Lecture notes on the third lecture of Evolution and Development on the development of a single cell to an organism. Document contains literature, theories and lecture notes.
23/3/2022: Evolution and Development: From single cell to the whole organism
Which whole organism?
Y chromosomes in male mammals and W chromosomes in female birds only contain a few
genes. They contain too few genes to directly cause all of the sex differences. In fact, most of
the genome is shared. In mammals, just 0.66% of genes are on the Y chromosome.
Dimorphism must therefore be the result of different regulations of autosomal genes in each
sex.
There are no sex chromosomes in hymenopterans → all individuals share all of the same genes
(males are haploid and females are diploid) → formation of morphologically distinct castes
(=bijenrang (queen or worker)) is an even more extreme example of how gene regulation
influences adult morphology→ due to environmental influences of the diet, queens, and workers
can be produced → diet influences speed of development → faster development results in
queen, slower development results in workers.
Timing of the change in gene expression seems important → can be influenced by social
environment and food → eg. in ants. (Social) environment (such as dominance feeding) of the
callow adult results in differential expression of toolkit genes → resulting in either
subordinate/worker ants or dominant/queen ants.
Early deviations in the developmental program seem to result in more significant morphological
effects than later deviations. The social environment (such as physical contact/feeding) of young
larvae results in differential expression of toolkit genes → resulting in either worker-destined
larvae or queen-destined larvae.
Bilateral gynandromorphs: individuals with half male and half female cells → this is due to the
loss of W/Y chromosome in early cleavage of the embryo → in birds, males are ZZ, females are
ZW ; butterflies: males are XY, females are XX.
Cell memory and identity
Cell memory is determined in early development → Daughter cells have the same identity as
the mother cells. Can result in gynandromorph: In a partial gynandromorph an error occurred
later in development → therefore the impact of the developmental mutation on future cells is
smaller. This suggests that cells are committed to becoming parts of the body very early, and
then this commitment is further refined during cell division → once committed, it stays that way.
This suggests that polarities are formed extremely early in development → this can be left-right /
front-back.
, Cell types: developmental potential becomes increasingly restricted as development continues
- Totipotent: earliest cells→ can differentiate into any cell.
- Pluripotent: can differentiate into certain types of tissues → cannot differentiate into all
cell types anymore
- Unipotent: has fixed cell identity
Cell type is specified by gene expression;
- Gene expression depends on the current environment (extracellular signals)
- Gene expression also depends on ancestry (= the extracellular environment experienced
by the cell’s ancestors)
Exactly the same signal can have different outcomes due to the environment of the cell.
2 mechanisms for generating different responses
to the same inductive signal:
A. Combinatorial signaling: the effect of a
signal depends on the presence of other
signals received at the same time
B. Cell memory: previous signals (or other
events) can leave a lasting trace that alters
the response to the current signal.
Spatial patterning via morphogens
It’s not enough to generate different cell types during embryonic growth and differentiation→
these have to be spatially organized to form tissues, organs, etc.
Therefore the spatial organization of gene expression is organized by morphogens.
Morphogens are secreted proteins that act as inductive signals → the activity of morphogens
depends on their concentration in the tissue → high concentration = high activity; low
concentration = low activity.
Example: source cell produces morphogen → concentration of
morphogen decreases as you are further away from the source
cell → cells have threshold → certain concentration morphogen is
needed to overcome the threshold.
Important for morphogens → they need to be soluble
Morphogenetic fields are established in response to the
morphogen. These are regions of the embryo that are distinguished by gene expression, but are
not committed to a specific cell fate yet. Morphogenetic fields can result in an axis in the embryo
→ dorsoventral axis as a result of morphogen gradient → results in differentiation within the
morphogenetic fields and thus yields cells with a fixed fate.
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