Lecture 12: Developmental Bias II

Question 1

Evolutionary Developmental Biology (Evo-Devo)

Answer 1

This field studies the interactions between evolutionary changes and developmental processes to understand the diversity of life forms. It explores how developmental constraints influence evolution, leading to a limited range of potential phenotypes (observable traits).

Question 2

Hourglass Model of Chordate Development

Answer 2

1) In chordates (vertebrates and their close relatives), early developmental stages are highly variable, but as development progresses, they converge into a conserved, middle stage known as the phylotypic stage.

2) Intense constraints on phylotypy: This conserved stage features the formation of crucial organs like the central nervous system, heart, and kidneys—organs essential for survival across chordates.

3) Developmental bias limits the possible range of variations during this phylotypic stage, creating a discontinuous distribution of traits and limiting how much these organisms can diverge from the conserved model.

Question 3

Developmental Constraints in Early Development

Answer 3

Significant constraints during early development restrict the potential morphological (structural) variations that organisms can exhibit, especially in the formation of fundamental body plans

Question 4

Explain example of developmental constraints in evolution: evolution of mammals (placental vs. marsupial)

Answer 4

Divergence: Placental and marsupial mammals diverged around 160 million years ago, each group evolving independently.

—> Placental Mammals:
- Characterized by a prolonged gestation period, allowing the fetus to develop complex structures within the womb.
- This extended prenatal period enables placentals to achieve a high degree of anatomical diversity, with a broader range of body forms and adaptations.
- Placental mammals are more widespread and display a greater disparity in skeletal structures compared to marsupials.

—-> Marsupial Mammals:
- Marsupials are born in an underdeveloped state (essentially late-stage embryos) and complete much of their development outside the womb, relying on advanced development of certain structures like the mouth and forelimbs.
- The early life history of marsupials limits their anatomical variation, particularly in limb and cranial structures, leading to lower morphological disparity compared to placentals.

Question 5

Explain example of developmental constraints in evolution: differences in Limb and Cranial Development.

Answer 5

1) Placental vs. Marsupial Limbs: Marsupials exhibit less diversity in limb structures than placentals, primarily due to constraints associated with their unique reproductive strategy and early postnatal development.

2) Cranial Disparity: Marsupials also show less cranial variation than placentals due to these developmental constraints.

Question 6

Explain example of developmental constraints in evolution: impact of prolonged gestation in primates.

Answer 6

1) Primates, including humans, exhibit proportionally longer gestation periods for their body size.

2) Stromal Invasion: Primates have a high degree of placental invasion into maternal tissues, which allows greater nutrient transfer but also increases susceptibility to metastatic cancer. This intense placental-maternal interface is partly why primates can support large brain growth but comes with health trade-offs like higher cancer risks.

Question 7

Developmental Patterns and Morphogenetic rules: Pattern formation in Development.

Answer 7

1) Developmental processes often follow morphogenetic rules—guidelines that govern how cells organize to form tissues and organs. For instance, patterns such as reaction-diffusion models can explain the distribution of pigments in animal coats (like cheetah spots).

2) Condensation and Scaffold Formation: Cells condense to form a scaffold for structures like cartilage, which later expands into differentiated tissues, shaping organs and skeletal elements.

3) Discrete Phenotypes: These developmental rules lead to distinct physical characteristics (phenotypes) without intermediate forms.

Question 8

Developmental Patterns and Morphogenetic rules: Reaction-Diffusion Models.

Answer 8

1) These models use equations to describe how molecules interact to create stable, repeating patterns across the body, such as stripes, spots, or skeletal formations.

– > Example: In limb development, altering molecular gradients can yield distinct skeletal patterns, suggesting potential mechanisms for evolutionary changes in limb structure.

Question 9

What is a summary of this lecture, the key points and concepts discussed.

Answer 9

1. Developmental Processes as Constraints:
   o Developmental rules limit the range of evolutionary changes possible in an organism, known as developmental bias.
   o These biases can impact structures like limbs, which show distinct patterns of evolutionary change and reduction.
2. Limb Development and Evolutionary Patterns:
   o Limb development highlights the relationship between evolutionary pressures and developmental rules, as seen in cases of repeated evolutionary reduction of digits.
3. Trade-Offs in Gestation and Health:
   o Prolonged gestation and intense stromal invasion in placental mammals, especially primates, allow for advanced development but increase the risk of conditions like metastatic cancer.
4. Advances in Developmental Imaging:
   o Technology has improved, enabling 3D imaging of embryonic development, which allows researchers to visualize developmental constraints and potential variations at the cellular level.