Alisa Poppen  Mira Loma High School  Sacramento, CA arpoppen@ucdavis.edu

 

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Students will:

1.    observe the two major developmental pathways (protostome and deuterostome)
2.    analyze data regarding differences in nucleotide sequences
3.    construct a phylogenetic tree
4.    observe the similar evolutionary history shared by echinoderms and chordates

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Classes:    Biology (probably most appropriate for honors/advanced classes)

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Notes to the Teacher: to top

1. Unless you want to provide additional diagrams/sketches/pictures for students to use, no preparation time is required.
2. This activity could be completed in one class period.  It may be more appropriate to send home with students and use as a discussion tool in the following class period.
3. This activity is most appropriately used in conjunction with a sea urchin fertilization lab, although it also could be used during a discussion of taxonomy or evolution.

No materials necessary, unless you have access to good diagrams/sketches/pictures of the early developmental stages of the five organisms listed.

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In order to complete this activity, students should:

1. have an introductory knowledge of evolution, including the ways in which evolutionary relationships are determined
2. be familiar with the structure and function of DNA
3. be able to identify the early stages of development in animal embryos
4. be familiar with the fate of the blastopore in different animal embryos
5. be familiar with the concept of a phylogenetic tree

Embryology as Evidence of Evolution

Introduction

The embryos of animals from different phyla share many common characteristics during the early stages of development.  Each embryo is formed when a sperm cell fertilizes an egg to create a zygote.  This zygote then goes through subsequent cleavages to form two cells, then four, then eight, and so forth.  Eventually the embryo is solid ball of cells called a morula.  Cells in the morula begin to migrate and the embryo changes from a solid ball to a hollow sphere of cells known as a blastula.  Soon thereafter a group of cells begins to migrate inward.  The point on the embryo where this migration takes places is called the blastopore.  Other cells will follow in the migration, eventually forming a tube called the archenteron.  This process of migration is called gastrulation, and an embryo in this stage is called a gastrula.

The stages described thus far can be observed in any animal embryo.  After gastrulation, however, the development of embryos varies from phylum to phylum.  One example of this variation can be seen when we observe the fate of the blastopore in different animals.

As mentioned earlier, a tube called the archenteron forms as gastrulation takes place.  In embryos of any phylum, this tube will develop into the gut of the animal.  It is the orientation of this gut that varies from phylum to phylum.  In animals such as mollusks, annelids and arthropods, the blastopore (the opening of the archenteron) will develop into the mouth of the gut.  In animals such as echinoderms and chordates, the blastopore will form the anus instead of the mouth.

Introductory Questions

1. In attempting to determine the evolutionary relationships between organisms, scientists look at several different types of data.  One such type is embryological data.  It is assumed that organisms with similar developmental patterns share a common ancestor.  Is this a safe assumption?  Why or why not?
2. If similarities in development are a reliable means by which to determine evolutionary relationships, what would you conclude about the different phyla in the animal kingdom?  Which are most closely related?
3. In an attempt to determine the relatedness of several different organisms, two researchers (Hiroshi Wada and Noriyuki Satoh) analyzed the nucleotide sequence in a segment of DNA from five different organisms.  The DNA used was a segment that codes for the production of 18S rRNA, a nucleic acid that forms part of the ribosome. Why would Wada and Satoh choose to examine DNA sequences to determine relatedness of organisms?
4. Think about the function of rRNA, and decide why the selection of 18S rRNA was a good choice for comparing the biochemical makeup of organisms.

The table below shows the results obtained by Wada and Satoh.  The numbers listed indicate the number of substitutions (or differences) in the DNA nucleotide sequence between any two possible combinations of organisms.
 

1.  Based on the data above, which two organisms are most closely related?  Which are least closely related?

1. Construct a phylogenetic tree that shows the relationship between the five organisms examined.  This tree should show in graphic form the relationships between organisms and how they diverged over time.  In making your tree, be sure you consider both embryological and molecular data.
2. Suppose you constructed a phylogenetic tree for these five organisms using only morphological (physical) characteristics.  Would this tree differ from the one constructed in question 1?  If so, how?
3. Discuss which of the two trees (embryological/molecular vs. morphological) shows a more accurate relationship between the five organisms.
4. If there was a conflict between embryological and molecular data, describe the conflict and how you decided which piece of data was more significant.
5. Make a statement as to why it is or is not appropriate to use sea urchin embryos in the study of animal (especially vertebrate) development.  If there is a more appropriate organism to study, name it and then list the pros and cons of using this organism in the classroom.  Also list the pros and cons of using sea urchins.

Methods of Evaluation/Assessment to top
 

1. Students' interpretation of data will be assessed in looking at the phylogenetic tree created.
2. Conclusion questions will assess the process used by students in constructing the tree.

1. Several activities produced at the 1995 Woodrow Wilson Summer Biology Institute in Evolution focus on the creation of phylogenetic trees using molecular evidence.  These activities can be found athttp://outcast.gene.com/ae/AE/AEPC/WWC/1995?

1. Wada, H. and Satoh, N.  1994.  Details of the evolutionary history from invertebrates to vertebrates, as deduced from the sequences of 18S rDNA.  Proceedings of the National Academy of Sciences of the USA  91:  1801-1804.