Nothing against honey bees, but is there anyone out there who doesn’t like bumble bees? They’re cute, fuzzy, often featured in children’s storybooks, and they also happen to be the most important pollinator of Solanaceous plants such as tomatoes and peppers. Yes, all you salsa and hot sauce aficionados out there are bumble bee fans, whether you know it or not!
But bumble bees are also declining at an alarming rate. In fact, long-term data on bumble bee populations provide some of the best evidence in existence that pollinator declines are real. Much better evidence than for honey bees, actually. For example, 13 of the 18 species of bumble bees here in New York are known to be experiencing range contractions. These numbers are representative of similar trends throughout much of the world.
Why are bumble bees (and other pollinators) declining? There are many opinions on this topic, and I’m sure most readers of this article have their own. But what if we could ask the bees? What would they have to say? This is the topic for our eleventh “Notes from the Lab,” where we highlight “Conservation genomics of the declining North American bumblebee Bombus terricola reveals inbreeding and selection on immune genes,” written by Clement Kent and colleagues and published in the journal Frontiers in Genetics [9:316 (2018)].
For their study, Kent and colleagues first searched throughout Ontario and Quebec for the yellow-banded bumble bee, Bombus terricola, at over 30 sampling locations. They collected several gynes, or potential queens, and brought them back to the lab. At the lab, the researchers extracted DNA from the bees, then sequenced the genomes using new sequencing technology.
As with any sequencing study, this is when the real work began. The authors spent lots of time putting the DNA puzzle together, specifically looking for a genomic signature of population size, inbreeding (which often occurs when populations are small and close relatives start mating with each other out of necessity), and any genes that had undergone recent and rapid selection (in other words, genes that may have been important for dealing with a recent stress).
So, are yellow-banded bumble bee populations still declining? Unfortunately, yes. Kent and colleagues found genomic evidence supporting a large (nearly 100-fold) decline in yellow-banded bumble bee populations over the past ~100 years. Furthermore, most of the bees analyzed showed signs of inbreeding.
Inbreeding is an especially bad problem for bees due to their haplodiploid genetic system (i.e., male drones typically have one set of chromosomes, while female workers and queens have two sets of chromosomes). When inbreeding occurs, some males are produced with two sets of chromosomes, which makes them sterile. This is due to homozygosity at the sex locus and single-locus complementary sex determination in bees (for more details, see Rusty Burlew’s excellent and easy-to-follow explanation of how this works on p. 916 of the August issue of ABJ!). This outcome of inbreeding has been termed the “diploid male vortex” since it increases the extinction risk of a species whose populations start to dwindle. In fact, it may be an important mechanism for why many Hymenopteran pollinators are currently declining – once their populations decline to a threshold, inbreeding ….