|Abstract or Summary
- Epichloë typhina (Ascomycetes: Clavicipitaceae) is an endophytic fungus that
infects perennial Pooid grasses and is the causal agent of choke disease. It is endemic
to Europe and was inadvertently introduced into orchardgrass seed production fields in
western Oregon. Choke disease, which was first recorded in Oregon in 1996, currently
infects ~90% of cultivated orchardgrass seed fields in the region, resulting in yield
losses >65%. Infective propagules (i.e. ascospores) are produced sexually by the
bipolar heterothallic fungus and gamete outcross has been shown to be facilitated in
the wild by Botanophila spp. (Diptera: Anthomyiidae), including Botanophila lobata.
The fly - fungus interaction is generally considered to be one of obligatory mutualism
based on studies conducted in areas endemism. However, recent evidence suggests
that the fungus is able to sexually outcross in cultivated Oregon orchardgrass fields
without the aid of fly "pollinators." Additionally, ascosporic fertilization has recently
been implicated as an alternative mechanism for gamete transfer and might have
important impacts on fungal reproduction in Oregon. The objectives of this study were
to: 1) explore how two tightly linked species, which appear to have an obligate
mutualistic relationship in areas of endemism, interact in a non-native context; 2)
quantitatively examine the seasonal and diurnal presence of E. typhina ascospores in a
cultivated Oregon orchardgrass field; and 3) test alternative transfer mechanisms of
fungal spermatia for E. typhina.
To address objective 1), the spatial variability and reproductive success of E.
typhina and B. lobata were estimated during surveys of ten cultivated orchardgrass
fields in 2008 and four fields in 2009. Fungal distributions were spatially aggregated
at five of the study sites in 2008 and three in 2009. Fly distributions were spatially
aggregated at three sites in 2008 and one in 2009. Botanophila lobata density
exhibited a positive linear relationship with E. typhina density, suggestive of positive
density dependence of fly oviposition with fungal density. However, fungal
reproductive success was not affected by fly density or fungal density within the range
of distributions observed in this study.
To address objective 2), airborne ascospores were monitored in a single
cultivated orchardgrass field during 2008 and 2009 using a Burkard volumetric spore
trap. Ascospore production began in early to middle May and continued into late July
during both years of the study. Daily ascospore production exhibited a circadian
rhythm, with production peaking on average at 1:08 am and 12:36 am, in 2008 and
2009, respectively. The prolonged duration and high intensity of ascospore production
during the growing season suggest a large window within which new plants are at risk
to infection, and within which preventative management strategies must be adopted.
To address objective 3), splash fertilization, contact fertilization and B. lobata
fertilization were tested in the greenhouse. Although only two replicates were
completed, both contact and splash fertilization appeared to be viable mechanisms of
sexual outcross for E. typhina.
The results of this study strongly indicate that E. typhina can successfully
reproduce without the presence of B. lobata. Splash and contact fertilization, as well
as ascosporic fertilization, provide opportunities for reproduction of the fungus in
absence of fly pollinators. It appears that the fly - fungus interaction has shifted from
an obligatory mutualism to facultative mutualism or simple fungivory within the
introduced range in western Oregon.