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Colleen Butler - Plant Ecology Lab

Boston University, MA

Fungal specificity and seasonal changes in fungal abundance in

the showy orchid Galearis spectabilis

Classical mycorrhizal symbiosis is typically regarded as a mutualism between a green plant and a fungus.  It was originally assumed that mycorrhizal plants could associate with a wide variety of fungi.  However, recent molecular data suggest that some mycorrhizal plants, especially orchids, are quite specialized in the fungi with which they can associate.   Unlike classical mycorrhizas, orchid mycorrhizas more closely resemble a parasitism because the orchid digests the fungus in its roots.  Thus far, there has been no evidence that the orchid provides the fungus with any type of benefit.  One hypothesis is that the fungus benefits by digesting plant tissue when the orchid begins to senesce, but this has not yet been demonstrated.  If this is true, we should expect to see a peak in fungal abundance right before the orchid senesces. 
My project focused on mycorrhizal symbiosis between the showy orchid, Galearis spectabilis, and Ceratobasidium fungi.  I sought to answer two questions: Does Galearis associate with a wide variety of mycorrhizal fungi or does it specialize?  Does Galearis's mycorrhizal fungus change in abundance over time?  During the summer, I sampled four orchids every two weeks from each of four sites on SERC property.  DNA sequencing data suggested a high degree of fungal specialization.  All Galearis plants tested were associated with a closely-related group of Ceratobasidium spp. fungi.  Sequence variation was similar within and among sites.  Root sectioning data from the 2004 summer were compared with similar data from 2002 (Rasmussen, 2002).  The 2004 data showed a peak in live and dead infection on July 19, followed by a decrease on August 2.  Root sectioning measurements will be continued until the plants senesce to see if there is a peak just before the plant senesces.  Dead infection and live infection were positively correlated, as were leaf area and total root length.              
Further root sectioning data should further elucidate the seasonal cycle of fungal abundance.  Further DNA analysis of fungi from Galearis and other orchids will help us to learn more about fungal specificity.  Finally, in order to fully understand orchid mycorrhizas, a methodology needs to be developed to determine if the fungus digests the senescing plant tissue. 
In May, I graduated from Boston University with a BA in Biology with specialization in Conservation Biology.  After my internship at SERC, I plan to return to Boston to work for a few years and then enter a PhD program in Biology.

Funding provided by a research grant from the National Science Foundation