Mechanisms Defining Ecological Range Limits
in a Plant Hybrid Zone
Sarah Kimball, UC Irvine
zones, areas where two divergent populations cross-fertilize producing
viable offspring, can be used to study the mechanisms that isolate
species. Studies of selection in hybrid zones indicate that intrinsic
factors like genomic incompatibilities as well as extrinsic factors like
pollinators and climate can be important in structuring species
boundaries. My study investigates the role of pollinator differences and
physiological trait differences in determining the range limits of
parent species and hybrids. My study system is an extensive hybrid zone
along an elevational gradient. Penstemon newberryi is a perennial
wildflower that grows at mid-elevations throughout the Sierra Nevada
mountain range. Its close relative, P. davidsonii, grows in the
alpine zone in the Sierra Nevada and the Cascade ranges. These two
species form hybrids wherever their ranges overlap, between 3000 and
3400 m elevation in the Sierra Nevada. The two parent species are
somehow maintained as distinct entities with different habitat ranges
despite constant gene flow from the other species.
I am addressing the following four questions:
1. Does intrinsic isolation occur in the stages between
pollination and seedling growth?
I conducted a hand-pollination
experiment in which the pure species and naturally occurring hybrids
were crossed in all possible combinations. Results indicated that the
two species hybridized readily, with no reduction in number or viability
of seeds resulting from hybrid crosses. The lack of intrinsic isolation
indicates that other factors must act to keep the species distinct.
2. Do pollinator differences limit hybridization or the elevational
range over which hybrids are found?
I surveyed pollinators at all
elevations along my study transect, identifying over 50 species of
animal pollinators. Bumblebees were found visiting plants at all
elevations, suggesting that they may transfer pollen between species.
However, some pollinators, like hummingbirds, hawkmoths, and wasps, were
only found visiting individuals along a portion of the elevational
transect. Thus, differences in pollinators may limit gene flow between
the parent species, but shared pollinators may transfer pollen at all
points along the cline.
3. Are there differences in some
physiological traits that may define the elevational range limits of the
parent species and the hybrids?
I measured physiological traits of
both species and hybrids and found differences that correlate to the
different habitats of each species. For example, P. davidsonii (alpine
species) reached maximum photosynthetic rate at a lower temperature than
P. newberryi. Penstemon davidsonii had lower water-use-efficiency than
P. davidsonii. Soil moisture increased with elevation and P. newberryi
was under greater water stress than P. davidsonii.
Do parental and hybrid genotypes survive equally well throughout the
I performed a large-scale reciprocal transplant
experiment this summer, planting both species and hybrids into three
gardens at low, middle, and high altitudes in the Lee Vining canyon.
Plants with P. newberryi as the mother did best in the low garden, where
P. newberryi naturally occurs. Hybrids did best in the middle elevation
garden, and all plant types grew in the high altitude garden. I think
that the P. davidsoniii is unable to survive at low elevations because
it is too hot and dry in the montane zone, and P. newberryi is able to
survive up high, but cannot reproduce in the short growing season of the
alpine zone. I plan to continue to study the gardens, measuring
physiological traits of garden plants and recording environmental
This study is significant because it helps to explain
species diversity. It is unique in its focus on the environmental limits
to the distribution of the parent species. Penstemon davidsonii and P.
newberryi appear to hybridize freely, raising the question of how they
remain distinct as species that occupy separate ecological niches. This
study will document how the characters that differ between the two
species correspond to pollinators and adaptations to the physical
environment. I hypothesize that such traits are involved in maintaining
the species. Little modern ecophysiological work has been conducted in
hybrid zones, and few studies have combined reciprocal transplant
experiments with physiological data to determine the mechanisms by which
plant hybrid types succeed or fail at different points along an
environmental gradient. I hope to understand an example of the
maintenance and origins of biological diversity in Penstemon, a genus
that has undergone an explosive radiation into 275 species in the
mountains of western North America. Most importantly, my study will
elucidate the selection pressures that act to maintain range limits of
species that hybridize when their ranges overlap.