Laboulbeniales: The Little Fungi that Could
While the order Laboulbeniales is a species-rich group of fungi brimming with diversity and fascination, they are often overlooked due to their microscopic size and difficulty in manipulating specimens. Scientists so far have had hard luck in growing and extracting DNA out of Laboulbeniales. Even though difficult to work with, it would be a shame to allow such interesting organisms to go unnoticed in mycology.
Laboulbeniales, often dubbed “labouls” by those most familiar with them, are microscopic ectoparasitic fungi that latch on to the exoskeleton of a diverse array of insects (1). Their hosts include ants, flies and cockroaches among other insects, but, as the famous British geneticist J.B.S. Haldane put it, these fungi seem to have “an inordinate fondness for beetles.” To better understand these microscopic organisms, biologists often turn to screening insects for a living. Thus, one would need to regularly screen insects in order to see these microscopic critters.
Unlike the well known moldy or mushroom-forming fungi, labouls exhibit determinate growth (2). This means that the fungal body, or thallus, develops from a two-celled ascospore by a restricted number of mitotic divisions, resulting in an individual with a set number of cells. However, determinate growth poses problems for researchers who want to study these organisms. The fungi’s minuscule stature makes detection and manipulation of the thallus almost impossible without a microscope, fine needles and steady hands. For me, screening beetles feels more like opening a Where’s Waldo* book. Because of the difficulty in handling the thallus, few biologists are interested, let alone brave enough, to confront these menacing fungi. My graduate supervisor who introduced me to the labouls is incredibly patient with his research, but given the size of the organisms he works with, his persistence is needed.
Considering this lack of attention, it is not surprising that only few have attempted to explore the world of the labouls, let alone tried to culture these fungal gems. Only one man, Howard C. Whisler, successfully grew thalli in the lab without the aid of other living organisms. Even Whisler, however, was unable to attain fully mature thallus (3). Forty-five years later, I find myself picking up this unsolvable puzzle.
Just recently, my supervisor presented me with a to-be-described species of Laboulbenia (Figure 1) that grows on a Curculio beetle (order Coleoptera, family Curculionidae). While describing a species has always been a dream of mine, I wanted to do something more with Laboulbeniales. Specifically, I wanted to grow them in order to learn about their mysterious physiology.
Figure 1: Scan of Roland Thaxter’s [1858-1932] original sketch of Laboulbenia sp. nov. [from the archives of the Farlow Reference Library of Cryptogamic Botany]
Growing Laboulbeniales in culture requires not only specific media but also a substrate on which the thallus can grow. Laboulbeniales are obligate parasites, meaning they require a host in order to survive and reproduce. To make things more complicated, nobody knows exactly which specific nutrients labouls take from their hosts. Mycologists often grow fungi on agar plates, petri dishes with jelly-like media containing agar and nutrients needed to grow. If the medium does not contain the right combination and balance of nutrients, then there is a smaller chance the fungi can grow and reproduce. While this challenge already sounds ominous, I ran into another problem with Laboulbeniales: there were not enough thalli for a workable culturing experiment. I have seen from past experience that screening insect populations often yielded a measly 2% infection rate. That would mean that several hours spent collecting and screening may only give me a couple of thalli to work with. Fortunately we discovered that cockroaches on campus have an ridiculously high infection rate with labouls. As a result, I have been collecting cockroaches around student dormitories in search of these ever-elusive fungi, and attempting to raise them in lab. Collecting cockroaches is duel-purposed as it allows me to create cockroach-infused media and offers a steady supply of fresh thalli of Laboulbeniales. Note: the infection frequency of the American cockroach with labouls in the Cambridge area is 95.5% (as of 10/10/2014).
One possible key in working this culturing experiment is to learn more about their food supply. It is my hope that by learning more about the circulatory system of the insects on which labouls feed, we will discover which ingredients are needed to culture these fungi. If all else fails, boiling the remains of dead insects could also be a viable way to approach the issue of feeding these fungi. It has been several decades since Whisler published his article on culturing Laboulbeniales, but it is my hope that by trying to grow these little wonders, we can understand and appreciate fungi a little more—one thallus at a time.
By Tristan W. Wang
Harvard College, Cambridge, Massachusetts
*Where’s Waldo is a children’s picture book in which the reader is challenged to find a character Wally hidden in a crowd of people.
1. Vega, F.E., and M. Blackwell. 2005. Insect-fungal associations: ecology and evolution. Oxford University Press, New York, USA. 333 pp.
2. Haelewaters, D., van Wielink, P., van Zuijlen, J.W., Verbeken, A., and A. De Kesel. New records of Laboulbeniales (Fungi, Ascomycota) for The Netherlands. Entomologische Berichten 72 (3): 175-183.
3. Whisler, H.C. 1968. Experimental studies with a new species of Stigmatomyces (Laboulbeniales). Mycologia 60 (1): 65-75.