Some families have heirlooms that get passed down from generation to generation but if you’re a nematode, you might inherit more than just your Grandpa’s pocket watch!
The phenomenon of transgenerational epigenetic inheritance (TEI) was first discovered in Caenorhabditis elegans, where environmental stressors can alter physiological responses in offspring up to 20 generations later! Since then, researchers have asked whether TEI applies to behaviors, but finding a mechanism that could explain how neural processes are gifted to grandchildren has been difficult.
Two recent papers have begun to unwrap how C. elegans epigenetically bestows behaviors onto their offspring through two separate small RNA pathways.
Neuronal siRNAs Transmit Behavior Through the Germline
Traditional TEI in nematode worms involves small interfering RNAs (siRNAs) in the germline, that bind to Argonaute proteins and guide chromatin modifiers to complementary regions of the genome to alter gene expression. The lab of Oded Rechavi (Tel Aviv University, Israel) asked whether this model could be applied to transgenerational inheritance of chemotaxis, or a worm’s natural movements towards or away from attractive or repellent chemical stimuli. The talented team of researchers created genetic knockouts of RDE-4, a protein that’s needed to generate endogenous siRNAs, and then used cell-specific promoters to re-express the gene in neurons alone. Using small RNAseq, they found that:
- 476 small RNA regions targeting a specific gene (STGs) are expressed in neurons when RDE-4 was rescued, and 744 STGs are downregulated, including 46 that target genes related to neuronal processes
- Rescuing RDE-4 in neurons alters the expression of 1287 STGs in the germline (GermSTGs) of the same animal
- 189 GermSTGs are found in F2 progeny, even though they lack RDE-4 expression in their neurons
The researchers found that RDE-4 knockout worms had difficulty with chemotaxis when they were heated to 25°C. When they placed worms in the center of a plate with a chemical attractant, they found that:
- Selective expression of RDE-4 in neurons can partially rescue their defective chemotaxis
- This behavioral benefit can be found two generations later, in worms without neuronal RDE-4 but whose grandparents had a functioning RDE-4 allele.
These exciting findings suggest that RDE-4 dependent siRNAs are necessary for normal worm behavior, and that neuronal siRNAs can alter which information gets written in the epigenetic will in the germline.
A separate study approached the question of behavioral TEI from a different angle and asked whether C. elegans ancestors could pass learned behaviors onto their progeny as well.
Worms Inherit Avoidance Behavior From Their Ancestors Through piRNA
Even though they are initially attracted to it, C. elegans learn to avoid a strain of bacteria (Pseudomonas aeruginosa; PA14) that is toxic to them. The lab of Coleen Murphy (Princeton University, USA) found that this specific learned behavior can be transmitted through 4 generations after a single PA14 exposure and that this phenomenon depends on physical contact and infection with the bacteria. Using RNA-seq, they found that:
- Exposure to PA14 leads to changes in immune, metabolism, and epigenetic-related gene expression in the F1 progeny
- 60-70% of up-regulated genes are expressed in neurons
- A large group of PIWI-associated RNA (piRNA) are downregulated in the exposed animal
They also found that DAF-7, a signalling molecule involved in transmitting information between sensory and motor neurons, is induced after PA14 exposure and although DAF-7 knockout worms learn to avoid PA14, they don’t transmit this behavior to their offspring. The talented team used a library of genetic knockouts to explore the molecular pathway involved, and found that:
- PRG-1, the elegans ortholog of Piwi Argonaute, is necessary for worms to inherit learned avoidance behavior from their ancestors
- Prg-1 mutants do not show the expected increase in DAF-7 in their sensory neurons
- mut-7 and rrf-1, which are genes downstream of prg-1 that act as double-stranded RNA polymerases, are also necessary for TEI of avoidance behaviors
- Worms without the H3K9 methyltransferases set-25 and hlp-2 won’t pass along PA14 avoidance to their progeny
When it comes to avoiding PA14 poisoning, multiple steps along the piRNA pathway are necessary to pass on avoidance behavior to great-great-great grandworms!
Although some studies suggest that the molecular effects of diet or stress can be passed down through the paternal germline in mice, it’s too early to tell whether we can blame any behaviors on our forefathers as mammals. Fortunately, the details that these studies provide into the two small RNA pathways of behavioral TEI in worms could give researchers something to look for.