How to Es­cape a Deadly Em­brace

This is the last of three in­stall­ments cel­e­brat­ing the Week of the Fungi on STC.

by Elio

The space around a sin­gle soil par­ti­cle can be one hell of a bat­tle­field. Here, ne­ma­tode worms con­sume bac­te­ria, bac­te­ria fight back and kill them, fungi suck the worms dry, and so on. Not a mo­ment of peace. These bat­tles are as in­tense as they are in­tri­cate, yet we know lit­tle about all this drama. Hon­est, if it weren't for C. el­e­gans, ne­ma­todes would hardly be in my con­scious­ness de­spite their enor­mous abun­dance and eco­log­i­cal im­por­tance in soils and wa­ters. They are found nearly every­where. They are ma­jor play­ers in the Mc­Murdo Dry Val­leys of Antarc­tica, and have re­cently been found one kilo­me­ter down in a gold mine, where they feed on bac­te­r­ial biofilms grow­ing on the walls. Ne­ma­todes have their bard: a de­li­cious 1914 ar­ti­cle by N.A. Cobb gra­ciously ex­tols their glory. But this be­ing Fun­gus Week at STC, I choose to re­late a ne­ma­tode story of life in the soil that in­volves fungi.

A net­work of fun­gal hy­phae mod­i­fied for ne­ma­tode- trap­ping. Source

In the ground, fungi and wa­ter molds are adept at feed­ing off ne­ma­todes. Like all car­ni­vores, these preda­tors must first catch their prey. They use a va­ri­ety of strate­gies for do­ing this. Fungi make gooey struc­tures, some lol­lipop-like, to which the worms stick ir­re­versibly, while cer­tain wa­ter molds shoot a pro­jec­tile con­sist­ing of a spore into pass­ing ne­ma­todes. All these mech­a­nisms are ex­ceed­ingly ef­fec­tive, but one stands out for its vi­sual hor­ror. Here, the fun­gus makes a lasso out of three hy­phal cells. When a ne­ma­tode sticks its body into this noose, the fun­gal cells in­flate within a sec­ond or two, trap­ping the worm in an in­escapable em­brace. In every case, what­ever the trap­ping mech­a­nism used, the fun­gus makes spe­cial­ized cells called haus­to­ria, which pen­e­trate into the worm and de­velop within it, even­tu­ally de­vour­ing its in­nards. Fun­gal cells then emerge and the spores they make are re­leased to spread in the environment.But why do the worms stick their front end in the trap? There is ex­per­i­men­tal ev­i­dence that they re­spond to some chemo­tac­tic bait made by the fungi, al­though the com­pounds in­volved have not yet been fully char­ac­ter­ized. (For an ac­tion-packed video of this drama, click here.)

A worm trapped in a con­strict­ing ring of a nema­to­pha­gous fun­gus. Source

So, how come not all the ne­ma­todes are eaten up? This is a li­ons-and-ze­bras ques­tion, with one dis­tinc­tion: the fungi (the "li­ons") are not oblig­a­tory car­ni­vores and do not de­pend for their liveli­hood on eat­ing ne­ma­todes (the "ze­bras"). And the ne­ma­todes are not to­tally de­fense­less. They have de­vel­oped mech­a­nisms to en­sure that at least some will es­cape the fun­gal traps. It turns out that when you gen­tly touch the front of one of these worms, it will quickly re­verse di­rec­tion. The short time be­tween worm en­try into the fun­gal ring and its con­stric­tion al­lows some worms to re­tract and es­cape the fa­tal squeeze. In fact, some 80% of the worms es­cape un­harmed. A re­cent study shows that C. el­e­gans mu­tants that are de­fec­tive in the re­sponse to touch are more likely to be cap­tured by the fun­gus. This be­ing C. el­e­gans about whose neu­ro­bi­ol­ogy so much is known, the de­tails of the neural cir­cuit in­volved in this be­hav­ior has been elu­ci­dated. C. el­e­gans afi­ciona­dos will en­joy read­ing the ex­cit­ing de­tails of this study. These re­sults sug­gest that se­lec­tive pres­sures im­posed by preda­cious fungi have shaped the evo­lu­tion of the C. el­e­gans "es­cape be­hav­ior." In the au­thors' words: Be­cause we know the mol­e­c­u­lar and neural un­der­pin­nings of the C. el­e­gans es­cape re­sponse in ex­quis­ite de­tail, com­par­a­tive stud­ies of soil ne­ma­todes pro­vide a pow­er­ful model to study the mech­a­nisms that un­der­lie the evo­lu­tion of es­cape be­hav­iors.

Oys­ter mush­rooms, Pleu­ro­tus os­trea­tus, a car­niv­o­rous species. Source

Back to the fungi. "Ne­matophagy" among the fungi goes back a long ways, at least to the age of the di­nosaurs. Well pre­served fos­sils in 100 mil­lion year-old am­ber show the char­ac­ter­is­tic con­strict­ing rings and other fea­tures of today's car­niv­o­rous fungi. In the present world, ne­matophagous be­hav­ior is found in over 200 species, dis­trib­uted in the main fun­gal branches (Zy­gomy­cota, Ba­sid­iomy­cota, and As­comy­cota). In­cluded are such ed­i­ble (to hu­mans) kinds as the oys­ter mush­rooms and the inky caps. The next mush­room you eat may be a car­ni­vore!

This is not all. Ne­matophagy has made it into sci­ence fic­tion. In a post in Kathie Hodge's ter­rific Cor­nell Mush­room Blog, I found the fol­low­ing bit of es­sen­tial in­for­ma­tion: Sci­ence fic­tion fans will re­call the fa­mous Piers An­thony novel Om­ni­vore, which fea­tures a planet where all or­gan­isms in all eco­log­i­cal niches evolved from fungi. In one dra­matic scene, the he­roes are men­aced by gi­gan­tic ne­ma­todes and then res­cued by the con­strict­ing ring traps of a heroic Arthro­bot­rys. Fungi to the res­cue!