No abstract

The aphid species Cryptomyzus galeopsidis (Kaltenbach) includes several distinct forms which have different host plant relationships and life cycles. Cross breeding was used to elucidate the taxonomic status of these forms and to investigate the inheritance of host preference, reproductive performance and host-alternation. One of the forms appeared to be a distinct species because of the reduced fitness of the hybrids. Other host-alternating and non host-alternating forms are considered conspecific and represent two life cycle strategies. Reproductive performance is probably controlled polygenically, since hybrids show an intermediate performance. Host preference in hybrids showed some degree of dominance and seemed to be determined by only a few genes. Host-alternation is presumed to be inherited monofactorially. The implications for speciation are discussed.

1. Host plant preference experiments were conducted with closely related taxa of the aphid genus Crypromyzus. Males, and presexual morphs (sexuparae and gynoparae), were used to determine the impact of host plant choice on reproductive isolation. In the case of host-alternating species these morphs are migratory and so will select the host plant.2. Host plant preference of t w~ closely related taxa of C.alboapicalis (Theobald) was found to promote their reproductive isolation. The preference of sexuparae of these monoecious taxa was more pronounced than that of the males.3. Host plant preference and subsequent production of oviparae showed that C. guleopsidis (Kaltenbach) consists of two host races restricted to Ribes rubrum L. and R.nigrum L.,respectively.The existence of clones, intermediate in their preference and reproductive performance on these plants, suggests that hybridization occurs.

Realistic values of population growth rates are needed when used in forecasting programmes, e.g., in a programme of integrated control. Therefore, comparisons were made in a chrysanthemum – aphid system between different methods of assessing population growth rates. The reproductive performances of the aphid species Aphis gossypii and Myzus persicae were measured on two chrysanthemum cultivars using three plant growth stages (young vegetative, budding and flowering). In the first set of experiments, development time and reproduction were used to estimate the population growth rate rm. The mean relative growth rates (MRGR) were also assessed. It was shown for the first time that the relationship between rm and MRGR was influenced by aphid species. In a second experiment, the aphid population increase on a whole plant was measured and rm was estimated by calculating the slope of the (ln transformed) population increase. It is shown that population growth rate is affected by the growth stage of the plant, and that cultivar and aphid species interact with plant growth stage in influencing population growth rate. Thus, no single growth stage of chrysanthemum for maximal aphid population growth can be assigned, but the budding and flowering stage are the most suitable in three out of four aphid × cultivar combinations. Comparison between the results from both experiments demonstrates clearly that more realistic values for rm are obtained when measured on whole plants.

Speciation by reinforcement, the process by which characters that increase assortative mating will he favoured because of selection against the production of less fit hybrids, is a much criticized evolutionary scenario, supported by little evidence. Previous studies have focused on hybrid zones, i.e. where different populations come into contact after diverging in allopatry, which might not be the optimal situation in which to look for reinforcement. We argue that reinforcement is more likely to occur in sympatrically diverging populations, like host races of phytophagous insects. In an attempt to demonstrate this we studied mate recognition of different life cycle forms, host races, sister species and more distantly related species of the aphid Cryptomyzus. All but one of these taxa mate on the same host plant, thus their mate recognition is subjected to similar ecological conditions, i.e. extrinsic selection pressures. The males of the life cycle forms and host races were not differentially attracted in an olfactometer by their respective females, which all released sex pheromones during the same period of the day. The females of the sister species, however, released sex pheromones at different times of the day, one 'calling' in the morning and the other in the afternoon. This difference is corroborated by a corresponding pattern of activity in the males, and a marked preference of the males for conspecific pheromone. It is argued that these sister species originated sympatrically following a shift in summer host plant utilization. As their hybrids are less viable than the F1 of intraspecific crosses reinforcement might have been a major force in the development of specific mate recognition, i.e. the specificity and difference in the daily cycle of release of the sex pheromones.