L29 Transposons

Question 1

Hybrid dysgenesis

Answer 1

see onenote

Question 2

Maternal factor controls transposon activity

Answer 2

see onenote

to maintain fertility - mother transmits some factor into oocyte that controls transposon activity in germline/adjacent somatic cells of the offspring

This female has been exposed to P-element

• Some factor deposited from the somatic tissue of the female into all of her eggs - transposon control factor
• If there is mutation in that factor, we would expect sterility that is associated with increased transposon activity in the germline

Question 3

Mutants with uncontrolled transposon activity - piwi

Answer 3

see onenote

piwi mutants have defects in gametogenesis

TE activity in germline of piwi mutants - mainly effects retrotransposons but some DNA transposons

Question 4

Piwi encode Ago-like proteins

Answer 4

Two groups of Ago proteins

1. non-piwi Ago - animals, pants, fungi
2. piwi Ago - animals only

Question 5

PIWIs only expressed in germline

Answer 5

see onenote

PIWI protein accumulate in both male/female reproductive tissue

Question 6

small RNAs associated with piwi proteins

Answer 6

see onenote

piwi-interacting RNAs = piRNAs

Question 7

Mammalian PIWIs and gonad development

Answer 7

see onenote

Mili/Miwi2

3 piwis in mammals

Question 8

Mammalian PIWIs and piRNAs

Answer 8

see onenote

Question 9

piRNA clusters in genome

Answer 9

see onenote

fly - in centromere/telomere regions
mouse - small clusters in euchromatin regions

• cluster length varies
• 90% clusters match TE transcripts, but match the inactivate remnants
• piRNA clusters expressed in germline/follicle cells

Question 10

De novo piRNA production

Answer 10

see onenote

piRNA not generated by dicer

• larger than siRNA/miRNA
• primary piRNA derived from ss piRNA precursor transcript
• bias for U at position 1 of primary piRNA

Question 11

piRNA mode of action

Answer 11

see onenote slides

cytoplasm - cleavage
nucleus - methylation

Question 12

Role of piRNA

Answer 12

see onenote

maintain genome integrity

• prevent TE mobilisation in germline and DNA damage
• prevent telomere loss

BUT…transposon activity is high but piRNA precursor processing is inefficient => requires amplification pathway to increase number of piRNA for effective transposon suppression

Question 13

Amplification of piRNA - secondary piRNA

Answer 13

see onenote

amplification generates highly abundant secondary piRNA in germline

TE sense and antisense piRNA associated with different subclasses of PIWI:
antisense - PIWI class 1 (Piwi/Aub)
sense - PIWI class 2 (Ago3)

10nt overlap between sense/antisense strands

Question 14

secondary piRNA biogenesis - PingPong cycle

Answer 14

see onenote slides

achieves two functions:

1. selective amplification of useful piRNAs
2. silences TEs through cleavage of their transcripts

Question 15

What are the maternal factors in hybrid dysgenesis?

Answer 15

see onenote

piRNAs

maternal deposition of P element piRNAs - control P elements in F1 germline

Sterile progeny receive piRNA cluster from father but doesn’t produce piRNA at very high levels

Question 16

Role of maternal piRISC

Answer 16

see onenote

1. enter nucleus of germline cells and install H3k9me3 marks on piRNA clusters => enhancers transcription of piRNA clusters
2. initiate ping pong amplification cycle in cytoplasm => enhances processing of precursor piRNA transcripts

Question 17

Restoration of fertility in dysgenic females

Answer 17

see onenote

• As dysgenic F1 females age, their fertility is partially restored in some egg chambers
• need to produce piRNAs to all TEs before fertility can be restored
• p element piRNA arise from activity of paternal piRNA clusters (not sufficient to restore fertility), other TE piRNAs arise from de novo insertions into piRNA clusters

Question 18

Defence against TE - key issues

Answer 18

1. diversity of TE with little similarity at primary sequence level
2. TE evolve rapidly
3. TE can jump species barrier

Question 19

How to generate piRNAs to new tranposons

Answer 19

see onenote

piRNA clusters are programmable silencing loci

acquire new content through innate mobility of transposons
- when TE lands in piRNA clusters - immunity is established

piRNA not only slice RNA from transposon, also able to silence it by modifying the histone, which is heritable => next generation is immune to that transposon.