VARIABILITY IN LIFE-HISTORY TRAITS OF THE APHID, ACYRTHOSIPHON-PISUM (HARRIS), FROM SEXUAL AND ASEXUAL POPULATIONS

Many aphid species have shown remarkable adaptability by invading new habitats and agricultural crops, although they are parthenogenetic and might be expected to show limited genetic variation. To determine if the mode of reproduction limits the level of genetic variation in adap tively important traits, we assess variation in 15 life history traits of the pea aphid, Acyrthosiphon pisum (Harris), for five populations sampled along a north-south transect in central North America, and for three traits for three populations from eastern Australia. The traits are developmental times and rates as affected by temperature, body we ights as affected by temperature, fecundity, measures of migratory ten dency, and photoperiodic responses. The most southerly population from North America is shown to be obligately parthenogenetic, as are the A ustralian populations, and the four more northerly North American popu lations are facultatively parthenogenetic with the number of parthenog enetic generations per year increasing from north to south. The broad- sense heritabilities of life history traits varied from 0.36 to 0.71 f or nine quantitive traits based on a comparison of within- and between -lineage variances. Using these traits, 7-13 distinct genotypes (i.e. clones) were identified among each of the 18 lines sampled from the No rth American populations, but the number did not differ significantly among populations. The level of genetic variation differed from trait to trait. For 4 of 12 quantitative traits, the level of variation in t he obligately parthenogenetic population from North America was lowest , but significantly lower than all the sexual populations for only 1 t rait. The obligately parthenogenetic population had the highest level of genetic variation for two traits, and had intermediate levels for t he others. The most northerly population, which was sexual and had rel atively few parthenogenetic generations each year, had the lowest leve l of variation for 5 of 12 traits and the highest level of variation f or 2 traits. There was no decline in variability from north to south c orrelated with the increase in the annual number of parthenogenetic ge nerations. The Australian populations showed no less variation than th e North American populations for two of three traits, although the pea aphid was introduced to Australia only 5 years prior to the study, wh ereas the aphid has been in North America for at least 100 years. The mode of reproduction has not had a substantial impact on the level of genetic variation in life history traits of the pea aphid, but there a re population-specific factors that affect the level of variation in c ertain traits.