Hox gene expression in E. scolopes


You Are Here:

I. If You Sleep in Class: Review of Hox Basics

II. What’s so Great?

III. Why Bobtail Squid are Better and How

  • Hox in Central Nervous System
  • Hox in Bracial Crown
  • Hox in Satellite Ganglia
  • Hox in Metabracial Vesicles, Buccal Crown, and Light Organ

IV. For Overachievers- Current Resource Analysis & More

V. Bob@tailsquid.hox- T-Shirt!

VI. Where I Got My Facts

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I. If You Sleep in Class: Review of Hox Basics

Hox genes are a group of genes found in metazoan organisms that determine the axis of the organism.  Hox genes have a DNA sequence called the homeobox, which is a 180 nucleotide long DNA sequence that encodes a 60 amino acid long protein domain called the homeodomain [7]. The Hox proteins are transcription factos that are capable of binding to enhancers and either activate or repress genes. The Hox protein binds at the homeodomain. The homeodomain protein motif is very much conserved with evolution [7].  Similarities in Hox genes among different organisms show their evolutionary relatedness. Hox proteins that are structurally similar show functional similarity as well [7].

II.  What’s so Great?

Bobtail squid (Euprymna scolopes) are simple model organisms that have shown important evolutionary innovations that put them miles ahead of their ancestors [1].  These innovations include sensory organs and central nervous system reorganization [2]. Understanding the Hox gene evolution in these organisms can give insight to how genes have been used to create other diverse organisms [1].  How have the E. scolopes Hox genes been organized for these new features?

III. Why Bobtail Squid are Better and How

Figure 1

Figure 1 a: Cephalopod evolution form a monoplacophoran-like ancestor. Shell reduction, expansion of mantle, and foot modification that forms brachial crown and funnel tube. b: E. scolopes embryo body plan. Arm pairs are I-V. Scale bar is 0.5 mm. c: Posterior view of the hatchling and adult body plan. Scale bar is 1 mm.

Callearts et al. used PCR- based approaches to find nine Hox genes that are expressed in E. scolopes [2]. In Lee et al., the Hawaiian Bobtail squid was used to study the developmental patterns of eight out of its nine total Hox genes by using whole-mount in situ hybridization [6].  Lee et al. conducted research to find out how the bobtail squid was able to form these new innovations that other mollusks lack. Cepholpods are evolutionary similar to gastropods; the most innovative changes cephalopods gained is well-developed eyes, extra-ocular photoreceptors, a vestibular system similar to vertebrates, different light organs, and a complex central and peripheral nervous systems (Figure 1)[6]. The most conserved feature of the Hox genes is the collinear pattern of expression (for more on collinearity).  In E. scolopes, modifications have been found in the conserved parts of Hox expression that has lead researchers to believe that this is how they evolved many of the innovations[2,6].  For full description of figure 1, see here.

Hox in Central Nervous System

FIGURE 2. Evolution of the cephalopod brain

FIGURE 2. Evolution of cephalopod brain a: Central nervous system of Archaeogastropod compared to b: central nervous system of cephalopod.

The innovations in the central nervous system are shown in figure 2a (ancient) and 2b (modern).  E. Scolopes has a fusion of the ganglia, the addition of the optic ganglia, and bracial lobes that comes from the pedal ganglia (Figure 2b).  For more on developing CNS in E. scolopes click here.

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FIGURE 2. Hox expression in the CNS

FIGURE 3. Hox expression in the CNS a: 3 in palliovisceral ganglia. b: 3 in pedal ganglia. (ys= yolk sac).

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In E. Scolopes, six Hox genes are expressed in the developing central nervous system [6]. The palliovisceral ganglia have three orthologous expressed: E. scolpes lab (Esx-lab), Esc-Hox3 and Exc-Sce (Figure 3c).  The pedal ganglia have the other three: Esc-Antp, Esc-Lox4 and Esc-Post-2 (Figure 3d) [6].  The CNS patterning shows collinearity and is consistent with ancestor gene patterning [2,6].   The difference from the modern and ancient is that E. Scolopes has lost the role of Esc-Lox5 and Esc-Post-1 in the CNS [6].  The cerebral ganglia did show Hox expression, but it may be controlled by the optic ganglia, which has Pax-6, or it could be patterned by gap genes (More on gap genes) [6].  For full description of figures 2 and 3, see here.

Hox in Brachial Crown

FIGURE 4

FIGURE 4 Hox expression in the developing brachial crown. The shading patterns indicated intensity of expression.

The brachial crown is where the arms , or tentacles, and the funnel tube (used for feeding) of the squid are found.  The arms and funnel tube are derived from regions of the foot.  Hox orthologous are not found in the foot, but they are found to be recruited later in the reorganization of the nervous system in the foot into the different ganglia [6]. Seven of the Hox genes explored in Lee et al. were expressed in the post-gastrula neurogenic sites that create the brachial ganglia (Figure 4) [6].   Each arm pair expresses a combination of the orthologous; the expression, however, is not collinear and the changes caused by evolution have created a new pattern in the brachial crown (Figure 4) [6].  For full description of figure 4, see here.

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Hox in Stellate Ganglia

Stallate Ganglia are only in higher cephalopods.  The stallate ganglia help to coordinate the muscular pump.  The muscular pump is used for locomotion and resperiation and helped these organisms diversify [2,6].  Here, Esc-lab, Esc-Hox3, Esc-Lox4, and Esc-Post-2 are expressed in many different neural cell types different from the brachial ganglia [6].  This leads researchers to believe that a subset of Hox genes have been recruited for specification of these neuronal cells [6].

Hox in Metabracial Vesicles, Buccal Crown, and Light Organ

The metabracial vesicles are the eyes of the bobtail squid.  They detect light changes; one is between arms I and II, the other is between arms IV and V on the brachial crown.  Esc-Hox3 is the only orthologue that Lee et al. found that is expressed in the vesicles but nowhere else [6].  Development of the metabracial vesicles does not use a cocktail of Hox genes like the arms do, but they are located in the same place [6].  The take away message from these results is that the brachial crown Hox genes are not specifying anterior-posterior.  They are retrieved independently for the arm development.

The buccal crown is a ring of lappets, seven small appendages, around the mouth that are derived from the foot.  Esc-Post-1 is the only othologue in developing lappets, and it is the only orthologue in the light organ (more on light organ) [6].  The Esc-Post-1 expression in these areas shows that creation of the innovations may include the same Hox gene [6].

IV. For Overachievers- Current Resource Analysis & More

In conclusion, the data has showed that Hox genes are not only used in anterior-posterior patterning.  A mixture and overlapping of them creates these innovations over time.  There are some model organisms that have a single Hox gene cluster but also complex body plan, like fruit flies[2].  Callearts et al. suggests that cephalopods have a single cluster but with recruitment of different target genes into regulatory networks [2].  Gert de Couet (part of Lee et al.) stated, “To really be able to say these genes are related to morphological novelties, we need to knock this gene out or interfere with its expression and see if the organism has failed to make the structure.”[1].

The most helpful experiment used in this blog was Lee et al. in Cephalopod Hox genes and the origin of morphological novelties.  It offered evidence to exactly what I was searching for and gave easy to read figures and language, but it did not explain the reason for choosing the specific Hox genes to study nor did it give a clear conclusion on what the results meant in terms of the innovation genes not following collinearity.  The paper by Callearts et al., HOX genes in the sepiolid squid Euprymna scolopes: Implications for the evolution of complex body plans, had crude anatomical diagrams of the organism, but it did provide a genetic diagram of the differences in homeodomain sequences between the Hox genes.  Callearts et al, has a useful discussion in what further studies could be done with the results.

Most of this research was done through the Department of Zoology at the University of Hawai’i.  They have a world renowned evolutionary laboratoy that’s always spitting out the latest and greatest discoveries [3].  If you are interested in bobtail squid as a model organisms, check out their site.

V. Bob@tailsquid.hox- T-Shirt!

$22.00.  Women and children sizes available.  Hipster hat and sunglasses not included.  Make checks payable to bob@tailsquid.hox.  Profits go toward more research with this model orgnaism…*

*The information in this section is not fully true.  Check out their website here.

VI. Where I Got My Facts

1. Altonnhaltonn@starbulletin.com, Helen. “Honolulu Star-Bulletin Hawaii News.” Hawaii Archives – Honolulu Star-Bulletin Archives – Starbulletin.com – Archives.starbulletin.com. Web. 01 May 2011. http://archives.starbulletin.com/2003/11/02/news/story7.html.

2. Callearts, Partik, Patricia N. Lee, Britta Hartmann, Claudia Farfan, Darrett Choy, Kazuh Ikeo, Karl-Friedric Fischbac, Walter Gehring, and H. G. De Coue. “HOX Genes in the Sepiolid Squid Euprymna Scolopes: Implications for the Evolution of Complex Body Plans.” PNAS(2001). Pnas.org. Web. 20 Mar. 2011. http://www.pnas.org/content/99/4/2088.full.pdf.

3. “Department of Zoology, University of Hawai’i.” University of Hawaii System. Web. 01 May 2011. http://www.hawaii.edu/zoology/.

4. “Gap Gene.” Wikipedia, the Free Encyclopedia. Web. 01 May 2011. http://en.wikipedia.org/wiki/Gap_gene.

5. Lee, Patricia M., Partick Callearts, and H. Gert De Couet. “The Embryonic Development of the Hawaiian Bobtail Squid (Euprymna Scolopes).” Cold Springs Harbor Protocol (2009).Cshprotocols.cshlp.org. Web. 14 Apr. 2011. http://cshprotocols.cshlp.org/cgi/content/abstract/2009/11/pdb.ip77.

6. Lee, Patricia N., Patrik Callaerts, Heinz G. De Couet, and Mark Q. Martindale. “Cephalopod Hox Genes and the Origin of Morphological Novelties.” Nature (2003): 1061-065.Nature.com. 28 Aug. 2003. Web. 20 Mar. 2011. http://www.nature.com.www.library.gatech.edu:2048/nature/journal/v424/n6952/full/nature01872.html.

7. Lemons, D. “Genomic Evolution of Hox Gene Clusters.” Science 313.5795 (2006): 1918-922. Sciencemag.com. 26 Sept. 2006. Web. 20 Mar. 2011. https://t-square.gatech.edu/access/content/group/XLS0101142838201102.201102/05%20Drosophila/Lemons_McGinnis2006-GenomicEvolHoxClusters.pdf.

8. “RadCakes » Shop » Hawaiian Bobtail Squid V-Neck Guys Shirt.” RadCakes. Web. 01 May 2011. http://www.radcakes.com/shop/hawaiian-bobtail-squid-v-neck/.

9. Tschopp, Patrick, Basile Tarchini, Francois Spitz, Jozsep Zakany, and Denis Duboule. “Uncoupling Time and Space in the Collinear Regulation of Hox Genes.” PLoS Genetics (2009).Plosgenetics.org. Web. 14 Apr. 2011. http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.1000398.

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