L24 - Sexual Reproduction

What was the least common eukaryotic ancestor predominantly made by?

Mitosis

What was the relationship between Sex and Reproduction in the evolution of the LECA?

Sex =/= reproduction

What is saccharomyces cerevisiae?

Haploid single cell organism

What do haploid cells do in response to starvation?

2 haploid cells fuse to form a transient diploid cell
Clonal reproduction by mitotic cell divisions then occurs
The diploid then immediately enter meiosis and produces 4 haploid cells

What were the features of early eukaryotes?

Single Celled

Haploid

How did early eukaryotes reproduce?

Normally clonally by mitosis

Occasionally by sexual fusion of 2 haploid cells, creating a transient diploid, followed by meiosis producing 4 haploid cells

What are the features of modern higher eukaryotes

Modern higher eukaryotes (plants and animals) are multicellular, distinct soma and germline cells which are diploid,

How do modern higher eukaryotes reproduce?

Meiosis of germline stem cells creates haploid gametes, fusion of sperm and egg (or pollen and ovule) creates diploid zygote – clonal expansion of cells via mitosis creates the new organism

Is there sexual reproduction between every generation of multicellular organisms?

Yes

What must modern asexual organisms have evolved from at some point?

Earlier sexual species

What is sex based on?

Meiosis

What is the only type of cell to have sex?

Eukaryotes

What are the three main points about asexuality in eukaryotes?

1. Most higher eukaryotes use sexual reproduction and asexual reproduction is not common

2. Most higher eukaryotes that can reproduce asexually can also do so sexually

3. Obligate asexual reproduction is VERY uncommon

What is fission?

Division into 2 equally sized offspring

What is budding?

unequal division; smaller offspring

What is fragmentation?

Parent breaks into many small new individuals

What unique mechanism does Daphnia (water flea) undergo?

Switches from asexual to sexual in different seasons

What are dandelions obligate asexual by?

“Apoxmixis”

What is the process of dandelion asexual reproduction?

Diploid ovules develop into seeds → Seeds are wind disseminated

What is the process behind the Lesbian Whiptail Lizards

Pseudo-mating promotes ovulation.

On top: estradiol low & progesterone high; small ovary.

On bottom: estradiol high, ovary increase in size and ovulate.

What is phylogeny?

Evolutionary descent with reflected levels of genetic similarity

What is key about the members of one singular genus?

They all share a more closely related common ancestor than any member of another genus

Where is obligate asexual reproduction ONLY found within an otherwise sexual genus?

Level isolated species

What were the sexes originally?

Fusion between cells

What drives outbreeding in yeast?

Yeast has two mating types - one expressing ‘a’ proteins and the other expressing ‘alpha’ proteins

What is anisogomy?

Big gamete and small gamete

What does anisogamy prevent and promote?

Prevents ‘selfing’ and promotes outbreeding

What does the reproduction number determine?

How much a population will increase per generation

What does an R of 1 mean?

Population stays the same per generation

What does an R of 2 mean?

Population doubles per generation

What is a negative of sexual reproduction?

Biologically Costly (R number Wise)

→ Complex behavior required to find a mate

What does the single genome encode in sexual reproduction?

2 different gametes from 2 gonads

Other anatomical differences between sexes

What must the two sexes both do during evolution?

Co-evolve - compatible behaviours, anatomies etc

How does meiosis allow for genetic variation?

Independent assortment of chromosomes

crossing over between homologous chromosomes

What promotes genetic variation in a species genome?

Sexual reproduction coupled to diploidy

Why is giving up sex an evolutionary dead end?

Sex drove evolution - multicellularity, developmental complexity, mind