Tioga Pass and Lee Vining Peak, Eastern Sierra, California

Kimball, S. 2008. In Press. Differential performance of reciprocal hybrids in multiple environments. Journal of Ecology.

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Links to Penstemon Research Topics:
Dissertation Summary

Pollinators

Physiology

Links to Additional Research Topics:

Traits influencing plant community composition

Restoration ecology at Carrizo Plain National Monument

Local ecology and geographic range limits

Closely related taxa may be maintained as distinct species by endogenous isolation in the form of genomic incompatibilities and by exogenous isolation in the form of environmental factors. To understand the relative importance of endogenous and exogenous factors, we performed hand-pollinations and reciprocal transplant experiments in a natural plant hybrid zone.

We measured reproductive isolation by making crosses between two parent species of Penstemon and naturally occurring hybrids. Inclusion of reciprocal hybrid crosses allowed us to determine whether fitness components differed depending on the identity of the mother, as would be expected if cytoplasmic genes were involved.

Hybrid performance was evaluated in the greenhouse and in a reciprocal transplant experiment in the field. We measured fruit set, seed set, seed weight, time required for fruits to mature, and seedling growth for potted plants. To test for exogenous isolation, we planted pure parents, reciprocal F1 hybrids, and later generation hybrids in a reciprocal transplant experiment, and measured survival.

On average, hybrids had fitness equal to or greater than parents in all environments, including the greenhouse and all field gardens, indicating a lack of endogenous isolation. Parent species and reciprocal F1 hybrids differed in many of the traits that were measured. In each field garden, the hybrid with the native cytoplasm had a higher survival rate, suggesting local adaptation to different elevations.

Exogenous factors that differ along elevational gradients can be more important than intrinsic genetic incompatibilities in determining the fitness of plant hybrids. Our 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.