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Elements of the Suitability/Defense Vector in Hybrids: Expression of S/D Traits
Most work on resistance expression (i.e., a_s_)in natural and artificial interspecies hybrids has focused on suitability/
defense (s) expression for particular insects, assuming that a plant's attractiveness (a) is high and invariant. Fritz et al.
(1994) hypothesized four expression scenarios: (1) the "additive" case where the hybrid's S/D ranking (s_) against an insect
is the simple mean of its parents, p_, and p: (sh_= (s_ + s2)/2), (2) the "dominance" case where the hybrid is consistently like
one of the parents, e.g., p_(sh_= s_), (3) the "susceptibility" case where the hybrid has more insects than either of the parents
(s_a< sh_> s2), and (4) the "resistance" case where the hybrid has fewer insects than either parent (s_ > sh_< s:).
Most studies with trees have shown that hybrid S/D expression (s) against an insect is not cleanly predictable but
varies with the particular insect and the host plant species (Boecklen and Larson 1994, Fritz et al. 1994). For example,
Manley and Fowler (11969)and Osawa (1989) discovered that the severity of spruce budworm, Choristonenrafumiferana,
defoliation of spruce trees was linked to the direction of their introgression between black and red spruces, P. maria_za and P.
rubens. Hybrids closer to pure red were severely defoliated as were pure reds, whereas hybrids closer to pure black were
only lightly defoliated as were pure blacks. Several hybrid elms, Ulmus, were consistently intermediate in resistance to the
elm leafbeetle, Xanthogaleruca luteola, whenever one parent species was susceptible and the other was resistant (Hall and
Townsend 1987, Hall et al. t987). Hybrids of P. sitchensis x P. glauca are usually resistant to Pissodes strobi, and so are
such backcrosses to white spruce, the resistant parent whereas sitka spruce is highly susceptible (Mitchell et al. 1974, Copes
and Beckwith 1977). Similarly, the Pinus coulteri x P jeffreyi hybrid carries resistance against the weevil, Cylirzrocopturus
eatoni, that the coulteri parent possesses, but thejeffreyi parent does not (Miller 1950). Hybrids of several species of
Japanese pines, Pinus, all apparently inherited their "resistance" against the scale, Matscoccus matsumurae, from the more
susceptible parent species (McClure 1985).
More recently, Floate et al. (1993) reported that Chrysomela confluens was invariably more abundant in a hybrid
swarm of Populusfremontii x P angustifolia than in the parental populations. Whitham et al. (1994) reported that of some
40 phytophagous taxa in a hybrid swarm of Eucalyptus risdonii x E. amygdalina only two were more abundant on FI hybrids
than on the parents. Specialist insects were most abundant on hybrid backcrosses to their favored parent species, and
generalist insects were generally more abundant on all hybrid types. Hanhimaki et al. (1994) reported that in the case of 14
insects tested on hybrids of Betula pubescens ssp. tortuosa x B. nana, the hybrids were consistently equivalent to B.
pubescens, but superior to B. nana. Fritz et al. (1994) tested 11 insects and one leaf rust on hybrids of Salix sericea x S.
eriocephala, and concluded that 3 supported the additive, 2 the dominance, and 7 the susceptibility expression hypotheses.
Boecklen and Larson (1994) measured densities of 8 species of galling wasps on hybrids of Quercus grisea x Q. gambelii,
and reported that 3 supported the additive, 2 the dominance, 2 the susceptibility, and 1the resistance expression hypotheses.
In the case of non-woody species, plant breeders have created innumerable interspecies hybrids which have become
the cornerstone of breeding programs for the development of insect resistance in food and forage plants. Consequently, most
resistance traits useful against native insects and pathogens have been derived from crosses to related but allopatric plant
species (Harris 1975, 1980 1982; Bailey 1983). These studies reveal a surprisingly large number of cases for which insect
resistance seems to be either monogenic or oligogenic, but polygenic inheritance is also important (Hanover 1980, Harris
1982, Zobel and Talbert 1984, Singh 1986, Carson and Carson 1989, Geiger and Heun 1989). Moreover, the resistance genes
are usually, but not always dominant in their expression (Harris 1982, Singh 1986), and evidence suggests that gene linkages
and multiple resistance alleles are uncommon. Resistance genes appear to be insect specific, not conferring cross protection
against many species (Harris 1982).
Elements of the Species A/R Vector in Hybrids: Expression of A/R Traits
General plant traits (e.g., crown form, height, growth rate, phenology, etc.) in hybrids are often intermediate between
those of its parents (implying that S,_= (a_ + a2)/2) (Hanover and Wilkinson 1969, Bongarten and Hanover t982, Yeh and
Arnott 1986) which suggests that interspecies hybrids will probably have the unique gestalt of properties to attract the all the
specialist insects from both parents (Miller and Strickler 1984). In t_ct, Whitham et al. (1994) and Morrow et al. (1994) have
found that this is true in a eucalypt hybrid swarm. They reported that the average hybrid tree supported 53% more phytophagous
insect and fungal species than equivalent trees from the pure parental stands.
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