The role of centromeres in the bouquet formation of Tetrahymena thermophilia.

Bouquet formation, homologous pairing and crossing over in early meiosis are all processes that are strongly dependent on the centromere.  

When meiosis is initiated one of the first structures that is seen to form in almost all organisms is the chromosome bouquet. The chromosome bouquet is an arrangement where telomeres bunch together in a confined area of the nuclear periphery with centromeres at a polar position to them, which as its name suggests resembles a bouquet of flowers. This structure allows for the the pairing of homolgous chromosomes within the cell. 

Tetrahymena thermophilia is a unicellular ciliated protist who’s micronuclei elongate and stretch dramatically during its meiotic prophase, which is the point at which Tetrahymenas exaggerated bouquet forms. Loidl, Lukaszewicz, Howard-Till and Koestler at the University of Vienna have released a paper that may help to explain what mechanisms are taking place during this process in the unicellular protist. 

This paper investigates the importance of Double stranded breaks (DSB’s) and centromere function in Tetrahymena’s bouquet formation, suggesting that centromeres have essential functions in recombination and chromosome pairing.

To begin their investigation Loidl et al attempted to understand the function of centromeres during Tetrahymenas nuclear elongation. They did this by constructing strains where the H3 histone Cna1p, was disabled through RNAi depletion. Under wild type conditions where telomeres and centromeres segregate during the formation of the bouquet, centromeres cluster at the periphery of the nuclei.  However with this RNAi mutant immunostaining detected only background staining, with no clear organisation of centromeres.

This paper also discusses how the bouquet arrangement of centromeres and telomeres at opposite poles of the nucleus is highly dependent on in the interaction of microtubules with the kinetochore. It was found that whilst microtubule interaction is the main contributor to nuclear elongation and that centromeres play no role in the elongation of the cell. Microtubules have two known functions in Tetrahymena; to elongate the nucleus and to hold the centromeres at a fixed position of the nucleus. 

It was already known that DSB’s were needed for the bouquet to form, as it is an ATR-dependent response. ATR being an enzyme that is activated in the persistent presence of single stranded DNA, which is a common intermediate for most DNA damage repair pathways.  Loidl et al set out to confirm that the bouquet was actually necessary for DSB repair by adding nocodazole- a microtubule inhibitor – to prevent the formation of the bouquet within the Tetrahymenas nuclei. 

As an additional measure DSB-formation and repair were monitored using pulsed-field electrophoresis in both the control and the nocodazole treated cells. Both of these experiments showed that DSB’s were repaired independently of bouquet formation. Therefore this proves that whilst DBS’s are need for the initial formation of the bouquet, the bouquet itself is not needed for the repair of those DSB’s.

To try and further understand the role of DSB’s and how they regulate bouquet formation Loidl et al created a scenario where DSBs were continuously produced. To do this they treated meiotic phase cells with cisplatin – an inducer of DSBs.

Cells treated with cisplatin were no longer able to exit the bouquet stage, suggesting that the trigger for the cells release from the bouquet stage must be an intermediate stage in the DNA repair.

The bouquet structure is highly conserved amongst a vast number of different species, but it’s function varies

slightly from organism to organism. For example in Arabidopsis, like with Tetrahymena, telomeres are linked with the nuclear periphery. However, in contrast to Tetrahymena the centromeres do not cluster at a single point at the opposite pole to the telomeres, but are more dispersed randomly across the nucleus with no evidence to suggest that they are involved in homologous chromosome pairing or recombination. Instead chromosome pairing occurs in zygotene, when a structure loosely similar to the bouquet is formed.  In mammals and budding yeast chromosome pairing has been proved to be led by telomeres and dependent on the protein SUN1 which anchors the telomere to the nuclear membrane. A similar protein of which could not be found by Loidl et al for Tetrahymena.

The next step in the investigation of Tetrahymenas exaggerated bouquet is a more in depth look at the function of telomeres and telomere associated proteins. By doing this there will be either confirmation of the views that this paper has put forward or give us a better insight into the function of telomeres allowing us to appreciate their involvement in the bouquet forming process.

By understanding Tetrahymenas bouquet completely and fully we will be able to apply this knowledge to other organisms to help us determine the processes that govern their bouquet formation. This would be especially important is we could apply this new knowledge to ourselves and the processes that go on in our cells during meiosis. This would give us a much clearer insight into how diseases and disorders may form, and therefore give us clues on how to prevent these diseases. 

Pretty soon our food is going to run out – but what can we do about it?

The human population of the earth has been increasing exponentially over the past few hundred years, a trend which is showing no sign of stopping. But with all these extra mouths to feed, do we have the resources to actually keep feeding them?

One of the biggest problems that the human race faces at the moment is the rapid development of global poverty and global hunger, issues which will only be worsened if the population increases to nine billion, as predicted.  Whilst these are two separate problems, they can be simultaneously resolved in a long term way through the promotion of agricultural growth in areas where poverty is particularly rife. However, communities which experience particularly harsh levels of poverty usually occur in areas whose land is incapable of sustaining a large number of crops. These areas therefore require new types of crop species which can grow with a much lower level of nourishment, whilst still producing the same yield of food.

Therefore, research geared towards producing these new varieties of plants is of critical importance. The main method which could be used to produce these variants is through increasing a plants level of genetic recombination.

Recombination is a molecular process which occurs within the cells of plants during meiosis, a specialised round of cellular division which produces gametes, cells with half the usual number of chromosomes (haploids). During this time, chromosomes which are genetically very similar to each other called homologues pair together and form cytological structures called chiasmata. When these chromosomes then resolve during a stage called anaphase, the resultant chromosomes often contain pieces of genetic information from each of the chromosomes which originally pair (recombinants). This introduces a level of genetic variation within a population, which is often the driving force behind evolution, and the adaptation to different environmental influences.

This process occurs in all sexually reproducing animals. However, in certain plant species recombination is kept under incredibly strict control in an attempt to ensure the stability of their genome. Whilst this is a positive outcome for these plants in their natural environment, when attempting to produce variants with more resilient phenotypes this produces a difficult obstacle. It is therefore the aim of many researchers to further understand the mechanisms which govern recombination, as well as any techniques which could be adapted to try and artificially induce much higher levels of recombination.

It is this type of research which I am currently involved in whilst completing my masters at The University of Birmingham, UK. During my time working in this lab I will be attempting to determine whether okadaic acid, a phophase 2A inhibitor, is capable of inducing a much higher level of recombination in Brassica napus between chromosomes which normally don’t recombine at all.

This type of research is much different to previous research which has produced genetically modified (GM) crops, which usually involves placing foreign genes into an organism which would never have been present in the wild. This type of technique is often poorly favoured by the public at large, with many suggesting that the repercussions of manipulating nature in this way could never be fully understood.

However, the research that I am involved in is interested simply in inducing the expression of genes that were are already present within the genome, but were never allowed to surface and influence the phenotype. This is a much more environmentally safe method of producing high yielding plants, and one which would be a globally accepted resolution to current issues around poverty and food security.

Could this type of research be the answer to some of the big questions that we are going to have to face in the near future? Whilst it certainly has potential, we are still a long way away from being able to completely know the truth. But don’t worry, I’ll keep you up to date if I ever find out the answer to my tiny scope of research, and let’s just hope that the hundreds of other labs around the world do the hard work for us.

What do you think about this research? Comment below with any thoughts or questions. 

Volunteers required for whole genome sequencing – Will you participate?

Today the UK has launched a personal genome project, urging 100,000 people to contribute their genetic information and have their genome sequenced to be put on public record. Is this a big step in the advancement of sequencing huge sample sizes? Or is making our entire genome available  to anyone online a step too far?

Our DNA is what makes us who we are; a sequence of bases in each and every one of our cells which contains the code to making what we see in the mirror every morning. It is possible for someone to look at your DNA and determine almost everything about us; the colour of our hair, our eyes or any genetic diseases we may have or be prone to, without ever having actually laid their eyes on us.

Whilst it would have previously been near impossible for many people to gain access to our DNA, a new initiative has been set in place by The Personal Genome Project UK (PGP-UK) urging 100,000 volunteers to donate their genetic information for their genome to be sequenced. This has already been undertaken by other countries, including America in 2005. However, this is the first time a project like this has been attempted in the UK, and many sceptics have their doubts.

The aim of those conducting this project is to accelerate researchers understanding of genes, both normal and defective, as well as how different environmental influences can affect those genes. However, the majority of companies sponsoring this research (one of which being google) are hoping to use this data for commercial exploitation, to specifically target advertising for drugs that each of us may need, based on our genetic code.

This is one reason why this project in controversial, but many also have issues with the broad spectrum of individuals who will be able to view your genetic code. Whilst names and addresses will not be included on record, the research group involved has warned that the security of participants is not guaranteed, and they could potentially be identified.

Because of this, a number of tests are included in the application of those who are applying to be involved, in which only a score of 100% will be accepted, which is designed to ensure that all those involved fully understand the potential risks.

If you were decide to join the project, and then be accepted, you will receive a kit to take cheek swabs, and also asked to attend a clinic to provide more extensive samples, with your genetic information published within a month.

It is hoped that this sort of extensive record of so many peoples genomes will allow for a number of major diseases, which have a large effect on public health, to be linked to genes which have previously been unidentified. This would be crucial information for those researching therapies for these diseases, and could dramatically advance our understanding of them.

Do you think you’d be interested in taking part in this project? Or do you think that having this kind of information about yourself on public record is a step too far? I for one know I’m definitely going to be signing up.