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the inheritance patterns of secondary chemicals, found that 13 of 32 compounds had dominant inheritance. Given dominant
inheritance, if the parent possessing the high levels of defensive chemical is resistant, then F1 hybrids should also be resistant.
Paige and Capman (1993) showed that hybrid breakdown in resistance to Pemphigus betae occurs only in backcrosses
of Populus hybrids to the susceptible parent, and F1 hybrids are as resistant as the resistant P. fremomii. This illustrates the
Dominance hypothesis in the cottonwood-aphid system.
There was also considerable support for the Hybrid Susceptibility hypothesis. Three herbivore species consistently
supported the Hybrid Susceptibility hypothesis, and over three years 9 of 33 cases supported this hypothesis. The Hybrid
Susceptibility hypothesis was also supported by the analysis of total herbivore densities in 1993 (Table 1, Fig. 2). Mosseler
(pers. comm.) reports that in his garden of F1 hybrids and parents of seven Salix species, hybrids were highly susceptible to
Melampsora rust, but parents were immune. Fritz et al. (1994) has also found for Melampsora rust that hybrids were much
more susceptible than either parental species. This observation shows that Fl's appear to show breakdown of resistance.
The Hybrid Resistance hypothesis was supported only by the Aculops gall mite in 1992. Density of mites was lower
on hybrids and on either parent. Boecklen and Spellenberg (1990) found some evidence for this hypothesis in their study of
leaf mining and leaf galling herbivores on oak. A problem with the support for the Hybrid Resistance hypothesis from that
study is that many herbivore species were combined into feeding guilds for analysis and they did not explicitly test for a
significant departure from the Dominance hypothesis.
The results of this study suggest that interspecific hybridization of plants has a variety of effects on resistance of
hybrids and it does not lead only to a pattern of breakdown of resistance (i. e., support of the Hybrid Susceptibility hypothesis).
The various herbivore responses suggest that different resistance traits of plants have different mechanisms of inheritance
and/or that different resistance factors could affect herbivores differently. An important question is: How can the
hypothesized herbivore responses be explained by the known patterns of inheritance of resistance traits? The Additive and
Dominance hypotheses both have fairly straightforward explanations. If resistance is dosage-dependent, then an Additively
inherited trait should lead to support for the Additive hypothesis. A dominantly inherited trait would be fully expressed in
hybrids and should result in a pattern of herbivore resistance that supported the Dominance hypothesis. Backcrossing to the
susceptible parent, as seems to have occurred in cottonwoods, could lead to the loss of one or more dominant resistance
genes, thereby resulting in Hybrid Susceptibility of the backcross progeny. Paige and Capman (1993) suggest that there is
more than one dominant gene involved in resistance to aphids in the cottonwood system.
Additively inherited traits could also result in herbivore densities that support the Hybrid Susceptibility hypothesis.
Herbivores that require a threshold amount of a chemical defense could have higher densities on hybrids (Hybrid Susceptibility)
if the dosage in hybrids was less than the threshold amount. Hybrid Resistance could result from a mechanism of
dominance inheritance. Hybrids with dominant inheritance of two different resistance traits from each parent would possess
complete expression of two different resistance mechanisms. If particular herbivores are negatively affected by both resistance
mechanisms, hybrids could be more resistant than either parent (i.e., Hybrid Resistance). It also seems possible that if
two additively inherited resistance mechanisms interacted synergistically to affect herbivores Hybrid Resistance could also
result. These alternative mechanisms suggest that various outcomes of interspecific hybridization on resistance are likely and
that mechanisms of inheritance of resistance and their effect on herbivores need to be investigated in hybrid systems.
The year-to-year variation in support for particular hypotheses was an unexpected result of this analysis. Seven of
the 11 species had at least one change in which hypothesis was supported between adjacent years. These changes could be
considered either: (1) random fluctuations in herbivore numbers on hybrids and parents or accidents of sampling, (2)
consequences of temporarily lower population sizes that made differences difficult to detect, or (3) real year-to-year changes
in the expression of resistance of hybrids and/or parent species due to variation in environmental factors. Point 1 is always
possible, but it is impossible to discount using the census data. A number of shifts in herbivore response occurred between
1991 and 1992 and then back to 1991 patterns in 1993. This may have something to do with the decreased abundance of
herbivores in 1992 (unpublished results). Lower numbers could have made differences difficult to detect. While point 2 is
possible, it would be wrong to attribute the shifts in support for hypotheses to this factor alone. Shifts in support for hypotheses
may be real changes in the relative expression of resistance between hybrid and parental plants. Environmental variation
(e. g., solar radiation, temperature, nutrients, etc.) is known to affect the expression of resistance among genotypes of many
plant species. It therefbre seems likely that changes in environment could affect resistance of hybrid and parent species of
plants in nature. There were some substantial shifts in the weather among the years of this study. The summer of 1992 was
cool and rainy compared to normal in the northeast, whereas in 1993 there were some long periods of high temperatures and
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