Extreme Life-Cycle and Sex Ratio Variation among Sexually Produced Clones of the Aphid Rhopalosiphum padi (Homoptera: Aphididae)

Abstract

The aphid Rhopalosiphum padi exhibits considerable life-cycle variation, showing coexistence between lineages that alternate asexual and sexual reproduction (cyclical parthenogens), lineages that are entirely asexual (obligate parthenogens), and male-prodacing obligate parthenogens. We collected 222 sexually produced clones, in two regions of France, from 1991 to 1994. In experiment 1, with three replicates per clone, we determined the life-cycle of the clones by placing them in conditions inducing the production of gynoparae (the precursors of sexual females) and males. A substantial proportion of the clones consisted of obligate parthenogens. These may result from occasional matings between females (from cyclical parthenogens) and males that are produced by obligate parthenogens that transmit their life-cycle character. Such matings would create bi-directional gene flow between the pools of sexual and asexual lineages and would be likely to modify the relative cost of sex and asex. We studied sex allocation variation in the cyclically parthenogenetic clones, and identified two main phenotypes, one male-biased and one strongly gynopara-biased. In experiment 2. with four of these field-collected clones and twenty replicates perclone, we confirmed that at least some clones are consistently male- or gynopara-biased. In experiment 3. we studied two parental clones (F and H) that were male-biased, and 13 F × F and 14 H × H self-crossed offspring clones. Nearly all of these clones were also consistently male-biased, strongly supporting genetic control of sex allocation trait.