Pilobolus crystallinus is a member of the phylum Zygomycota. Zygomycota are grouped together because of their sexual reproductive structures. Zygomyctoa form sexual spores called zygospores by fusing two hyphae of different mating types. Zygospores are diploid (2n) and have a hard, dark coating. Although Pilobolus crystallinus is a member of this phylum its prominent form of reproduction is asexual. Zygomycota also have an asexual reproductive state that involves the formation of haploid spores. There will be more detail on this from of reproduction later on this page.
Reproduction is the area where P. crystallinus has developed the most complex strategies in the crusade with Dung the Great. The rest of this page has been devoted entirely to uncovering the battle tactics that P. crystallinus has evolved in order to win the upper hand in battling Dung the Great...
First of all recall from the nutrition page that P. crystallinus and its army need a dead organic material source for nitrogen and other essential elements and in the case of P. crystallinus that source is dung. This dung can come from any number of herbivores but cows are the most commonly studied provider (along with rabbits and elk). P. crystallinus infects a new dung pile and completely destroys it. The problem is that when the dung pile is destroyed the P. crystallinus cannot survive without a new food source and fungi cannot move to a new food source. Fungi can grow to a new food source but cows do not eat near their own dung (who does?) and fungi cannot grow very far, very fast. This is where the evolutionary adaptations come in. When a food source starts to run out P. crystallinus begins to produce a fruiting body meant to disperse its spores to another food source. Beneath the surface of the rapidly diminishing dung pile two hyphae of different mating types fuse together in a process called plasmogamy and form a zygospore (n+n) (like the one produced by Rhizopus pictured above). Immediately following the formation of the zygospore the fungi undergoes a process called karyogamy where the nuclei of the cells that make up the zygospore fuse to from a diploid spore. The diploid zygospore then goes through meiosis and a sporangium grows out from the zygospore. Inside the sporangium the haploid cells from meiosis become haploid spores. The fungi is now ready for spore dispersal. This is where P. crystallinus is really inventive! P. crystallinus grows a stalk. On top of the stalk is a vesicle. At the base of the vesicle are caretenoid pigments. On top of the vesicle is the sporangium. The vesicle swells with water and creates pressure. When enough pressure is built up the sporangium is launched into the air! Hydraulics in nature baby! But P. crystallinus has realized that precious p sporangium cannot just be shot at random in hopes of landing in dung. And perhaps an even bigger problem is that they cannot be shot straight up or they will simply land directly back onto the same dung pile. That's where the caretenoid pigments come in and the sun proves to be a helpful ally. The vesicle of P. crystallinus is relatively transparent and the water filled vesicle acts like a lens focusing the sporangium at the sunrise. The caretenoid pigments absorb the light at the base of the vesicle causing the fungi to lean towards the sunrise, thus angling the shot. Enough water builds up in the vesicle and the sporangium is shot at the perfect angle into the sunrise. When the sporangium lands a sticky film adheres it to a blade of grass and the cow eats the sporangium for lunch. The spores pass through the cow and land directly in a brand new food source!
Those are the basics now for a look at some of the details. According to Steven Vogel's research a Pilobolus sporangium accelerates at a speed of 100,000 m/s^2 which approaches that of a rifle bullet! The distance traveled by the sporangium is extremely difficult to calculate. A sporangium that is 0.3mm in diameter and with the same density as water has an optimal launching angle of 17 degrees and at this angle will travel about 0.82m. Were you expecting it to go further? Well this is a calculated distance with out taking into account the 98% drag tax. In fact the sporangium will travel two or three times that far because of the cell sap that travels with it. When the sporangium is launched off the vesicle a tiny bit of cell sap travels with it. This sap adds mass with out adding a whole lot of diameter and may even produce a faintly stream lined tail. Along with having a high acceleration rate and traveling an extremely long distance sporangium also have the ability to aim. It is known that sporangium aim at a light source so a study was conducted to see how accurate this fungi could aim. P. crystallinus was aloud to grow in a clear glass jar. Tin foil was placed over the jar and a small whole the size of a dime was cut out of the tin foil. When the tin foil was removed the majority of the spores were stuck right in the spot where the light showed through. The results were proof that the sporangium could hit a target the size of a dime!
©2007 by Ashley Seidler
This page was developed for Biology 203 (Organismal Biology) at the University of Wisconsin La Crosse
Last updated April 27, 2007
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