Younger cycad seeds can contain more toxins than older seeds, and their widespread consumption by people on occupied Guam during World War II could be the environmental origin of the rare sporadic amyotrophic lateral sclerosis (ALS) cluster found there, according to a recent study.
Guam residents emerged from their wartime occupation, 1941–44, as the epicenter of an ALS-like neurodegenerative disease (ALS-parkinsonism dementia complex) that was 50 to 100 times more prevalent than ALS in North America. To this day, it is one of only two known ALS clusters.
The study, “Fresh and Dry Weight Relations Are Predictors of Cycas micronesica Seed Age,” was published in the journal Horticulturae.
Studies suggested an environmental cause for this postwar prevalence, and cycads have since emerged as a possible source of an environmental toxin.
During the occupation, residents turned to non-farm sources of food, such as seeds of the native cycad tree, known scientifically as Cycas micronesica and called “fadang” in the local Chamorro language.
“The increased reliance on cycad seeds as a source of dietary starch during the years of the occupation has been one of the proposals for what appears to be an ephemeral increase in exposure to an environmental factor,” Christopher Shaw, a professor of ophthalmology at the University of British Columbia and a study author, said in a press release.
Cycad seeds are typically harvested to be eaten once their shell turns a dark brown color.
Shaw and a colleague discovered that this color change is a less reliable way of estimating a seed’s age than previously thought. This matters because they have also shown that neurotoxic compounds found in the seeds break down and decline with age, making younger seeds potentially much more toxic than older ones.
Cycad seeds begin to turn dark brown as early as 17 months of age, but can remain on a tree for more than 30 months, meaning that similarly dark brown seeds can differ in age by over a year.
Of 30 seed traits they evaluated, only two measures — based on fresh and dry weight ratios — demonstrated an ability to accurately estimate the age of cycad seeds.
These were the ratio of the fresh weight of the gametophyte — the plant in its sexual phase of life — to that of its seeds, and the ratio of the dry weight of the fleshy outer layer of the seed coat — called the sarcotesta — to that of the middle “stony” layer, known as the sclerotesta.
The investigators suggest that past research has overlooked this source of variation by not reporting seed age.
Although many toxins have been found in cycad tissues used for human consumption, none have emerged as the clear culprit of neurodegeneration.
“Results from this research inform the ongoing attempts to identify environmental toxicants that pose a risk through chronic low-level exposures in many regions of the globe rather than acute high-level exposures in one location,” said Adrian Ares, associate director of the Western Pacific Tropical Research Center at the University of Guam.
As such, this work might broaden our understanding of similar neurological diseases found elsewhere.
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