A worm species destroys their own internal organs to make “yolk milk” which is passed out of their vulva for their offspring to consume. Isn’t nature wonderful?
The nematode worms, Caenorhabditis elegans, have two sexes: hermaphrodite and male. Hermaphrodite C. elegans can reproduce with males or self-fertilize their eggs with their own limited stock of sperm, a reproductive strategy called androdioecy.
After reproduction ceases, worm mothers undergo rapid senescence (deterioration with age). Oddly, this senescence is promoted by the same signaling pathway that promotes yolk production, with yolk-rich fluid pooling around the bodies of the waning worms. This process seems futile, and was assumed to be a state of disease associated with old age. However, a new study in the journal Nature Communications uncovered the very selfless reason behind it.
“It is both a form of primitive lactation, which only a few other invertebrates have been shown to do, and a form of reproductive suicide, as worm mothers sacrifice themselves to support the next generation,” explained lead study author Professor David Gems of the University College London Institute of Healthy Aging in a statement sent to IFLScience.
“The existence of worm milk reveals a new way that C. elegans maximize their evolutionary fitness: when they can’t reproduce anymore because they have run out of sperm, they melt down their own tissues in order to transfer resources to their offspring,” added first author Dr Carina Kern.
In the paper, the authors describe this process as a “destructive repurposing of internal biomass” which contributes to the mother’s senescence and atrophy. The resulting yolk is vented out of the vulva along with masses of unfertilized oocytes (egg cells), starting immediately after egg-laying stops.
This ordeal, though grim, has huge benefits for the wormy babies, with the paper noting that “As a means of transferring resources from mother to offspring after egg-laying, vented yolk serves a function similar to that of mammalian milk”.
The researchers observed yolk in the intestines of worm larvae and removing yolk-producing mothers reduced the larvae’s growth. As for the ejected oocytes, Dr Kern explains that “all those unfertilised eggs are full of milk, so they are acting like milk bottles to help with milk transport to feed baby worms.”
They may be small at only 1 millimeter long, but C. elegans have a huge role in biology research as a model organism. The worms were the first multicellular organism to have their genome sequenced, and the developmental fate of every cell in their body has been mapped. C. elegans was also the first animal to have its entire connectome mapped, meaning the connections between all 302 of their neurons (compared to the 86 billion in humans) are known.
One field that widely uses C. elegans is aging research. Surprisingly, these yolky findings are very relevant here – senescence, yolk production, and yolk venting are all promoted by the same signaling pathway, which has been shown to increase worm lifespan when turned off.
“In the end, what is critically important is to understand the principles that govern the process of C. elegans ageing and explain the causes of age-related disease more generally,” explained Professor Gems.
“We don’t yet understand this for any organism. But for C. elegans we are getting there, and the discovery of worm milk gets us another step closer.”