Arthropod Embryogenesis

FlyFISH hps://www.mpi-cbg.de/research-groups/current-groups/pavel-tomancak/open-access/ Lecture Outline

• Two (main) modes of embryogenesis • • Drosophila axis formaon and segmentaon • What c aspects of arthropod development are conserved? Short vs. Long

• The mechanisms of embryo arthropod embryogenesis

occur via one of two embryo mechanisms, the “long- embryo

germ” mode, or the embryo “short germ” mode. embryo embryo – Short germ: only a small

poron of the egg embryo embryo embryo becomes the embryo (more ancestral). – Long germ: the majority embryo embryo embryo of the egg becomes the Long Germ-Drosophila (Parkhurst Lab) embryo (derived and has evolved in mulple lineages secondarily). embryo

Short Germ-Tribolium (Tautz et al. 1999) Short vs. Long

Nakamura et al. 2010

Tomer et al. 2012 Embryogenesis in • Why Drosophila? – The genec tools available for Drosophila have allowed for the discovery of genec mechanisms controlling development. – Early work by Ed Lewis, Chrisane Nusslein-Volhard and Eric Weischaus led to them being awarded the Nobel Prize in Physiology or Medicine in 1995. Insect Ovaries

• Insect ovaries are composed of many smaller units known as “ovarioles.”

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Hyalophora cecropia, Telfer 2009 Three Types of Insect Ovarioles

• Three insect ovariole types • Polytrophic meroisc (A), in which each oocyte is associated with nurse cells. • Telotrophic meroisc (B) in which the oocytes are aached to the terminal nurse cells. • Panoisc (C) in which there are no nurse cells. Jeremy A. Lynch, and Siegfried Roth Dev. 2011;25:107-118 The Drosophila Ovariole The Drosophila Life Cycle Early Embryonic Events: Pre-ferlizaon Nurse Cells Load Axial Determinants and Germline Components into the Oocyte

St. Johnston 2005 Dorso/Ventral Axes Formaon

•Gurken/Egfr signaling is used to establish the dorso-ventral axis before ferlizaon in Drosophila. •gurken mRNA is localized around the asymmetrical oocyte nucleus. •Aer translaon, Gurken protein signals to the overlying follicle cells via the Egfr receptor to demark the future dorsal side of the embryo.

Gilbert et al. 2014 A/P Axis Formaon in the Ferlized Egg

Sanson 2002 bicoid and Nanos mRNA translate Into Protein Gradients

Porcher et al. 2010

nanos mRNA Nanos Protein

Dahunakar and Wharton 1996 Bicoid Phenotype

•Depleon of bicoid mRNA causes a mirror image of the posterior in the anterior. •Arrowheads point to the “filzkorper,” which are structures on the 8th abdominal segment.

Staller et al. 2015 Bicoid and Nanos Protein Gradients and the Establishment of Gap Gene Boundaries

Sanson 2002 hunchback (hb) and cuadal (cad) Are Maternally Ubiquitous

•Bicoid protein inhibits caudal translaon •Nanos protein inhibits the translaon of hunchback mRNA •Bicoid protein also acvates zygoc hunchback translaon in the anterior. •The hunchback gene contains mulple occupancy sites for Bicoid protein which are needed for its anterior translaon.

Carroll et al. 2004 Drosophila Gap Genes

• These genes are named for the “gap” in segments caused by their mutaon. • The coordinates of gap are controlled by the read-out of maternal gene expression boundaries • Controlled by the differenal use of enhancers in the gap genes.

Nuslein-Volhard and Wieschaus, 1980 Pair-rule Genes

Sanson 2002 The Pair-rule Genes

• The expression of these genes occur in stripes that correspond to future segments. • There are 7 stripes, Carroll et al. 2004 each alternang for every other segment boundary even-skipped (eve) Mutaons

even-skipped mutaons cause a loss in every other segment

Manoukian and Krause 1992 How Do We Go From an Aperiodic Gap Gene to Periodic Gene Expression?

• Aperiodic gap and maternal genes establish the periodic expression of the 7 stripes of the primary pair-rule genes.

• This is accomplished, again, by the “read- out” of the maternal and gap gene coordinate system on the pair-rule gene enhancers.

• Each stripe of expression is formed by the binding of maternal and gap gene protein products.

Carroll et al. 2004 Gilbert et al. 2014 Segment Polarity Genes

Sanson 2002 Segment Polarity Genes

• These are expressed in 14 stripes (7x2). • These demark the anterior and posterior boundaries of the final segments. • These are inially controlled by pair-rule gene acvity, but later interact with one another to refine their boundaries. Carroll et al. 2004 Segment Polarity Genes

Kornberg 1980 In Summary…

Carroll et al. 2004 How Much of This is Conserved in Other Arthropod Groups?

Peel et al. 2005 Maternal Effect Genes

• bicoid is a derived Hox3 gene, as seen in other , like the mite, where it is expressed as a . • Many other rely on the funcon of orthodencle orthologs to paern their head regions; bicoid may have been co-opted for this in the fly lineage leading to Archegozetes longisetosus; Telford and Thomas 1998 Drosophila. • The role of nanos in other arthropods is less clear, but it is localized to the posterior of Schistocerca oocytes. Schistocerca belongs to a basally branching Schistocerca americana; Lall et al. 2003 group of insects. Gap Genes- “Gap” funcon is Likely Restricted to Insects

engrailed

abdominal-A

hb RNAi in the Tribolium castaneum; Bucher et al. 2004 Pair-rule Gene Conservaon?

• Orthologues of Drosophila pair- rule genes are expressed in

segmental paerns in every paired arthropod group, however the do not always display double-segment periodicity. • The cenpede Strigamia marima uses paired and sloppy-paired orthologues in a pair-rule manner. sloppy-paired

Green and Akam 2013 Segment Polarity Genes?

• The expression of the segment polarity genes has the highest level of conservaon across arthropods

engrailed expression; Carroll et al., 2004 Conclusions

• Drosophila provides a useful model to understand the maternal contribuon to axial paerning

• Drosophila dorso-ventral paerning is iniated in the oocyte via Gurken and Egfr signaling.

• Drosophila antero-posterior paerning progresses from maternally-derived factors to the acvaon of gap genes, to pair-rule genes and lastly to the segment-polarity genes.

• The acvaon of these gene classes stems from their “read-out” of the posional informaon that was provided previously.

• In terms of evoluonary conservaon, the gap genes likely evolved in the insect lineage, however the engrailed/ wingless segment-polarity pathway is widely conserved in all arthropod groups.