Baby Bees Grow By Eating Their Fungus With Their Pollen

Among biologists who study social insects, the development of sophisticated fungus agriculture in leafcutter ants and some termites is considered one of the landmark events in social insect evolution. In the most advanced species, the insects organize massive foraging trips for vegetation, converting it into a suitable medium on which fungal gardens proliferate. In exchange, the fungi compose the main diet of their gardeners, supporting vast nests of hundreds of thousands of individuals. We continue to make interesting discoveries on the ecology and evolution of these mutualisms, even though we’ve studied these systems to death in the past few decades. As it turns out, other animals as ecologically and phylogenetically diverse as bark beetles, sea snails, and damselfish have evolved ‘farming’ of some kind.

Being mainly subterranean insects that live in dark, humid nests, it would make sense that ant and termite lineages would have coevolved ancient and advanced partnerships with equally specialized fungi over millions of years. But what about other social insects? In The Insect Societies (1971), E.O. Wilson, noting that social bees and wasps seem to lack the accompanying communities of inquiline insects (highly specialized freeloaders) that inhabit ant and termite nests, proposed that this might be because bees and wasps tend to build more tightly sealed and inaccessible nests, sanitize nests more thoroughly of detritus, use more durable materials to build their nests, and construct them in arboreal environments rather than underground. I suspect that Wilson would have extended this hypothesis to microorganisms as well. Aside from my humble opinion that this explanation is excessively reductive and hand-wavy, Wilson’s observation is true. After all, the bees, despite their overall uniform ecology as nectar feeders and pollen hoarders, have spawned eusocial lineages just as advanced and varied as the ants and termites, so we ought to expect that some of these lineages have likewise evolved intimate associations with fungi in a convergent matter. Thus, my excitement at the news that we now have our first fungus-cultivating bee, as described by Menezes et al. in Current Biology.

Scaptotrigona depilis, an already known and rather well-studied tropical stingless bee, has a fairly typical social life compared to its relatives in the Meliponini. The colony’s home life is centered on the nest, a complex structure built out of cerumen, a mixture of wax secreted by young workers and resin brought in from outside. It is this cerumen that is used to build individual cells to store food and house bee larvae, which will develop into the next generation of adult workers. Cerumen is recycled throughout the hive to build and repair various other structures, and when a new queen leaves the mother colony to found a new colony elsewhere, a swarm of workers uses cerumen from the old hive to begin construction on the new nest. Atypically, however, the fungus, an unidentified species in the genus Monascus, also inhabits the cerumen, and as the bees reuse and transport cerumen, they spread the fungus throughout the nest and disperse it to new sites. In fact, the fungus seems to be so dependent on bees for propagating it that the authors found no evidence of the fungus producing conidia, the durable clonal spores generated by normal fungi. Instead, it grows exclusively as a webby mass of active growing cells called mycelium.

large Image
Progress of bee larva development from newly laid egg to mature larva. The mycelium only grows after the egg hatches, and completely disappears with the food store as the larva eats and grows.

Also like other social bees, after a new brood cell is completed, S. depilis workers fill it with a barfed-up soup of pollen and honey, enough to fuel the development of one bee larva to maturity. After the enormous queen deposits a single egg on the mess and shuffles away, the cell is completely sealed off; the baby bee hatches literally swimming in its own exclusive food mass. As Monascus grows on the food–in fact, the brood cells are the only place where the fungal mycelium actively grows–the larva actively consumes the fungus as well, so that nothing remains in the cell by the time the larva matures and begins spinning its cocoon. To test the importance of the fungus to bee larva survival, the researchers grew 300 S. depilis larvae in the lab on either UV-sterilized food or sterilized food that was then retreated with the fungus. While 76% of larvae fed food containing the fungus survived, only a piddling 8% of larvae fed sterilized food did the same.

large Image
In all of the colonies tested, bee larvae survived at much higher rates when raised on fungus-infested food than on sterilized food.

Clearly, the fungus was playing a key role in bee development. But what was it doing? During the lab experiments, they found that sterilized bee food quickly spoiled, but food containing Monascus stayed fresh. Incidentally, other species of Monascus are known to secrete antimicrobial agents, and the fungus has been long used by human cultures to preserve and ferment food. However, when Menezes et al. tested the effectiveness of bee food against E. coli and staph bacteria compared to the antibiotics penicillin and streptomycin, they found it was not effective all. (Newly made bee food was as effective as the antibiotics in inhibiting growth, but this was probably due to properties independent of the fungus, similar to the way that honey in your cupboard doesn’t spoil). Perhaps the fungus is only effective in excluding other microbes more specialized in growing on the bee food or plays another role entirely.

Overall, this paper’s main significance was in providing the first proof of a bee relying on a close association with a fungal ectosymbiont, and in this respect the authors did demonstrate that bee larvae require the fungus to develop. However, the results were preliminary, and I was dissatisfied that no experiments were performed to better confirm the mutualistic nature of this relationship. I would have liked to know how well the fungus grows outside of S. depilis hives and on media besides bee larva food, or if larval mortality without the fungus does result in decreased worker recruitment and lower colony fitness. I was also personally skeptical of the comparison of the bee-fungus mutualism with advanced agriculture in ants and termites, since the bees’ adaptations to inoculate the fungus, cultivate it, and utilize it in their diet were either not proven or not noticeably different from other stingless bees in general. I agree with the authors that the disappointing lack of obvious and elaborated anatomic and behavioral adaptations is more akin to other examples of proto-farming.

Sourdough miche & boule.jpg
To make an analogy, it seems S. depilis’s version of fungus cultivation is more similar to our species’s propagation of sourdough culturesfor bread, rather than wheat or corn agriculture. After all, you don’t see bakers weeding their dough crops very often.

Of course, these problems can be addressed in the many interesting and potentially fruitful future directions that can be taken with this research. I’d especially like to know if the unknown Monascus in question is a distinct species exclusively associated with S. depilis bees, or a unique form of an unknown that grows facultatively lives alongside other bee species or in other environments. I’m also confident that we will uncover more examples of fungal ‘agriculture’ in the large genus Scaptotrigona, among the diverse meliponine bees, and in other social bee lineages in general. The lesson is that when we don’t know about species natural history in ‘well-studied’ groups, we are prone to overgeneralizations and overlooking major discoveries. In this respect, this paper deserves to generate more buzz among scientists.

Advertisements

My thoughts on the GRE Biology subject test

I took my GRE Biology subject test just this morning. For those unfamiliar with the Biology subject test, it is given only a few times every year, once in the spring and twice in the fall, and contains 190 multiple choice questions to be answered in 2 hours and 50 minutes without breaks. The material is as broad as the field of biology itself, with some questions about very specific concepts and others more general, and is roughly equally divided among the subfields of cellular/molecular biology, physiology/organismal biology, and ecology/evolutionary biology.

Questions are multiple choice, and grouped by question type. The first section of the test mostly asks for straight information recall and rapid calculation/figure analysis, the second asks for matching entries in lists or diagrams with respective descriptions, and the third mainly tests critical analysis of experiments, tables, and figures. In calculating raw total and subfield scores, 1 point is given to each correct answer, 1/4 point is deducted for each incorrect answer, and 0 points are deducted for questions left blank. Raw scores are converted into scaled scores, which are assigned a percentile rank.

Here are my personal reflections on the test:

  • This summer, I’ve been focusing all my attention on the MCAT. It was only after that was over that I realized I still had time to register for the October subject test. Since I’m a little unsure about my future plans, I figured this was worth a shot, especially since the next test date wouldn’t be until April 2016. Unfortunately, because I hadn’t bothered to register until 5 weeks before the test, all the seats closest to me were all out; I had to sign up for a test location at a small college an hour’s drive away. It was somewhat unpleasant to have to wake up in the darkness of 6 am for a 3 hour test on a Saturday morning. Obvious advice: Don’t register for tests without planning ahead. 😦
  • I was surprised at the rather casual atmosphere just before the test. Part of it was the fact that a lot of the test takers were students at the university who knew each other, part of it was the very understanding, laid back, and experienced proctor we had, and part of it was the fact that I was stressed out in the minutes before. Even so, having taken the MCAT, I had expected a whole array of security and procedural ceremonies coming into the test, but we all just talked in the lobby and then sat down when it was time for business.
  • The material was more predictable than what I expected. ETS itself has said that the scope of the Biology subject test is so extensive that “no one is expected to be familiar with the content of every question.” Thus, I expected to be tested on very different material than the official practice test. Though there were some topics that I should have studied more intensively, the surprise was actually how well the material corresponded, not only with my old high school Campbell & Reece textbook, but also with the practice test. It was definitely worth going through the practice test again to compare my answers with the key and learn what I got right and what I did wrong.
  • Then again, I probably should have started studying a little earlier. Part of it was because I did quite well during my first pass through the practice test, so I got lazy and turned my attention to other work. I lingered learning plant anatomy and growth, and didn’t end up actually *studying* until maybe 2 days prior to the test. I was still scrambling to get the basics of plant transportation and development this morning before my trip. It also didn’t help that I slept less than 6 hours last night due to my anxiety.
  • Guessing on plant bio like
“I got this, the answer is A!” *aggressively bubbles in C*
  • My approach to taking this test was a little different than I think most people would expect. I started in the middle of the test in the matching section, to settle my brain into exam-taking mode. I went all the way through into the analysis section, where each question takes the most time (over a minute per question at least to parse out figures and numbers). After reaching the end of the test, I went back and only then started at the beginning of the test; I gave myself 15-30 seconds for each recall question. I’ve found that starting the test in medias res allows me to complete the analysis section, the most difficult and time-consuming questions, before I tire out or run short on time at the end of the test. The recall questions themselves are usually straightforward and stand alone, so I find it vastly preferable to be going through these rapid-fire know-or-don’t-know problems during the last few minutes of a test, rather than be stuck on analysis problems I could have gotten right if I only had more time.
  • I’m still unsure whether taking this subject test was worth the time and money. At least a few top graduate programs in EEB ‘highly recommend’ submitting Biology subject test scores, which I take as basically being the same as ‘required’. But it seems many more programs don’t care about the subject test scores in admissions (and for good reason, in my opinion), and after surfing various boards and talking to graduate students, I believe that most applicants don’t bother with the subject test either. Of course, scoring very high will be impressive regardless, and I think I need to have performed well all the more to at least partly compensate for the deficiencies in my potential graduate applications.
  • Overall, I am quite satisfied with my performance. I felt about the same taking the real thing as I did the practice test, so I don’t believe I did too shabby. After the test, I had a snack, looked over what material I might have missed, then had a nice drive back home. In about a month I’ll find out how I did!

Helpful resources:

GRE Biology practice test

GRE subject test percentiles

GRE subject test subscore percentiles