Sordaria: Exploring Genetics in a Haploid Organism

[Ester]

The following is an assignment for homework for an intoductory biology class in college:

Sordaria is a member of the ascomycete fungi, a group often used in genetics experiments. Yet, Sordaria is haploid. How can a haploid organism be used to test genetics problems when such things as crossing over require meiosis as the cell goes from a diploid to a haploid stage? How, then, can we use this haploid organism to test the same genetic problems as with Mendel's peas? Use an example to support your argument. Is it possible to use the same statistical procedures used in diploid species such as corn? Describe the experimental and statistical proedures that you would recommend to test the hypotheses that you would develop.

It is supposed to be detailed. I will post later what my response is (i'm still working on it). In the meantime, please check it out and feel free to post comments about it.

 

[Ester]

Needs editing and maybe more info?

It is supposed to be detailed. Please edit this essay. I'd like to know if I got all the questions covered. I don't know how to approach this writing assignment, but I have come up with the following,
Here it goes:

Even though Sordaria is haploid, heterotrophic organism with a zygotic life cycle, it is nevertheless useful in genetics experiments because it can undergo mitosis and meiosis. Therefore, it is possible that crossing over occurs in Sordaria fimicola. Sordaria meets the requirements, and therefore, can have genetic recombination. In order to see how Sordaria is used to test genetics problems, one must understand its life cycle during which reproduction and ascus formation takes place. A zygotic life cycle means that the zygote is the only diploid stage. This happens within a dikaryon.

During sexual reproduction, “gametes are not produced” in Sordaria. (p.64 Lab #5) What does happen is that the zygote will undergo first and second meiotic divisions to form four haploid ascospores. Mitosis is then followed in which there is a production of the ascus. An ascus is a sac of eight spores. These spores can take on different arrangements of grey/tan and black colors. The 4-4 arrangement represents the first division segregation, while the 2-2-2-2 and 2-4-2 arrangements represent the second division segregation. During our laboratory, we counted 40 asci. In our calculations, we were interested in the second division segregation because that let us know crossing over has taken place. It is possible, as we have done in the laboratory, to find the map distance between the mutant locus and the centromere. The map unit, or centimorgan (cM), represents the recombinant frequencies. (p. 204 text)

This haploid organism can test the same genetic problems as with Mendel’s peas. Mendel contributed to the science of genetics by using statistics and probabilities in analyzing data. (p. 195 text) Mendel’s main contributions were his first and second laws. The first law was the law of segregation that says alleles segregate during production of gametes. The second law was the law of independent assortment that says that “alleles of different genes assort independently of one another during gamete formation.” Both of his laws involve inheritance. Even though Mendel never knew of chromosomes and meiosis, he was able to reject the “widely held belief that inheritance is always a blending phenomenon.” (p. 190 text) Moreover, Mendel observed recombinant phenotypes in a dihybrid cross.

Even though there is no gamete production in Sordaria, there is meiosis. As mentioned earlier, crossing over takes place in second division segregations, and this is because we know that alleles of different genes segregate independently. Analysis of the different spore arrangements within an ascus is like an illustration of the phenomenon of crossing over centromere linkage and Mendel’s laws of segregation and independent assortment.

In addition, the same statistical procedures can be used in diploid species such as corn. When doing lab #4, we counted kernels of four corncobs. These kernels were a result from various crosses. We differentiated specific color (purple and/or yellow) and specific texture (smooth and/or wrinkled). By doing so, we were able to determine what the phenotypic ratio is and therefore predict whether or not it underwent a monohybrid or a dihybrid cross. This is very similar to what Mendel was experimenting with when he worked with his peas.

 

[Ester]

Please help. Thank's in advance. :)

 

[Moonbear]

What you have written so far looks good (with the disclaimer that I don't know your teacher and whether s/he might be looking for something different).

However, I don't see an answer to this part of the assignment:

Describe the experimental and statistical proedures that you would recommend to test the hypotheses that you would develop.

Did you have to develop an hypothesis for your lab that this refers to?

Unfortunately, I'm going to be scarce around here for the rest of the week. I'll see if I can scare up some other biologists to pay attention over here if you need further help in developing your hypothesis and experimental design.

 

[Ester]

I assume they want me to come up with a hypotheses about Sordaria, and then describe what needs to be done in order for the hypothesis to not be rejected. I'm thinking, what if I want to find out why Sordaria is sometimes grey and sometimes tan? What causes this? can we minupulate the environmental conditions so that we get tan instead of grey?
Another hypothesis I'm thinking about is: Which one is better: tan or grey? and I guess we can figure this out by looking at how they live in life, which one is more efficient?

What do you think Moonbear, can these be my hypothesis?

 

[Moonbear]

I think for your assignment about genetics, it's probably better to consider your first question, why it is sometimes gray and sometimes tan. I'm guessing that your teacher is expecting you to approach this from the perspective of Mendelian genetics rather than environmental influences, based on the wording of the rest of the assignment.

Once you have a question, you will need to phrase it differently to make it an hypothesis. Remember, an hypothesis is a statement predicting an outcome (X is the reason Y happens). You then design your experiment to manipulate your variable (X) in a way that would allow you to disprove your hypothesis if it isn't true ("I will do X, and if Y doesn't happen, I will have disproven my hypothesis," or "I will remove X, and if Y still happens, then X is not required for Y").

 

[Ester]

Well, there actually three colors, but for second division segregation, I'd see either black and grey or I would see black and tan.
So if I say my hypothesis is:
Grey is dominant over tan by Mendelian genetics.

To test this, I would have to interbreed them using the same technique that Mendel did with his peas?


or maybe the hypothesis should be:
Tan occurs less often than grey.

I would test this by repeating the lab, by counting and getting many people to do the same thing, and find the map distance. currently it is 33 cM for grey and 27 cM for tan. -- based on a large sample data.

I'm really confused because I also get this out of the website: http://www.lehigh.edu/~jas0/102-Module%203-fungi.PDF

"Sordaria, like other organisms, exhibits genetic variants that result in phenotypes that are
qualitatively different from the common (wild-type) phenotype. One character that exhibits
several different variant types is the color of the ascospores. The common "wild-type" has black
ascospores, but strains with red, pink, gray and tan ascospores also exist. In the strains we will
use, one strain (G) has gray ascospores, and the other (T) has tan ascospores. The different
colors are the result of simple allelic differences. However, since several loci have the potential
to determine ascospore color, you cannot assume that the gene that causes tan color is allelic to
the one that causes grey color. It may be that they are variants at totally different gene loci."

 

[Ester]

I'm really out of ideas. Help.

 

[Ouabache]

However, since several loci have the potential to determine ascospore color, you cannot assume that the gene that causes tan color is allelic to the one that causes grey color. It may be that they are variants at totally different gene loci.

My genetics is a bit rusty, if I am interpreting your quote correctly, it seems ascospore color is a polygenic trait.

 

[Ester]

Thanks for the idea Ouabache. I also found out that Sordaria has 7 chromosomes, but I don't know if that is useful here.
Please tell me if this hypothesis is ok:
Hypothesis:
The color of the spores of Sordaria are different because it is polygenetic. It is due to many genes.

If this is ok, please tell me what experimental and statistical procedures are needed to test this hypothesis.

 

[Ouabache]

Let me preface by telling you, the last time I thought about genetics in the direction you are taking me, was more than two decades ago. Bear that in mind as you read my comments.

In a first year course, my suggestion would be to concentrate on tetrad analysis. You might try searching the net, using that phrase in quotes, and these additional words: ascospores, Sordaria. There are a number of good websites that discuss this, some more technically and others in easier language. I don't think an introductory biology class would delve very far into polygenic analysis. That's a more advanced topic. At most, they may introduce the concept. You might talk with your teacher before suggesting experiments in that direction. To give you an idea of what I mean, here is a http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TCY-44FBRWW-W&_coverDate=12%2F01%2F2001&_alid=326432303&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=5183&_sort=d&view=c&_acct=C000035679&_version=1&_urlVersion=0&_userid=657938&md5=9c2d609533deade313b52394b7e20abd that discusses polygenic analysis. As you will see, it is not easy.

 

[Ester]

I think the simplest hypothesis to work on is that there is greater number of crossing over when there is a bigger map distance. but this has already been tested, and i don't know if I can use it but just in my words.
http://www.biology.uAlberta.ca/courses/biol207/uploads/lab/PDF/LAB_SESSION_FIVE.pdf

 

[Ester]

if anyboday has an idea please help me, because the paper is due very soon

 

[Ouabache]

At this point, let me add a similar caveat as Moonbear's. I don't know your teacher. I am making suggestions relative to how I interpret your questions. Your teacher may be looking for something different.

You're right. Hypothesizing that more crossing over occurs with greater map distances, is already known. It is a useful tool, but not the central idea of this assignment. As a suggestion. I have found it really beneficial to work out my labs on my own or with a lab partner(s). Do you have a lab partner you can get together and discuss with? If not, you may consider asking a few others in your lab to get together in a study group, find a place to regularly meet (library, study room) and brainstorm about your lab assignments, just like you are doing here.

Even though Sordaria is haploid, heterotrophic organism with a zygotic life cycle

Why did you mention Sordaria is heterotrophic? Is it an important, relative to its genetic expression?

How, then, can we use this haploid organism to test the same genetic problems as with Mendel's peas?

Are you trying to prove or disprove whether inheritance for this haploid organism behaves according to Mendelian genetic expression? Are there features about your work with this organism, that suggest an hypothesis to test this? Could genetic crossover be useful to demonstrate that Mendelian principles are at play?

From what Moonbear and I have suggested, you have enough information to develop your own hypothesis.

 

[Ester]

thank you guys for all your help