Can endosymbionts drive speciation in their hosts?
In the Gerardo lab I use squash bugs (Anasa spp.) and their Burkholdaria endosymbionts test how mutualist bacteria can influence the diversification of their hosts. Squash bugs are widespread agricultural pests of cucurbits (eg. zucchini, pumpkin, and cucumbers) and contain host-specific strains of Burkholderia. These specialized microbes provide essential physiological benefits for the squash bugs. Importantly, Burkholderia can also have direct effects on squash bug traits that are critical for mating, like body size.
Isolated strain of Burkholdaria from squash bug crypt. Microbial symbionts provide both survival and developmental benefits for their hosts. Strains of symbionts within and between species can be cultured in the lab and experimentally transferred to squash bugs. These symbiont “swaps” reveal how Burkholderia can drive host mating behavior.
Squash bugs can be easily reared and observed in the lab. They can be kept in large colonies and their behavior can be filmed and quantified.
Fluorescent tags used to confirm symbiont colonization of squash bug crypts. This method allows for rapid identification of symbiont acquisition. Image by Jason Chen
Tagged symbionts can be easily be seen in live squash bugs to track transmission dynamics in real time. Our novel method also allows us to identify how symbiont acquisition influences mating and feeding dynamics within and between pest species. Image by Jason Chen.
Body size in squash bugs in important for reproductive compatibility. Data suggest that female egg production is directly influenced by the size of the male she mates with. Body size is also a potential prezygotic barrier preventing hybridization.
The key the squash bug-Burkholderia system is that the endosymbionts are horizontally transmitted as new hosts pick them up from the environment. This is different from many insects, including aphids, that have maternally inherited beneficial symbionts. Thus, I can experimentally manipulate the symbiont community through the natural route of infection to monitor their effects on squash bug phenotype and reproductive compatibility.
So far, experimental manipulations have resulted in increased hybridization! These preliminary results suggest the right combination of endosymbionts can break down species barriers. More to come soon!
Right: Mating between Anasa trusts female (top) and Anasa andresii female (bottom). Left: Mating between F1 hybrids.
Experimental hybridization between squash bug species
My ultimate goal is to develop this system to show how environmentally acquired symbionts can dictate host community structure and diversity.