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Biology – Sleeping Chironomid (Polypedilum vanderplanki)

Biology –  Sleeping Chironomid (Polypedilum vanderplanki)

| On 23, Sep 2018

Darrell Mann

Polypedilum vanderplanki or the sleeping chironomid, is a dipteran in the family Chironomidae (non-biting midges). It occurs in the semi-arid regions of the African continent (e.g. northern Nigeria and Uganda). Its larvae are found in small tubular nests in the mud at the bottom of temporary pools that frequently dry out during the lifetime of P. vanderplanki larvae. Under these conditions, the larvae’s body desiccates to as low as 3% water content by weight. In the dehydrated state the larvae become impervious to many extreme environmental conditions and can survive temperatures from 3 K to up to 375 K, very high (7000 gray) levels of gamma-rays, and exposure to vacuum. It is one of few metazoans that can withstand near complete desiccation (anhydrobiosis) in order to survive adverse environmental conditions. This species is considered the most cold-tolerant insect species, able to survive liquid helium (−270 °C) exposure for up to 5 min. with a 100% survival rate when desiccated to 8% water content.

The larva of the sleeping chironomid, has long been known for its post-desiccation recovery skills. However, the genetic mechanisms the insects use to achieve this feat, and, especially is the identity of the master gene that induces desiccation tolerance have remained largely elusive. Now, researchers from an international collaboration including Oleg Gusev of the RIKEN Innovation Center and collaborators from NARO, Kazan Federal University (Russia) and Skoltech University (Russia) have discovered that a gene called heat shock factor – which is present in some form in nearly all living organisms on earth – has been coopted by the species to survive desiccation.

Heat shock factor – which exists in a single form in invertebrates but multiple forms in vertebrates (e.g. Late Embryo Abundant (LEA), anti-oxidant) – is an essential part of the ability of living cells to survive stressful conditions such as heat, cold, radiation, and, it  turns out, desiccation. In desert insects, the researchers found, the gene is able in certain conditions to upregulate itself, and this upregulation leads to a number of downstream processes, including the synthesis of heat shock proteins that are able to protect proteins in the cell from misfolding.

To perform the research, published in the Proceedings of the National Academy of Sciences, the researchers compared data on RNA expression in the sleeping chironomid with a closely related species, Polypedilum nubifer, which is not capable of surviving desiccation. They found that in the sleeping chironomid, hundreds of genes, including genes known to be involved in forming a “molecular shield” against damage due to dehydration, were already expressed during the early stages of desiccation. They discovered that a certain DNA motif, TCTAGAA, which is the binding site for HSF, was strongly enriched around the transcription start site of the genes activated by desiccation in the sleeping chironomid, but not the other species. Intriguingly, they found that in the desiccation-tolerant species, but not the other, genes responsible for the synthesis of trehalose – a sugar that can stabilize cells in a dry state – contained the TCTAGAA motif.

To shed further light on the role of trehalose, they treated a cultured cell line from the sleeping chironomid with the sugar, and found that many of the genes activated by desiccation were also activated, and further, that the trehalose treatment led to the activation of the HSF gene. This effect of trehalose was prevented by knocking down the HSF gene, showing the HSF was clearly involved in the response.

According to Oleg Gusev, who led the group, “The discovery that heat shock factor is an important regulator of gene expression in response to desiccation was very interesting for us. It seems that these extremophilic insects in the process of evolution have coopted a very conservative transcription factor and its action for their own needs to survive without water by evolving a special gene structure and “adjusting” their genome sequence for these “needs.” Our data suggests the following story: HSF is activated during dehydration, and then HSF actually self-activates by binding to the upstream region of its own gene. This leads to the activation of the downstream genes that allow the insects to survive desiccation. What was very surprising to us was the finding that trehalose itself can activate HSF.”

Looking to the future, he continues, “One potential application of this finding will be in preserving cells outside the body in a dry state, if we can active the HSF gene. We now have a good understanding of how it works in this insect, so we will want to investigate if this is true for other organisms as well.”

From a TRIZ perspective, the sleeping chironomid’s desiccation-recovery trick can best be modelled as a conflict between the desire to preserve life and the loss of the substance water. Here’s how we can map that problem onto the Contradiction Matrix:

Not sure what your opinion is, but by my reckoning the ‘Heat Shock Factor’ and chironomid story carries elements of all five of the most frequently used Inventive Principles used by others. Call it a ‘full-house’. That’s a first, I think.

Reference:

  1. Pavel V. Mazin, et al., ‘Cooption of heat shock regulatory system for anhydrobiosis in the sleeping chironomidPolypedilum vanderplanki’, Proceedings of the National Academy of Sciences, 2018; 115 (10): E2477 DOI: 10.1073/pnas.1719493115

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