Question 1: Methodology and Logical Deduction A researcher identifies a specific epiRIL (Epigenetic Recombinant Inbred Line) of Arabidopsis that exhibits significantly delayed flowering compared to the wild-type parent, despite being genetically identical at the DNA sequence level. • Task: Outline how you would use bisulfite sequencing to identify the molecular cause of this phenotype. • Logic Challenge: If the sequencing reveals a Differentially Methylated Region (DMR) in the promoter of the FWA gene, but the DNA sequence itself matches the wild-type, what does this suggest about the stability of the trait over eight generations of selfing?

What is an epiRIL?

An Epigenetic Recombinant Inbred Line. A plant line that is genetically identical to the wild-type parent at the DNA sequence level but shows heritable phenotypic differences due to epigenetic variation.

What phenotype does the epiRIL in question exhibit?

Significantly delayed flowering compared to the wild-type parent.

What is the first step in bisulfite sequencing methodology?

Treat genomic DNA from both the epiRIL and wild-type parent with sodium bisulfite.

What does sodium bisulfite treatment do to DNA?

It converts unmethylated cytosines into uracil (read as thymine during sequencing), while methylated cytosines remain unchanged.

After bisulfite treatment, what is the next step?

Perform whole-genome sequencing on the treated DNA.

How do you determine the methylation status of every cytosine?

Compare the sequenced DNA to the original reference genome. Unmethylated cytosines appear as thymine; methylated cytosines appear as cytosine.

What is a DMR?

A Differentially Methylated Region. A specific segment where methylation levels differ significantly between the epiRIL and wild-type despite identical DNA sequences.

After identifying DMRs, where do you focus to find the causal epimutation for delayed flowering?

On DMRs located in the promoters or gene bodies of known flowering regulators such as FWA or FLC.

If bisulfite sequencing reveals a DMR in the FWA promoter but the DNA sequence matches wild-type, what does that suggest about the trait's stability over eight generations of selfing?

That the trait is stably inherited through meiosis and the epigenetic state is self-perpetuating.

What is the normal methylation state of the FWA promoter in wild-type Arabidopsis?

It is typically methylated and silenced.

What methylation change at FWA would cause delayed flowering?

Hypomethylation (loss of methylation) at the FWA promoter, causing ectopic expression of FWA.

How many parental DMRs at loci like FWA are stably inherited over eight generations in ddm1-derived epiRIL populations?

Approximately one-third are inherited according to Mendelian laws and remain stable for at least eight generations (F8) and beyond.

Why does the persistence of the FWA DMR through eight generations prove the epigenetic state is self-perpetuating?

Because the original inducing mutation (e.g., ddm1 or met1) was crossed out several generations ago, so the DMR persists in a genetically wild-type background independent of the original signal that caused the loss of methylation