EvoDevo Body plans Flashcards
What are the theories proposed by the founding fathers? What is the hourglass model?
Ernst Heackel in 1866 theorised that Ontogeny recapitulates phylogeny, while Karl Ernst von Bear in 1828 theorised that the embryo successively adds the organs that characterize the animal classes in the ascending scale. These two theories are opposite to each other, since the first one means that an organism, in the course of its development, goes through all the stages of those forms of life from which it has evolved, while the second one means that organs are added one by one, in a specific order, during development.
These contrasting theories have been surpassed by the hourglass model for morphological diversity, in which the various step of embryonic development can be seen and put in comparison:
- At early embryogenesis a great reproductive diversity can be found (placenta, egg etc)
- At mid embryogenesis, the embryos look all pretty similar. This stage is also called phylotypic stage.
- At late embryogenesis, a great adaptive diversity is seen, since the animals that the embryo will develop into are now recognisable.
What is meant for evolutionary conservation of development?
For evolutionary conservation of development is meant the presence of similar genes, portions of genes, or chromosome segments in different species, reflecting both the common origin of species and an important functional property of the conserved element.
We can see these conserved genes in two forms:
1. Homologs genes: genes inherited in two species by a common ancestor.
2. Paralogs genes: duplicated genes that may or may not evolve to have different functions and compositions.
What are morphogens? How do they function regards AP and DV axis formation?
Morphogens are compounds with a non-uniform distribution establishing portions of specialized cell types and tissues.
The morphogens for the AP axis are:
1. Bicoid (head and thorax)
2. Nanos
These two morphogens work at the same time, and suppress other compounds to form the anterior and posterior part of an embryo:
- Bicoid suppress Caudal in the anterior part
- Nasos suppress Hunchback in the posterior part
The DV axis’s morphogen is Spätzle, which – in the Toll-Spätzle pathway – activate Dorsal in the dorsal region of the embryo. The phosphorylation (deactivation) of Dorsal in the ventral region prompts translocation to the nucleus. When Dorsal is activated, it binds to the DNA and regulate the transcription of target genes:
- Decapentaplegic
- Sog
- Snail
The morphogens seen above are specific for the fruit fly, but there are homologous morphogens in mammals:
- Bicoid = Goosecoid (AP =PA)
- Decapentaplegic = BMP4 (Dorsal = Ventral)
- Sog = Chordin (Ventral = Dorsal)
- Hedgehog = Shh (Specification of segments)
How is organised the Hox complex?
Hox genes are genes resposible for the specification of segments position in an organism. The process of transformation from one body part to another is called Homeosis, in which Homeotic genes are involved. There are two types of homeotic genes:
- Hox genes
- Selector genes (resposible for segment identity)
Hox genes can be found in complexes in the genome, causing them to have a fixed position in the cluster, linked to a specific function. Their transcription sequence reflects the AP axis (positional collinearity), while the transcription of these genes always in the same sequence is the temporal collinearity.
How Hox genes regulate the process of segmentation among various animals? Are they responsible also for the formation of limbs? And their elongation?
All animals are characterised by segments – repeatitive sequence of the same segment.
In humans, segmentation can be seen in the spine, which is formed by:
- 7 Cervical vertebrae
- 12 thoracic vertebrae
- 5 lumbar vertebrae
- 5 fused sacral vertebrae
- 4 fused caudal vertebrae
We can observe the loss of one SV and one LV due to adaptation to bipedalism.
The CV are a highly conserved trait in mammals, since all animals present 7 of them. The number of CV is regulated by Hox 4-5 (responsible also for the regulation of stem cells proliferation); the overexpression of these Hox genes, can cause congenital cancer.
The snake present a very peculiar body: no appendages and 200+ segments: this is caused by the unleash of the Hox genes and a high rate of somitogenesis, which is regulated by the segmentation clock: the higher the rate of the clock, the higher the number of segments the organism will present.
Hox 6 is the gene specific for the thoracic segments, which is usually downregulated by Hox 10 in order to form 10/13 segments: in the snake, this downregulation is notpresent, causing it to have the very high number of segments seen earlier. The absence of limbs is characterised by a different regulation of the morphogens: snakes do not present Shh, causing the absence of the zone of polarity, and they lack of FGF8 expression, causing the absence of the Apical ectodermal ridge, which are two essential areas that are needed for the formation of limbs.
The bat present very long metacarpals: this extreme elongation is caused by the second wave of Shh, which cause a double elongation.
How do limbs develop?
Limbs develop from what are called limb buds, which have two areas resposible for the direction and length of the limb:
- Zone of polar activity, responsible for the correct direction.
- Apical ectodermal ridge, resposible for the length of the limbs.
The ZPA is regulated by the activity of Shh, which cause the formation of the AP axis in the limb bud; the AER is regulated by FGF8, which regulate the completeness of the limb.
How hox genes regulate limb development?
The hox complexes A, B, C and D are responsible for the overall limb structure:
- Hox 9 and 10 regulate stylopod
- Hox 10 and 11 regulate zeugopod
- Hox 12 and 13 regulate autopod
The presence of various Hox genes in the regulation of limbs means that if a mutation inhibit one gene, the structure won’t be completely removed.
Autopods are often seen divided in digits in mammals: the separation of the digits is regulated by BMP4, which induce apoptosis in the epidermal tissue. In webbed animals, BMP4 is inhibited by Gremlin.
