BIOL 371 - Animal Reproduction and Development

How are new individuals primarily produced in animal reproduction?

sexual reproduction

What is animal reproduction generally the dominant stage and what happens?

dominant diploid stage producing haploid gametes which fuse to produce new diploid offspring

What must the zygote undergo after its fusion?

mitosis and gastrulation

What happens to cells that give rise to gametes?

they are isolated early on in development

What are the different kinds of asexual reproduction?

• fission

• budding

• fragmentation

• parthenogenesis

What is fission?

genetically-controlled, two identical individuals result

What is budding?

smaller offspring arises from parent

What is fragmentation?

Each fragment gives rise to a new organism

What is parthenogensis?

new individual develops from unfertilized egg

What are the pros and cons of asexual reproduction compared to mitosis?

Pros

• don’t need males

• rapid reproduction

• saves energy

Cons

• lack of genetic variability

What is sexual reproduction?

genes from two parents

→ eggs (oogenesis) and sperm (spermatogenesis) - the gametes that are produced by meiosis

→ ALWAYS involves fertilization and development

What are dioecious reproduction? (gonochoristic)

involving of two seperate sexes

What is monoecious/hermaphroditic reproduction?

an individual has both male and female reproductive organs either simultaneously or at different stages

What are the pros and cons of sexual reproduction?

Pros

• generates genetic variation

Cons

• fertilization is necessary which can be complicated

• development must proceed form the zygote

• costs of egg and sperm

• males (half the population) can’t produce offspring

What are gametes?

cells that give rise to gametes (germ cells) developing separately from somatic cells

Is there a relationship between somatic and germ cells?

no, differentiation and development of somatic cells do not affect germ cells and experiences of an organism do not affect genetics of sex cells

Where does gametogenesis take place?

in primary reproductive organs or gonads

What is the haploid stage of an animal’s cycle?

Gametes are the only haploid stage of an animals life cycle

What does gametogenesis consist of?

mitosis, meiosis, and cell differentiation to produce gametes - haploid sex cells which must combine to give rise to zygote - new diploid individual

What produces sperm cells and what is the metabolic characteristic of sperm?

• spermatogenesis produces sperm cells

• sperm are highly metabolically active

Where do sperm get the energy for movement?

Mitochondria provide ATP for flagellum

What does the head of a sperm consist of?

• acrosome

• nucleus

• centrioles

• few or no stored nutrients

What is the male strategy of sperm production?

to produce abundant small, motel, cheap gametes

What is the structure of sperm?

What are eggs produces by and what is their main characteristic?

• produced by oogenesis

• eggs contain large amounts of protective components and nutritive contents (yolk)

What is the structure of eggs?

• contained with vitelline membranes (zone pellucid) - carries receptors for sperm

• may be wrapped within jelly layer

• hard shell

What is the female strategy of egg production?

produce few large, non-motile, well-provisioned gametes

(reciprocal of male)

Male reproductive system parts

• testes produce sperm

• urethra/ducts carry sperm genital opening

Female reproductive system parts

• ovaries produce eggs

• oviducts carry them to genital opening

What determines external reproductive structures?

Whether fertilization is external or internal

What is hermaphroditism?

• an adult organism possessing functional reproductive systems of both sexes, either simultaneously or sequentially

• monoecious

Who is hermaphroditism common in?

taxa like annelids, gastropods and nematodes

How is sex determined in placental mammals?

• two sex chromosomes, X and Y

  • Y chromosomes carries few genes, carrying SRY gene which controls development of new individual into a male

  • Y carried only on males

• without the Y and SRY, the new individual develops into a female

How is sex determined in bird, some reptiles, some fish and butterflies?

• two sex chromosomes, Z and W

• female determined

  • ZW - female

  • ZZ - males

How is sex determined in some insects?

• by aneuploidy (monosomy) in males

  • males X

  • females XX

How is sex determined in Hymenoptera?

• ploidy dictates sex

  • females - diploid

  • males - haploid (parthenogenesis)

• basis for sociality in wasps, ants and bees

What organisms does external fertilization occur in?

only aquatic animals

External fertilization

• Gametes generally small (egg still larger)

• Frequently high egg mortality

• Number of eggs released varies greatly among species

• Release of gametes by both sexes into environment requires coordination of gametogenesis in response to seasonal environmental cues

• Coordinated gamete release may be due to stimulus through courtship movements, calls, or pheromone release

What organisms is internal fertilization found in?

found in ALL terrestrial and in some aquatic animals

Internal fertilization?

• Large differences in gamete size

• Number of eggs relatively few compared to sperm

What are some challenges that go along with all fertilization?

making sure that the right sperm encounters the egg, and that only 1 sperm fertilizes it

What are some challenges that go along with external fertilization?

• the possibility of interspecific sperm-egg encounters

  • timing of breeding

  • molecular mechanisms of identifying appropriate sperm

What are some challenges that go along with internal fertilization?

• pre and post zygotic reproductive isolating mechanisms

  • behavioural

  • physical constraints

  • genetic incompatibility

What two ways does fertilization prevent polyspermy?

• acrosomal reaction (fast block)

• cortical reaction (slow block)

What is the acrosomal reaction (fast block)?

sperm cell membrane fuses with egg cell membrane initiating a wave of depolarization - subsequent sperm unable to fuse with egg

What is the cortical reaction (slow block)?

cortical granules fuse with egg membrane, vitelline coat becomes impenetrable to sperm - external and internal fertilization

How are species prevented from interbreeding and producing hybrids?

• pre-zygotic barriers: eggs and sperm never encounter one another, or if they do fertilization does not take place

• post-zygotic barriers - fertilization can take place, but hybrids unlikely to reproduce in turn

How does internal fertilization protect the developing embryo?

Internal fertilization allows for different ways of protecting the developing embryo

What do all amniotic vertebrates have?

the amanita

What are the parts of the amniotic egg?

• variation in all amniotic vertebrates

• allantois and chorion - carry out life support functions for embryo

• shell - protects internal environment

What type of fertilization do all organisms containing an amnion use?

internal fertilization

Oviparous meaning

fertilized eggs are laid and hatch outside the body (e.g. most birds, reptiles and monotremes)

What is viviparity?

• a form of reproduction where embryo develops inside the parent's body and the parent gives birth to live young

• nutrients, gases and metabolic wastes are exchanged between the mother and embryo requiring some form of a placenta

What is the placenta derived from?

extraembryonic membranes - The placenta develops from tissues associated with the embryo but outside the embryo itself.

When does the parent invest more in viviparity?

• The parent has to provide a lot of resources while the embryo is developing inside them.

• Giving birth to a live offspring doesn't automatically mean the parent invests heavily afterward. Different species vary in how much parental care they provide.

What are types of animals are placentalia?

placental mammals - some fish, amphibians and reptiles (rare)

What is ovoviviparity?

a reproductive strategy where the embryos develop inside eggs that are retained within the parent's body until they hatch.

• retention of the fertilized eggs within the mother’s body, with varying degrees of exchange of gases, nutrients or wastes between mother and developing embryo

• principle reproductive effort tends to be before birth

Describe the continuum of reproduction

Reproductive strategies can be viewed as a continuum based on where the embryo develops and how much nutritional support the parent provides:

Oviparity → Ovoviviparity → Viviparity

Difference between Oviparity, Ovoviviparity and Viviparity?

Oviparity: Eggs are laid outside the parent; embryo is nourished primarily by yolk.

Ovoviviparity: Eggs are retained inside the parent; embryo is still nourished primarily by yolk and young are born/released alive.

Viviparity: Embryo develops inside the parent and receives substantial nutritional support directly from the parent, often through a placenta.

Reproductive strategy	Where does the embryo develop?	Main source of nutrients	What happens?
Oviparity 🥚	Outside the parent's body	Yolk	Parent lays eggs → embryo develops in egg → hatches
Ovoviviparity🥚	Inside the parent's body, within an egg	Yolk	Eggs are retained inside parent → embryo develops → young are released/born
Viviparity 👶	Inside the parent's body	Parent	Embryo develops inside parent and receives nutrients from parent → live birth

What is the intermediate step between oviparity and viviparity?

ovoviviparity

What are the animal and vegetal poles of an egg?

The animal pole contains the egg nucleus and less yolk, so cleavage is faster and produces smaller cells. The vegetal pole contains more yolk, so cleavage is slower and produces larger cells.

Where is developmental information stored?

• the nucleus

• the cytoplasm

  • contains maternal cytoplasmic determinants which are mRNA and proteins from the mother’s genome that contain molecules that help direct development and differentiation in embryo

What is Ontogeny?

he development of an individual organism throughout its life.

What is blastulation?

cleavage the zygote undergoes immediately following fertilization

mitotic division with no increase in mass, using egg’s resources

What are the different types of blastulation?

radial and spiral

What are the stages of the zygote following gastrulation?

• process results in the morula - solid ball of cells

• gives rise to blastula -hollow ball of cells (blasters)

Does the blastula contain animal and vegetal poles?

Yes

What are cytoplasmic determinants?

Maternal mRNAs and proteins placed in the egg cytoplasm before fertilization that help direct cell differentiation and embryonic development.

How does cytoplasmic differentiation alter the way embryos develop?

• cleavage divides cytoplasmic determinants up - different blastomeres have different combinations and amounts of cytoplasmic determinants

• blastomeres undergo differentiation - become different types of cells depending on position

Cytoplasmic differentiation in relation to snail shells?

spatial distribution of cytoplasmic determinants determines direction of cleavage spiral in snails

What is morphogenesis?

the process by which an embryo develops its shape and organization.

What 7 cellular processes are important in morphogenesis?

1. mitosis

2. cell movement

3. selective cell adhesion

4. apoptosis

5. indiction

6. determination

7. differentiation

Why are mitosis, cell movement, selective cell adhesion, apoptosis important for morphogenesis?

1. Mitosis

Cells divide to increase the number of cells in the embryo.

→ One cell → two cells → four → eight → etc.

2. Cell movement

Cells move to different locations in the developing embryo.

This is important because cells need to end up in the correct place to form the appropriate structures.

3. Selective cell adhesion

Cells stick to some cells more strongly than others.

This helps cells sort themselves into appropriate groups and tissues.

4. Apoptosis

Programmed cell death.

Some cells are intentionally eliminated to help shape structures during development.

For example, removing certain cells can help separate developing structures.

What is apoptosis?

programmed cell deathHW

What is induction?

one group of cells affects developmental path taken by another group of cells, through signal molecules or direct contact

“Cell group A tells cell group B how to develop.”

What is determination?

A cell becomes committed to a particular developmental pathway.

switching of most of the genes of the cell, only allowing certain developmental pathways to be followed

What is differentiation?

After determination, the cell actually develops the specialized characteristics of its particular cell type.

After determination, cells follow specific developmental pathways

Easy way to think about induction, determination and differentiation

Induction: “You should become X.”
↓
Determination: “I'm committed to becoming X.”
↓
Differentiation: “I now develop the features of X.”

What does gastrulation establish (very early on)?

• process by which blastula forms 2 or 3 germ layers (embryonic tissues) - ecto-/meso-/endoderm giving rise to gut, mouth/anus

• initial establishment of body axes

What do the 3 germ layers develop into?

• endoderm - lining of gut and associated organs

• mesoderm - muscle, other internal organs, connective tissue and skeleton

• endoderm - outer surface, nervous system

What do the three embryonic tissue layers determine?

three embryonic tissue layers broadly determined fates in developing embryo

How do organisms, like drosophila develop?

Fertilized egg
↓ repeated nuclear divisions without cytokinesis
Syncytium (many nuclei, one cell)
↓ cytokinesis
Blastula
↓ gastrulation + cell movements
Segmented embryo

What is the key idea to how development of the embryo occurs?

The nuclei contain the genetic information, but maternal determinants and signaling establish which genes are expressed where, helping determine the embryo's development. The nuclei aren’t exclusive. The nuclei themselves don't determine the embryo's development by themselves.

When are body axes information present?

Even before fertilization takes place - body axes are established early, during the syncytial blastoderm stage, largely through maternal cytoplasmic determinants, before gastrulation.

Why is Drosophila useful as a developmental model organism?

Its larval segments have a one-to-one correspondence with regions of the adult, allowing researchers to easily track developmental changes from embryo/larva to adult.

What is the relevance of the bicoid gene to maternal-effect inheritance?

Mother makes bicoid mRNA → puts it in egg → Bicoid protein gradient → establishes anterior development.

So even though the embryo eventually has its own bicoid gene, the crucial early bicoid information was provided by the mother before fertilization.

How does the maternal bicoid gene establish the anterior-posterior axis in a Drosophila embryo?

Maternal bicoid mRNA is concentrated at the anterior end of the egg. After fertilization, it is translated into Bicoid protein, which diffuses toward the posterior to form a concentration gradient. Different Bicoid concentrations provide positional information that helps determine anterior vs. posterior development.

What do Hox genes do in Drosophila?

Hox genes are a group of genes arranged in a particular linear order on the chromosome.

They produce transcription factors—proteins that regulate which other genes are turned on or off.

These transcription factors give cells positional information along the embryo's body axis.

Different regions of the embryo express different Hox genes, causing them to develop different segment-specific structures.

What is direct development?

non-metamorphic - new individual similar to adult

What is indirect development?

metamorphic - new individual dissimilar from adult undergoes metamorphosis

Do animals observe allometric or isometric growth?

Both direct and indirect may display allometric

Isometric growth is uncommon