inner layer is greatly expanded compared to the Chlamydomonas inner layer. It makes up the bulk of the ECM that is not present in Chlamydomonas , and it helps cement the Volvox cells together. Researchers believe that the explosion in cell wall genes, and the morphing of some of those genes into different kinds of cell wall genes, is what drove the creation of ECM in Volvox.

Clearly, pure comparative genomic approaches have their limitations; they cannot tell us everything there is to know about how developmental processes and multicellularity evolve.

But genetic screens are possible for Volvox and Chlamydomonas. What insights have these screens provided into how multicellularity evolved in the volvocine lineage? All four genes have easily recognizable orthologs in Chlamydomonas that are very similar to their Volvox counterparts.

Researchers have cloned Chlamydomonas orthologs corresponding to two of the Volvox developmental genes.

One set of investigators showed that the GAR1 gene of Chlamydomonas , which is orthologous to glsA , is able to function just like glsA : When transformed into glsA mutants, it repaired, or rescued, their asymmetric division defect Cheng et al.

Likewise, another set of researchers found that IAR1 orthologous to invA can rescue the inversion defect of invA mutants Nishii et al. Figure 3: Gene and pathway co-option and the origins of asymmetric cell division and cellular differentiation in Volvox A The function of glsA appears to have been co-opted without change from an unknown function in the unicellular ancestor of Volvox, so that it is now part of a pathway shaded green that is required for asymmetric cell division.

This may have happened because some not yet identified gene X that acted in the same pathway shaded gray as the ancestor of glsA proto-glsA changed to take on a new function, generating the new asymmetric division pathway. The dashed arrow indicates that the ancestral pathway may or may not exist in Volvox. B The evolution of the somatic cell fate appears to have involved gene duplication and then change divergence of one of the gene copies, regA.

Scientists hypothesize that the ancestor of regA, proto-regA, acted in a stress-activated pathway shaded gray that led to the repression of growth and cell division.

Thus, regA could have gained its cell fate function because it changed in a way that permitted it to co-opt an existing pathway that repressed growth and cell division. A beige oval at the top of panel A shows the gls-A pathway in a unicellular ancestor. A bold arrow aimed downward points to the altered pathway in Volvox. The original pathway is still present in Volvox and is shown in a beige oval. In addition to this ancestral pathway, Volvox has a new pathway, which is shown in a green oval that overlaps with the beige oval and slopes downward to the right.

Panel B shows how duplication of an ancestral regA could lead to a new pathway in Volvox. A beige oval at the top of panel B shows the ancestral regA pathway. A stress stimulus acts on proto-regA, which inhibits growth and cell division. A bold arrow leads to an intermediate pathway in which regA is duplicated. This intermediate pathway is also shown in a beige oval. A second bold arrow points downward to the Volvox pathway.

Volvox has the unicellular ancestral pathway, which is shown in a beige oval. A second pathway, which is shown in green below the first pathway and points upward to the right where it intersects with the first pathway, partially overlaps the end of the original pathway.

In this second pathway, a developmental cue acts on regA, which inhibits growth and cell division. Thus, the duplication of regA has allowed two different stimuli stress and a developmental cue to lead to the inhibition of growth and cell division.

One way to think about how existing genes like glsA and invA might be incorporated or co-opted without change into a new developmental pathway is to consider the analogy of the gas-electric hybrid car. All cars have brakes. Hybrids are engineered to convert the potential energy generated during braking into electricity.

The brakes on hybrids still function as brakes, but they have also been co-opted into a new "pathway" that generates electricity. Take away the brakes from a hybrid car and it no longer produces electricity.

Think of glsA and invA as the brakes in this analogy; they likely have the same function they had in the unicellular ancestor of Volvox , but take them away and Volvox can no longer do asymmetric division or inversion Figure 3A. Additional insights of a different sort have come from analysis of the somatic regenerator, or regA , gene.

This gene is required for maintenance of the somatic cell fate in Volvox ; regA mutant somatic cells develop normally at first, but instead of remaining somatic cells their entire lives and then eventually dying, as somatic cells usually do, they enlarge and regenerate as gonidia that eventually divide to produce new spheroids Kirk Therefore regA somehow prevents somatic cells from growing and dividing, and keeps them from having the stem cell-like potential that gonidia possess.

Think of regA as a tumor suppressor gene that prevents the sort of uncontrolled growth that cancer cells exhibit. On analyzing the Volvox and Chlamydomonas genomes to determine how many regA -like genes they have, investigators discovered that both algae have a large family of paralogous genes that encode proteins resembling the regA product. But using phylogenetic analyses and other methods, they also found that Chlamydomonas does not have a regA gene Duncan et al.

Why not? In addition, where did regA come from in the first place, and how did it come to take on its role as a master regulator of the somatic cell fate? Researchers found answers to some of these questions through further archaeological analysis of the Chlamydomonas and Volvox genomes. Their analyses revealed that regA likely was generated when a progenitor gene in the ancestor of Chlamydomonas and Volvox was inadvertently copied to produce two paralogous genes: one that eventually gave rise to regA , and one that gave rise to a related gene.

While Volvox retained both regA and the other gene a paralog , Chlamydomonas lost regA. In terms of how the regA function evolved, the modern-day versions of that other gene offer the best place to look for clues. Investigators studying this question found that the Chlamydomonas version of that regA -like gene, named RLS1 , is turned on when Chlamydomonas is deprived of light or certain nutrients Nedelcu This correlation suggests that perhaps RLS1 functions when cells are deprived of energy or nutrients.

Since regA represses reproduction, it seems logical that RLS1 probably does too. This could have happened when the gene that controls that pathway was copied and then used to co-opt the entire pathway to repress growth and division in a developmental context Figure 3B.

Think of the hybrid car analogy again, except in this case the entire stress response pathway is the brake system. Something like this — the co-option of an existing genetic pathway so that it causes a cell to do something it would ordinarily do only under different circumstances — might explain, in general, how organisms evolve new cell types. What Volvox and Chlamydomonas have taught us so far is that multicellularity, at least certain aspects of it, can evolve through relatively minor modifications of the unicellular blueprint see Figure 3.

Presumably not just any unicellular blueprint will do; no doubt the unicellular ancestor of Volvox already had many of the requisite genetic and cell biological raw materials for multicellularity: a multiple fission cell division program, a cell wall that could be modified into ECM, and possibly a stress response pathway that could be adapted to repress growth and division of a subset of cells, causing them to lose the ability to reproduce.

But there is still much to learn. What new gene functions evolved to permit the evolution of asymmetric division and inversion? How did the other novel developmental traits of Volvox evolve?

And are there similarities between the way multicellularity evolved in the volvocine algae and the way it evolved in other kinds of organisms? With the rate of recent progress in this field, answers to these questions, and more, should be on their way soon. Cheng, Q. The role of GlsA in the evolution of asymmetric cell division in the green alga Volvox carteri. Development Genes and Evolution , —