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Insect Body Plan and Hox genes Lyn Harder and Claire Kuelbs

What are Hox genes?

●  Discovered in the 1980s in Drosophila ●  Homeobox-containing genes

●  Play a big role in segment identity

➔  Encode transcription factors that activate or repress different genes

●  If changed in any way, can cause homeotic

transformation

What are homeotic transformations? Replacement of one body part with another

What are features of Hox genes? ●  Clustered ●  Colinear

Do other organisms have Hox genes?

What would happen if Hox genes never mutated?

How are Hox genes studied? ●  Often studied in insects

➔ Why are insects ideal for studying Hox genes?

●  RNA interference

➔  Uses double stranded RNA to target a gene product causing degradation

➔  Creates a silencing of the gene

Insect body plan ●  What are the most defining features of the

insect body plan?

HOX genes and the insect body plan ●  Ultrabithorax (Ubx)

➔  Expressed in the abdominal segments

➔  Suppresses thoracic leg development

➔ What happens when ubx is ectopically expressed in the thoracic segments?

How did Ronshaugen et al. (2002) study the function of Ubx? ●  Comparisons between Drosophila and the

crustacean Artemia franciscana ➔ Why did they choose A. franciscana?

Is there a difference in function between the two species? ●  Artemia Ubx was only able to suppress 15%

of thoracic limb development when ectopically expressed in Drosophila

➔  Drosophila suppressed all limbs

●  Artemia Ubx transformed thoracic denticles towards an abdominal identity

Why is there a difference in the ability to repress thoracic limbs?

●  Two hypotheses

➔  The Drosophila Ubx C-terminal has a limb repressing domain while the Artemia Ubx C-terminal is not sufficient for limb repression

➔  The Artemia C-terminal sequences regulate a limb

repression domain elsewhere in both Ubx proteins

Figure 3: Repression of thoracic limbs by hybrid proteins

Artemia Ubx inhibits a limb repression domain ●  When the Artemia C-terminus was deleted,

80% of thoracic limbs were repressed

➔  Big shift from 15%

●  When the Drosophila C-terminus was deleted, 80% of thoracic limbs were repressed

Discussion Question What environmental or ecological constraints might have caused the need for limb repression within hexapods and therefore cause the divergence of the taxa?

Discussion Question What are the benefits of evolving, as insects have from other crustaceans, to having your limbs restricted to developing on one segment of the body, while repressing growth on other segments?

What contributes to the further diversity of insect body plans?

●  Changes in gene function

●  Changes in gene sequence

●  Changes in gene expression

➔  Location ➔  Timing

Bicoid ●  Required for head development in

Drosophila and other higher dipterans

Anterior Posterior

➔  Anterior-to-posterior concentration gradient ➔ Maternal effect gene → mRNA comes from

mother

Bicoid ●  Many other insects use other genes for

anterior patterning

Rosenberg, M., Lynch, J., & Desplan, C. (2009). Heads and Tails: Evolution of Antero-Posterior Patterning in Insects. Biochimica et Biophysica Acta, 1789(4), 333–342.

➔  bcd gene is absent from these insects ➔  Bcd has a role in higher flies that was

previously accomplished by other genes

●  Then, how did bcd come about?

Bicoid ●  Comes from duplication of Hox3/zen in

cyclorrhaphan flies in the past 140 million years and gained a new function ➔  Neofunctionalization: one copy of a gene takes on an

entirely new function after a duplication event

Hox3 ●  Lost the original function of specifying

segmental identity ➔  Required for extraembryonic tissue development in

winged insects

●  Duplicated in higher dipterans

Hox3 ●  D. melanogaster has

two types of Hox3

Stauber, M., Prell, A., & Schmidt-Ott, U. (2002). A single Hox3 gene with composite bicoid and zerknüllt expression characteristics in non-Cyclorrhaphan flies. Proceedings of the National Academy of Sciences of the United States of America, 99(1), 274–279.

➔  zerknüllt (zen): still produces extra- embryonic tissue

➔  bicoid: required for

embryonic patterning

Discussion Question How could the bcd gene take over the function of another protein and still allow for a healthy animal to exist and function?

Discussion Question How might the studies of Hox genes in insects such as Drosophila affect our knowledge of vertebrate development or even modern medicine?

Conclusion ●  Hox genes are important for development

➔  Changes in Hox gene expression and function underlie morphological diversity in all of nature

●  Ubx facilitates one of the most basic features of the insect body plan: no abdominal appendages

●  Many other Hox genes create further diversity among insect body plans