Reproductive epigenetics

Question 1

What are the key epigenetic marks involved in plant reproduction?

Answer 1

1) DNA methylation: Regulates gene expression and transposon silencing.
* Occurs in three contexts: CG, CHG, CHH.

2) H3K27me3 (Histone 3 Lysine 27 Trimethylation): Repressive euchromatic mark deposited by Polycomb Repressor Complex 2 (PRC2).

• Important for maintaining gene silencing during development.

Question 2

How does epigenetic reprogramming occur during plant reproduction?

Answer 2

Involves global chromatin reorganization in gametophytes.

Meiosis marks the start of reprogramming in both male and female germlines.

• In the female lineage: The megaspore mother cell (MMC) undergoes chromatin decondensation.
• In the male lineage: Pollen mother cells (PMC) exhibit similar chromatin changes.

DNA methylation and histone modifications are partially reset.

Question 3

What happens to H3K27me3 during gametogenesis?

Answer 3

• In the female gametophyte (central cell): H3K27me3 increases, repressing genes.
• In the male gametophyte (sperm cells): H3K27me3 is lost, due to the presence of an H3 variant (HTR10) that cannot be methylated by PRC2

Question 4

What is apomixis, and how does it relate to epigenetics?

Answer 4

Apomixis = Asexual reproduction via seeds (offspring are genetic clones).

Involves three key processes:
1) Apomeiosis (no reduction in chromosome number).
2) Parthenogenesis (embryo forms without fertilization).
3) Autonomous endosperm development (endosperm forms without fertilization).

PRC2 (FIS-PRC2 complex) normally prevents seed development without fertilization.

• Mutations in FIS-PRC2 (fis mutants) allow endosperm to develop autonomously, resembling apomixis.

Question 5

What is genomic imprinting, and why does it occur?

Answer 5

Genomic imprinting = Parent-of-origin-specific gene expression.

• Some alleles are expressed only from the maternal or paternal genome due to asymmetric epigenetic marks.
• Driven by H3K27me3 (maternal) and DNA methylation (paternal).
• Thought to have evolved due to conflict between maternal and paternal resource allocation in the seed.
• Genomic imprinting is a phenomenon driven by asymmetric deposition of epigenetic marks between the
  parental alleles.

Question 6

How does the maternal vs. paternal genomic conflict shape imprinting?

Answer 6

• Paternal alleles favor increased resource allocation to their own offspring.
• Maternal alleles balance resources among all progeny.

Crossing experiments (e.g., tetraploid mother × diploid father) show that paternal excess leads to larger seeds, while maternal excess leads to smaller seeds.

Question 7

How do transposable elements contribute to imprinting evolution?

Answer 7

Over time, new imprinted genes can evolve due to
* TE insertion near genes.
* Leading to local epigenetic modifications
* with differences in the maternal and paternal epigenetic langscape (H3K27me2 in females and DNAmethlation in male gametes)

Question 8

How do companion cells silence transposons in gametes?

Answer 8

Vegetative cells (male) and central cells (female) lose DNA methylation -> activating transposons.
* This leads to the production of small RNAs (siRNAs).
* These siRNAs are transported to gametes (sperm cells/egg cell), ensuring transposon silencing in the gametes.