The relative role of cospeciation and host switching in the phylogenetic history of
ascomycete foliar symbionts is addressed in the orders Leotiales and Rhytismatales, fungi
associated predominantly with Pinaceae (Coniferales). Emphasis is placed on comparing
the evolution of the sister genera Pseudotsuga and Larix (Pinaceae) with that of the
pathogenic and endophytic fungi in the genus Rhabdocline. Pinaceae evolved during the
Mesozoic and divergence of all extant genera and several infrageneric lineages (esp. in
Pinus) occurred prior to the Tertiary, with subsequent species radiations following
climatic changes of the Eocene. The youngest generic pair to evolve from Pinaceae, Larix
and Pseudotsuga, diverged near the Cretaceous-Tertiary boundary in East Asia or western
North America. Rhabdocline is comprised of seven species and subspecies, six known
from two species of Pseudotsuga and one, the asexual species Meria laricis, from three
species of Larix. Evidence from host distributions and from nuclear ribosomal DNA
suggests that Rhabdocline speciated in western North America and has been involved in
several host switches. The ancestor of Meria laricis appears to have switched from P.
menziesii to its current western North American hosts, L. occidentalis, L. lyallii, and very
recently may have extended its host range to the European species, L. decidua. The
occurrence of two lineages of R. weirii ssp. weirii on both North American species of
Pseudotsuga is also probably the result of a recent host switch. Evidence of hostmediated
divergence is seen in R. parkeri, which has different internal transcribed spacer
types in the geographically isolated coastal and interior forms of P. menziesii. The wide
host ranges of fungal genera closely related to Rhabdocline indicates that host switching is
a prevalent pattern in the evolution of foliar symbionts in Leotiales and Rhytismatales.
The prevalence of host switching in this group relative to other endosymbiotic organisms
can probably be attributed to differences in dispersal mechanisms. Spores of foliar fungi
are dispersed horizontally by wind and rain, rather than vertically from parent to offspring.
Over evolutionary time, this provides more opportunities to shift to new hosts, particularly
when the hosts are closely related and have overlapping distributions. / Graduation date: 1998
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/33897 |
| Date | 07 May 1998 |
| Creators | Gernandt, David S. |
| Contributors | Stone, Jeffrey K. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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