Fish's memory not only exceeds 7 seconds, some fish may also have prehistoric memory

Produced by: Science Popularization China

Produced by: Wang Guanlin

Producer: Computer Network Information Center, Chinese Academy of Sciences

In life, we often hear friends jokingly saying that they are like a fish with only a 7-second memory, and lamenting the fact that they have a bad memory.

However, is it true that fishes only have a memory of 7 seconds? The fish babies said: We are not responsible for this!

Image source: Photographed by the author

Fish actually have a very long memory

In addition to the saying in our country that fish have a memory of only 7 seconds, there is also a saying abroad that there is a '3 second memory fish'. So how long is the memory of fish?

As early as 1965, scientists from the United States conducted a study on the memory of goldfish. The researchers first emitted a bright light from one end of the fish tank, and then gave the goldfish an electric shock at this end.

After repeated training, the goldfish soon memorized the "light-electric shock" stimulation cycle. When the light was emitted again, the goldfish would quickly escape to the other side of the fish tank. The researchers found that goldfish can remember the "light-avoidance" behavior pattern for up to 1 month.

In addition to goldfish, researchers also found similar results in zebrafish, a model organism, through the "light-food" stimulation method. Not only that, but later scientists observed that farmed fish can retain memory of feeding stimulation for several years.

In fact, in life, if you carefully observe the ornamental fish in the ponds of scenic spots, once someone approaches, they will swim over quickly, blow bubbles, and try to get food. This is also a memory under the stimulation of food.

Fish not only have long memories, but can also be passed on to the next generation

A study published in Nature Communications has overturned people's understanding. Scientists from the University of Otago in New Zealand found through research on zebrafish that the epigenetic memory of zebrafish can be continuously inherited by offspring by preserving DNA methylation.

In short, the fish's memory can be passed on to the next generation, or even several generations.

Image source: Photographed by the author

Previous studies have shown that in addition to "genetic" factors that regulate our memory, epigenetics is also closely related to learning and memory.

Epigenetics refers to the heritable chemical modifications that occur without changing the genetic material DNA, which mainly include DNA methylation modification, histone modification, etc. If these modifications occur in gametes, they will be passed on to offspring.

The epigenetic modifications (including DNA methylation, histone modification, etc.) acquired by offspring often respond to events (stimuli) experienced by their parents (or ancestors several generations ago), which is also called epigenetic memory.

Specifically, offspring are able to remember and respond "transgenerationally" to events that happened to their parents but which they themselves have not experienced.

For example, in 2016, scientists from Tel Aviv University discovered that nematodes that experienced starvation would pass on the "memory" of starvation to their offspring through small RNA transmission. The researchers found that the offspring of nematodes, although they did not experience "starvation", were able to prepare for starvation in advance.

Most of this type of memory is epigenetic modification of environmental conditions, especially adverse environmental factors, by parents, which helps their offspring survive better.

How do zebrafish pass on memories to their offspring?

As one of the most important ways of epigenetic modification, DNA methylation refers to the covalent binding of a methyl group to a DNA nucleotide (such as the 5-carbon atom of cytosine) through a specific enzyme (DNA methyltransferase) without changing the DNA sequence, thereby changing the conformation of DNA and inhibiting the mutual binding of the major groove and protein in the DNA double helix structure.

Because of its important role in regulating gene expression, DNA methylation modification is also called the "footnote" of the DNA Bible.

In previous studies, scientists have found that in mammals (including humans), this type of DNA methylation modification is not stably inherited to the next generation, but instead a large-scale "methylation erasure phenomenon" occurs after fertilization.

Image source: Veer Gallery

Compared with the methylation levels of sperm and egg cells, the methylation level of DNA in embryos is greatly reduced, and large-scale DNA methylation erasure occurs across the entire genome. In the subsequent development of offspring, as environmental conditions change, DNA methylation levels rise again, and the process of "(parent) DNA methylation erasure - (offspring) DNA methylation reconstruction" occurs.

In a study on zebrafish, researchers used low-coverage whole-genome sulfite sequencing to analyze DNA methylation levels during the development of the zebrafish germline, and found that zebrafish do not experience genome-wide DNA methylation deletions like mammals during germline development. DNA methylation events in zebrafish can be inherited to the next generation, or even to subsequent generations.

That is, in fish (at least zebrafish), it seems that memories can be passed on to offspring by transferring DNA methylation, which may help zebrafish better adapt to changes in the environment.

So, don’t say anymore that fish’s memory is only 7 seconds. The fish you see may still have residual prehistoric memories!

References:

Oscar Ortega-Recalde, Robert C. Day, Neil J. Gemmell, Timothy A. Hore. Zebrafish preserve global germline DNA methylation while sex-linked rDNA is amplified and demethylated during feminisation. Nature Communications, 2019; 10 (1) DOI: 10.1038/s41467-019-10894-7

Guo, H., Zhu, P., Yan, L., Li, R., Hu, B., Lian, Y., ... & Jin, X. (2014). The DNA methylation landscape of human early embryos. Nature, 511(7511), 606.

Houri-Ze'evi, L., Korem, Y., Sheftel, H., Faigenbloom, L., Toker, IA, Dagan, Y., ... & Rechavi, O. (2016). A tunable mechanism determines the duration of the transgenerational small RNA inheritance in C. elegans. Cell, 165(1), 88-99.