Arthropod Segmentation and Engrailed gene

What is the key difference in leg dev mechanism compared to others in evolution?

Unlike most mechanisms which work via cis regulatory changes, arthropod segmentation involves protein functional evolution.

Is segmentation example of convergence?

Yes segmentation is an example of convergent evolution where although inverts and verts both have segmented body plans where terminal genes are conserved the upstream systems to create segmentation differs. This is developmental systems drift.

Explain segmentation in Drosophila pt 1

Segmentation involves spatial hierarchy governed by simultaneous top down system. This system can occur due to early state of embryo as syncytial blastoderm which allows maternal protein gradients to diffuse freely.

Explain segmentation process in Drosophila

Process = maternal effect proteins pattern primary axes of embryo and this triggers gap genes that divide embryo into spatial domains. Pair rule genes then add 14 distinct stripes and segment polarity genes such as engrailed are used to mark A-P boundary for each indv segment.

Explain segmentation in vertebrates

Vertebrate segmentation operates using a temporal oscillator described by Clock wavefront model. Unlike drosophila which have simultaneous patterning, verts pattern in sequential manner from anterior to posterior end within pre-somitic mesoderm.

Explain clock wavefront model

The clock consists of synchronised rhythmic oscillations from wnt, notch and FGF signalling pathways. These pulse at species specific intervals

The wavefront acts as moving threshold of maturing, defined by posterior to anterior gradient of wnt and FGF + opposing AP gradient of retinoic acid. These gradients create determination front (gatekeeper).

Posterior to front = undifferentiated

Anterior to front = permitted to form somite

How do somite form?

Somites form when oscillating clock enters specific phase exactly as cells move past determination point. At this intersection, cell state is frozen and this triggers transition of mesenchymal cells to epithelial, pinching off new somite.

What does the engrailed gene do?

The engrailed gene acts a highly conserved transcription factor that marks segment boundaries, acting as a molecular mark of compartmentalisation. This establishes the segments as independent developmental modules. Found across most arthropod lineages therefore e.g. of deep homology

How is engrailed gene example of co-option?

Engrailed gene was originally used in neurogenesis as neural boundary marker and regulator in axon pathfinding. Its recruitment into segment boundaries = evidence of evo as tinkerer

What is the role of Ubx and AbdA here?

Segment identity is managed by Hox cluster. The Ubx and AbdA proteins have evolved to act as transcriptional repressors of distal-less gene which promotes appendage development.

This transition involved the acquisition of a C terminal repression domain within the Ubx protein itself. This protein-level change ensures that whenever Ubx is expressed in a segment, it automatically silences distal-less and the "leg-building" program.

What is the evidence? Lewis et al. 2000

Researches conducted an experiment on Flour beetles (Trilobium), where knocking out Ubx and AbdA led to leg growth on every segment. This demonstrates that segments are serially homologous modules with a latent potential for limb formation that is suppressed by the specific Hox identity of that segment. Lewis at al. 2000

What is a constraint seen in segmentation?

In the centipede order Lithobiomorpha and others, he segment number is always an odd number. This suggests that the genetic pathway governing segment addition is "hardwired" in a way that it can only produce odd no of segments.

Explain why in Strigamia maritimia, centipedes in North have fewer segments than those in the South?

Temperature influences the rate of the segmentation clock where warmer June  (nesting) temperatures in the South allow for more rapid growth process, but the colder temperatures in North lead to slower growth rate where fewer segments form. The higher segment no in the South represents an environmentally induced peramorphosis specifically acceleration where higher temperatures in the south increase metabolic rate, allowing for extended segment phenotype.

Explain heterotopy link

In the centipede Henia vesuviana, the venom-sac module which typically develops on the first pair of appendages is shifted ten segments posterior to its usual position.