Aphid performance in an alder hybrid zone

Ecology, Oct, 1995 by Alan C. Gange

Key words: alder; aphid; herbivory; hybrid zone; physical defense; Pterocallis alni; trichomes.

INTRODUCTION

When two plant species naturally hybridize, the hybrid zone thus created can provide invaluable insights into evolutionary processes (Barton and Hewitt 1985). However, ecologists studying plant-herbivore interactions have only recently considered the consequences of hybridization on levels of herbivory, even though these represent excellent opportunities to understand how herbivores can respond to genetically assorted hosts (Strauss 1994). In view of the fact that hybridization is thought to account for the origins of between 80 and 80% of all plant species (Stace 1987), hybrid zones are rich areas in which to explore interactions between herbivores and plants at a variety of levels (Strauss 1994).

In trees, it is known that hybrids can be superior or inferior to their parents in teens of growth and reproduction (e.g., Mosseler 1990). Clearly, such characters may also be translated into resistance or susceptibility to insect attack. For example, Boecklen and Spellenberg (1990) found that hybrid oaks (Quercus) supported lower densities of leaf mining and gall-forming insects than did the parental species. Drake (1981) recorded intermediate levels of herbivory on some Eucalyptus hybrids, while Whitham et al. (1994) found that the hybrid zone between Eucalyptus amygdalina and E. risdonii in Tasmania was a center of insect bio-diversity with the majority of insect taxa being more abundant in the hybrid zone. In addition, it has been shown that hybrid Populus are more susceptible to the aphid Pemphigus betae and the beetle Chrysomela confluens than the parents (Whitham 1989. Floate et al. 1998). These results have led to the suggestions that hybrid zones act as "phenological" and "hybrid" sinks for insects. The hybrid plants act as phenological sinks for the beetle by providing food at a different time of the year than either parent does (Floate et al. 1993) and as hybrid sinks for the aphid because of a breakdown in the resistance mechanisms of the parents (Whitham 1989).In addition, Whitham (1989) has suggested that hybrids act as evolutionary sinks for insects, because if herbivores perform very well on hybrid hosts, then there may be weaker selection to overcome the resistance in parent plants.

Recently, a theoretical framework has been applied to studies of insect populations on hybrid plants, and several hypotheses have been developed (Fritz et al. 1994). Five possible scenarios of insect attack were identified, in which hybrids are more susceptible than either parent, less susceptible than either, equal to both, intermediate, or equal to one. The diversity of insect/plant relations was exemplified by Strauss (1994) who recorded over 20 examples for each category, except that of hybrids being less susceptible than either parent, where there were only six examples. Considering the diversity of outcomes, more studies are needed on well-defined systems, in order to understand how insects respond to the genetic mixing of their hosts.

The increased insect abundance on hybrid trees has been attributed to a longer availability of food rather than any increase in food quality (Floate et al. 1993). However. as nitrogen is critical to the diet of the majority of phytophagous insects (Mattson 1980), we cannot determine whether the reported increases and decreases in insect abundance are due to increases or decreases in the food quality. Such a situation can be alleviated by considering an insect species that is relatively unaffected by the quality of its food supply. The growth and reproduction of the aphid Pterocallis alni when feeding on alders (Alnus sp.), are determined by temperature. rather than by food quality, at least as measured by soluble nitrogen (Gange and Pryse 1990). Alders are nitrogen-fixing trees (Bond 1967) and it has been shown that the levels of soluble nitrogen in alder leaf tissue during summer remain nearly constant. in direct contrast to many other deciduous trees (Dawson and Funk 1981). P. alni responds to this continually good food supply by showing traits more characteristic of herbaceous dwelling aphids than arboreal species (such as constant ovariole number from generation to generation and prolonged summer reproduction) (Gange 1985). Therefore, confounding factors such as seasonal variation in aphid physiology can be eliminated, when studying the performance of this aphid on hybrid trees.

Naturally occurring hybrid zones often contain a bewildering variety of genotypes, including parents, F1 hybrids, and complex backcrosses (Keim et al. 1989). On some occasions, the degree of hybridization has been measured with a hybrid index (Moorehead et al. 1993), which describes the similarity of hybrids to one parent. In complicated situations, such an index is an excellent indicator of hybrid affinity. In the current study, I identified a hybrid zone that was of a known age and where the parent trees had been planted at a known time. By selecting a simple system, I was able to eliminate the potential difficulties of tree identification. In addition, I also simplified the system by studying one insect species. The aphid Pterocallis alni is restricted to the genus Alnus in Britain and its population biology has been well studied (Gange 1985). In addition, it is a free-living insect, thus providing a contrast to the majority of insect/plant hybrid studies, which have involved endophagous insects, such as gallers or leaf miners. Therefore, this system provided a natural "laboratory." while controlling for as many potential confounding factors as possible (Strauss 1994).