Evolutionary Biology 7: Comparative development 2

Question 1

Overview of double fertilisation event in angiosperms

Answer 1

• Pollen tube inserts 2 haploid sperm
• 1 sperm fertilises egg, forming diploid zygote
• 1 sperm unites with 2 polar nuclei, forming triploid endosperm

Question 2

4 key developmental stages in embryogenesis

Answer 2

1) Globular
2) Heart
3) Torpedo
4) Mature

Question 3

Describe apical-basal axis formation in zygotic stage

Answer 3

• First zygotic division is perpendicular to future axis
• Asymmetric division establishes axis
• Results in two cell lineages with different fates (basal, apical)

Question 4

Which early stages of development indicate the origin of tissue layers?

Answer 4

Octant stage (8 cell) -> Dermatogen (16 cell) stage
- change in orientation of division plane in protoderm leads to new axis (radial) patterning

• > distinct formation of epidermis and ground tissue (cortex)
• > New radial axis leads to early globular -> globular stage

Question 5

What is the Quiescent centre (QC) and what is its purpose? Which cell becomes the QC?

Answer 5

A region in the apical meristem of a root where cell division proceeds very slowly or not at all, but the cells are capable of resuming meristematic activity should tissue surrounding them be damaged

Top of basal cell lineage of development becomes QC

Question 6

3 regions of the apical region in embryo and what to they give rise to?

Answer 6

Upper tier > cotyledons, SAM, upper hypocotyl

Upper lower tier > hypocotyl

Lower lower tier > root + root meristem

Question 7

When is the root meristem pattern fully established?

Answer 7

Late heart > Torpedo stage

Question 8

Embryo develops in tandem with what?

Answer 8

Endosperm

Question 9

Difference between Arabidopsis and barley grain (cereal) endosperm?

Answer 9

Arabidopsis - Transitive endosperm (virtually disappears by germination)

Barley - Persistent endosperm that supports germinating seedling

Question 10

What are coenocytes? Why are the endosperm and coenocytes a challenge to cell theory?

Answer 10

Coenocyte - a body of cytoplasm containing several nuclei, enclosed in a single membrane

Endosperm initially has dividing nuclei within the seed/seed before cell formation

-> challenges general structure of a ‘cell’

Question 11

Why may have double fertilisation been advantageous on an evolutionary basis?

Answer 11

Rudimentary double fertilisation, presence of supernumerary embryo -> higher chance of germination and survival
-> supernumerary embryo became endosperm via nourishing behaviours

Question 12

5 Main developmental stages in vertebrates

Answer 12

• Cleavage
• Blastula
• Gastrula
• Pharyngula
• Growth/sexual maturation

Question 13

What occurs in the cleavage stage?

Answer 13

Division of single fertilised zygote

Cell divisions are unique

• No growth phase
• cell size halves with each successive division
• rapid division compared to later stages

Sister cells identical

Question 14

Purpose of yolk?

Answer 14

Contains key components that nourish embryo (fatty acids, proteins, vitamins)

• concentration and distribution of yolk affects cleavage patterns

Question 15

Levels of yolk density? Types of cleavage in each type? Examples of animals with this type

Answer 15

Isolecithal: Sparse, evenly distributed

• Holoblastic cleavage (complete cleavage throughout cell), constant
• mammals

Mesolecithal: Moderate, concentrated at vegetal pole

• Holoblastic cleavage, slower in yolky regions
• amphibians

Telolecithal: Very dense yolk, separate compartment to embryo

• Meroblastic cleavage, incomplete, cleavage occurs in superficial region relatively yolk free
• birds, fish, reptiles

Centrolecithal: Very dense yolk in centre of embryo

• Meroblastic cleavage
• insects

Question 16

Characteristics of blastula stage

Purpose of blastocoel?

Answer 16

Cell divisions differ from cleavage

• growth phases now present
• division rate slower

Formation of blastocoel - fluid filled cavity within embryo

• sister cells identical

Blastocoel provides important surface for cell migration
- ensures different germ layers are physically separated during gastrulation

Question 17

Characteristics of gatrula + gastrulation

- why is it important?

Answer 17

Formation of primitive gut (archenteron)

• Blastula transformed from mass of undifferentiated cells to multilayered structure (gastrula)
• Complex cell migration within blastula
• Resulting layers foundation for tissue and organs
• Cells begin to differentiate -> identical sister cells to specific cell types

Ectoderm, (mesoderm)* Endoderm
* only in triploblastic animals