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RESISTANCE OF HYBRID AND PARENTAL WILLOWS TO HERBIVORES:
HYPOTHESES AND VARIABLE HERBIVORE RESPONSES OVER 3 YEARS
ROBERT S. FRITZ l, BERNADETTE M. ROCHE I, and COLIN M. ORIANS 2
_Department of Biology, Vassar College, Poughkeepsie, New York 12601, USA
2Department of Biology, Williams College, Williamstown, Massachusetts 01267, USA
INTRODUCTION
Analysis of herbivory on hybrid plants in natural populations has recently gained attention (Whitham 1989, Keim et
al. 1989, Boecklen and Spellenberg 1990, Paige et al. 1991, Whitham et al. 1991, Aguilar and Boecklen 1992, Fritz et al.
1994, Whitham et al. 1994), and it may be important for understanding plant-herbivore interactions (Whitham 1989). To
assess the significance of interspecific hybridization of plants for plant-herbivore interactions, it will be important to know
how interspecific hybridization of plants influences resistance of hybrid plants in relation to resistance of pure parents. It has
been hypothesized that hybrid plants may act as an "ecological sink" for herbivorous insects (Whitham 1989). In mixed
stands of parental and hybrid plants, this hypothesis predicts that a large fraction of the herbivore population would reside on
hybrid individuals (Drake 1981, Whitham 1989, Floate et al. 1993). Whitham (1989) referred to highly susceptible hybrid
poplars as being an ecological sink for a galling aphid, but in terms of metapopulation dynamics, hybrid plants could function
as a "source" rather than a sink (e.g., Ericson et al. 1993). Hybrid plants have also been hypothesized to act as "evolutionary
sinks" to pests (Whitham 1989). The "evolutionary sink" idea predicts that if herbivore fitness was higher on hybrids
compared to their fitness on parental plants, there would be selection for specialization on the hybrids, perhaps preventing
selection tbr herbivore virulence on the parental species. If so, then hybrid plant-herbivore interactions may be critical to
understanding why plants seem to maintain pest resistance in the face of a greater evolutionary potential for virulence in
pests. Finally, Floate and Whitham (1993) suggest that hybrid plants may facilitate host shifts of herbivores onto new plants,
because they could act as a selective "bridge" favoring adaptation of an herbivore to previously non-host species. Tests of
these hypotheses will require knowing the general patterns of herbivore responses to hybrid versus pure parental species.
Hypotheses
Fritz et al. (1994) proposed several alternative hypotheses of phytophage response to hybrid plants. Those hypotheses
assume that pure parent species are being compared to F1 hybrids. The reason to focus on F1 hybrids is that this
comparison could suggest: (1) how resistance mechanisms are inherited from parental species, and (2) how these traits may
be expressed upon backcrossing. The Additive hypothesis (Fig. 1A) predicts that hybrids do not differ from the mean of the
resistances of the two parents (i.e., the midparent value). Thus, F1 hybrids are intermediate between the resistances of the
parental species, which suggests that hybrid resistances are due to the additive inheritance of resistance traits from both
parents.
The second hypothesis is the Dominance hypothesis (Fig. 1B). If hybrid resistance differs significantly from the
mean resistance of both parents but does not differ significantly from that of one of the parents, the Dominance hypothesis
would be supported. Hybrid resistance could resemble that of either the more resistant or the more susceptible parent.
(Dominance in this context refers to phenotypic similarity between a parent and hybrids, not necessarily genetic dominance,
though it may imply dominant inheritance of resistance traits.) Herbivore densities could be intermediate between the
Additive and Dominance patterns, which would support an hypothesis of partial dominance.
The third hypothesis is the Hybrid Susceptibility hypothesis (the Hybrids-as-Sinks hypothesis of Whitham 1989; see
also Keim et al. 1989, Boecklen and Spellenberg 1990). This hypothesis predicts higher herbivore densities and/or higher
herbivore pertbrmance on hybrids compared to parental taxa (Fig. 1C). The Hybrid Susceptibility hypothesis can be distinguished
from the Dominance hypothesis in that the susceptibility of the hybrid must exceed that of the most susceptible
parent.
Mattson, W.J., Niemelfi, R, and Rousi, M., eds. 1996. Dynamics of forest herbivory: quest for pattern and principle. USDA
For. Serv. Gen. Tech. Rep. NC-183, N.C. For. Exp. Sta., St. Paul, MN 55108.
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