Reconstructing the Evolution of Cauliflower and Broccoli
through its collaboration with the SERC Pedagogic Service.
This laboratory exercise allows students to make connections between plant morphology and the genetic differences affecting plant development. Students look for gene and protein sequence differences that explain differences in morphology between Brassica oleracea subspecies, using the Student Interface to the Biology Workbench.
- Students will experience and understand one approach to studying the evolution of development.
- Students will apply their understanding of nucleic acids, proteins, transcription factors, development, evolutionary selection, and the species concept to a specific problem: the origin of domesticated cauliflower and broccoli.
- Students will develop an understanding of basic flowering plant morphology.
- Students will make and test hypotheses about a mutant CAULIFLOWER (CAL) gene using gene sequence information from different Brassica oleracea subspecies.
- Students will gain bioinformatics skills working with gene and protein sequences from a public database.
Context for Use
This activity is a part of an introductory-level undergraduate biology course on genes, evolution, and development. The lab is designed to help students apply and cement their understanding of genes and proteins as they answer an evolutionary question situated in the context of plant development. Students complete this activity during a four-hour lab session, and usually finish in under three hours.
The lab is highly adaptable to different teaching situations. The lab could be used as part of mid-level courses in developmental biology, genetics, evolution, or plant biology. Any or all of the three sets of observations could be completed using photographs (available in Teaching Notes and Tips below) instead of live material. The vegetative structure observations could be easily modified to utilize locally available examples of Brassica oleracea subspecies. The computer portion of the lab could be assigned for students to complete outside of class time.
Before completing the lab, students should have been exposed to the concepts of phenotype, genotype, nucleotide sequence, protein sequence, codon, stop codon, intron, exon, mRNA, and phylogenetic tree. Knowledge of plant morphology and the gene sequence databases are NOT prerequisite for the lab; students will learn about these within the context of the lab.
Description and Teaching Materials
- observation of vegetative structures in various Brassica oleracea (30 minutes)
- observation of floral and inflorescence structures in broccoli (Brassica oleracea ssp. italica) and cauliflower (Brassica oleracea ssp. botrytis) using a dissecting microscope (20 minutes)
- observation of floral and inflorescence structures in broccoli and cauliflower using a scanning electron microscope (demonstration; 30 minutes)
- investigation of a gene sequence (CAL) related to cauliflower's phenotype in which students make and test hypotheses by comparing the gene's sequence in different subspecies of Brassica oleracea using the Student Interface to Biology Workbench, a free online bioinformatics resource (60-90 minutes)
Student Lab Handout (Microsoft Word 1.4MB Apr26 07) Student Lab Handout (Acrobat (PDF) 1MB Apr26 07) This is the lab handout given to the students, which includes specific instructions for observing vegetative structures in various B. oleracea subspecies, observing inflorescences in cauliflower and broccoli, and using a free online resource to investigate the CAL gene sequence.
The following image links can be used to enhance the student lab handout and, if desired, replace the in-lab microscopy components of the learning experience.
Teaching Notes and Tips
Integrating the Lab Experience
We integrate learning from this laboratory experience into in class work on evolutionary, developmental, and genetic approaches to understanding biology. This includes speciation; gene duplication and animal developmental genetics; paralogous and orthologous genes; heterochrony and homeosis; plant developmental genetics; and plant morphology.
- Speciation. The morphological diversity among subspecies of Brassica oleracea provides a point of departure for discussing the concept of species and speciation. We have already introduced students to speciation before the lab and have chosen to use the examples in the lab to both emphasize the challenges of distinguishing species and illustrate a mechanisms that could be part of the speciation process.
- Gene duplication and animal developmental genetics. An in class problem solving session on the evolution of the Hox genes links to student understanding of gene duplication and the role of transcription factors in development.
- Paraologous and orthologous genes. Gene duplication serves as a segue into understanding the difference between paralogous and orthologous genes. Gene duplicates such as AP1 and CAL in a Brassica species are paralogues, while the CAL genes in different Brassica species are orthologues.
- Heterochrony and homeosis.Both the laboratory experience and the discussion of Hox genes provide solid examples to discuss the distinction between homeosis (structures developing in the wrong place) and heterochrony (structures developing at the wrong time). The overall animal body plan is in place early in development while the plant body develops iteratively over time, making distinctions between homology and heterochrony a bit more challenging. To structure student learning, we introduce homeosis and heterochrony in the context of fly development and then work on applying these concepts to what the students saw in the laboratory experience.
- Plant developmental genetics. As we go more into depth on the role of flowering genes in development in Arabidopsis, including the ABC model for flower development, we refer back to the lab experience and build on the concept of homeotic mutants. We also build on the role of transcription factors in development.
- Plant morphology. The laboratory experience helps our students understand the basics of flowering plant morphology, something we can build upon in the classroom.
- Sequencing topics. Sequentially the lab could come before or after the topics are introduced in the classroom. For example, we begin building the concept of gene duplication before the lab experience and then return to it after the students have completed the lab. As mentioned earlier, we reflect back on the lab when speciation and also, homeotic and heterochronic mutations are introduced.
- Integrating assessment. As we have chosen to tightly link the lab and the class, much of our assessment is built into the classroom portion, including formative and summative (exam) assessments.
Background Information for Instructors This provides a description of the research findings used to develop this lab, an explanation of why the approach in this lab was selected, and details on ways the concepts in the lab can be used an introductory course.
Information for Lab Instructors and TAs
TA and Instructor Notes (Word) (Microsoft Word 90kB Apr11 07) TA and Instructor Notes (pdf) (Acrobat (PDF) 564kB Apr24 07) This provides a detailed description of how to run the lab, and includes a key for the vegetative structure observation table and all the questions and tables related to the CAL gene.
The lab is structured so that students are periodically required to reflect on their work (see the assessment section below) before they can proceed. Allowing ample time for this reflective work is an important consideration in the pacing of the lab and in ensuring students are engaged in the inquiry process.
Equipment, Supplies, and Lab Set-Up
A detailed list of the set-up necessary to perform the lab, including equipment, supplies, and sources of material.
Throughout the lab, students are asked to develop hypotheses and make calculations and predictions. There are spaces for them to write out their answers, and in several places they are instructed to discuss their ideas with a lab instructor or TA before proceeding with the lab. The students work in groups, and these discussions with group members and lab instructors provide time for them to reflect on their ideas and adjust their thinking as needed. The questions are available in the "Student Lab Handout" above; common student answers and a key for calculations are available in the "TA and Instructor Notes" above.
We assess application and transfer of knowledge through in-class problem sets and exam questions focused on Hox gene evolution and the developmental genetics of flowering (see "Integrating the Lab Experience" above).
References and Resources
- Ehrenreich, I.M., M.D. Purugganan (2006) The molecular genetic basis of plant adaptation. American Journal of Botany 93: 953-962.
- Kempin, S., S. Savidge, M.F. Yanofsky (1995) Molecular basis of the cauliflower phenotype in Arabidopsis. Science 267: 522-525. Lowman, A.C., M.D. Purugganan (1999) Duplication of the Brassica oleracea APETALA1 floral homeotic gene and the evolution of domesticated cauliflower. Journal of Heredity 90: 514-520 electronic resource
- Purugganan, M.D., A.L. Boyles, J.I. Suddith (2000) Variation and selection at the CAULIFLOWER floral homeotic gene accompanying the evolution of domesticated Brassica oleracea. Genetics 155: 855-862. electronic resource
- Smith, L.B., G.J. King (2000) The distribution of BoCAL-a alleles in Brassica oleracea is consistent with a genetic model for curd development and domestication of the cauliflower. Molecular Breeding 6: 603-613.