recap

19.3 recap

Cascades of transcription factors govern pattern formation and the subsequent development of animal and plant organs. Often these transcription factors create or respond to morphogen gradients. In plants, cell fate is often determined by MADS box genes; in animal embryos, cell fate is determined in part by Hox genes.

learning outcomes

You should be able to:

  • Summarize the function of floral organ identity genes.

  • Use an example to explain how organ identity genes work in combination to produce flower organs.

  • Determine the appropriate homeotic mutation to use to generate flowers with particular alterations in their organs.

  • Summarize how mutations were used to identify organ identity genes.

  • Predict the consequences of altering nanos expression for the developing fly embryo.

Question 1

What are organ identity genes, and how are they identified by mutations? How do they act in combination to produce different organs in a developing plant?

Organ identity genes specify different organs of the flower in plants. They are identified by loss-of-function mutations (mutations that result in a missing organ) or gain-of-function mutations (a promoter for the gene specifying one organ can be artificially spliced to a gene specifying a different organ, causing the latter gene to specify the first organ).

Which particular organ will be generated depends not just on the expression of a single organ identity gene but on the combination of the expression of other genes. For instance, the whorl that generates petals (whorl 2) and the whorl that generates stamens (whorl 3) both express class B genes. The difference between these whorls lies in the other organ identity genes that they also express, with whorl 2 also expressing class A genes and whorl 3 also expressing class C genes.

Question 2

If you wanted a rose plant to make flowers with only petals, what kind of homeotic mutation would you seek in the rose genome?

A mutation that caused expression of class A genes instead of class C genes would lead to an AB combination instead of AC, and petals would develop instead of stamens.

Question 3

Biotechnology enables the insertion of foreign genes into host cells along with promoters that maximize its expression (see the discussion of expression vectors in Key Concept 18.5). What would happen if nanos were inserted and over-expressed at the anterior end of the Drosophila embryo?

All neuronal precursors might undergo apoptosis, and no neurons would form. The Hunchback protein gradient would not form properly, and the embryo would not establish its anterior–posterior axis.

We have described some of the details of the role of gene expression in the control of development in two model organisms—Arabidopsis and Drosophila. Both involve genes whose expression as transcription factors control the expressions of other genes that result in differentiation and organ formation. Are these mechanisms basic to developmental biology, and have they been conserved through evolution?