Inyo National Forest
Mechanisms defining ecological range limits in a Penstemon hybrid zone.
Ph.D. dissertation, UC Irvine.
to Penstemon Research Topics:
to Additional Research Topics:
influencing plant community composition
ecology at Carrizo Plain National Monument
Local ecology and geographic range limits
zones, areas where divergent species cross-fertilize, can be used to study
the mechanisms that isolate species. Studies of selection in hybrid zones
indicate that endogenous factors like genomic incompatibilities as well
as exogenous factors like pollinators and climate can be important in
structuring species boundaries. I studied the role of endogenous and exogenous
factors in determining the range limits of two perennial wildflowers,
Penstemon newberryi and P. davidsonii, and their hybrids,
along an elevational gradient in the Sierra Nevada mountains.
First, I measured floral morphology and surveyed pollinator visitation
along the elevational gradient, identifying over 62 species of floral
visitors. Morphological measurements were used to construct a plant hybrid
index and examine the correlation with elevation and with floral visitors.
The visitor community that visited P. davidsonii was somewhat
distinct from the community that visited P. newberryi and hybrids.
Eleven common pollinator species visited both parent species and could
be contributing to hybrid formation.
In the second chapter, I identified physiological differences between
both species and hybrids. Penstemon davidsonii had less negative
pre-dawn and mid-day water potential, lower water-use-efficiency, and
took less time to produce mature fruits than its montane relative, P.
newberryi. Hybrids were mostly intermediate, but gas exchange traits
differed depending on genotype.
In the third chapter, I investigated the relative importance of endogenous
and exogenous isolation. I conducted a hand-pollination experiment in
which the pure species and hybrids were crossed in all possible combinations.
Hybrid crosses did not result in few seeds or less viable seeds. I performed
a reciprocal transplant experiment, planting both species and hybrids
into gardens at low, middle, and high altitudes. In each garden, the hybrid
with the native cytoplasm had a higher survival rate, suggesting local
adaptation to different elevations.
This study indicates that exogenous isolating mechanisms, like physiological
and pollinator differences, act to maintain P. newberryi and
P. davidsonii as distinct at the elevational extremes of their
ranges. My results illustrate the importance of studying hybrid performance
in multiple environments and in generating reciprocal hybrids to effectively
test for isolating mechanisms in natural hybrid zones.