Thursday, August 7, 2008

PS 62-108: Dissecting the association between a gall midge and its symbiotic fungus

Jeremy J. Heath and John O. Stireman III. Wright State University


Asteromyia carbonifera (Diptera:Cecidomyiidae) is closely associated with the fungus Botryosphaeria sp.  Together the larva and fungus form blister galls on goldenrod (Solidago) leaves.  These blister galls can be easily separated by eye into morphological groups (“morphs”).  One may find all these morphs on the same ramet.  Recent work in our laboratory found some level of genetic structure among these morphs, which strongly suggests an incipient adaptive radiation event.  One goal is to understand the evolutionary forces that have genetically structured and maintained these morphs.  Given the degree of sympatry, it is unlikely that host-plant associations have played a major role.  There remain two other obvious selective forces to consider: parasitoid pressure and this intimate midge-fungus association.

Our main question was to understand the nature of this association. Does it represent a mutualism?  If so, is it obligatory or facultative?  What benefits do each partner provide?  Larvae within the galls of cut stems were processed as negative controls, mock removals (positive control), or removals.  Mocks were only dissected to the removal point.  Removals were treated like the mocks, but the larva was removed.  Galls (n = 17/treatment) were incubated on cut stems for 10 days and their areas subsequently measured.


Results indicate that the midge-fungus relationship is obligate with respect to the fungus and possibly for the midge.  Control- and mock-treatment fungi grew normally (0.63 and 0.59 mm2/day/larva, respectively), while the removals grew very little and ultimately died (0.002 mm2/day/larva).  Preliminary experiments testing the ability of larvae to grow and form galls on cultured fungus completely isolated from plant tissue strongly suggests that the larvae subsist solely on fungal tissue.  Although, more comprehensive experiments are underway to test this hypothesis further, we suspect that this association is reciprocally obligate.