COEXISTENCE BETWEEN CYCLICAL PARTHENOGENS, OBLIGATE PARTHENOGENS, ANDINTERMEDIATES IN A FLUCTUATING ENVIRONMENT

We evaluate the cost of sex in cyclical parthenogens coexisting with o bligate parthenogens that produce a few sexuals, a situation found in numerous species. In aphids and cladocera, sex is generally the only w ay to produce a resistant form; this gives a potential ecological adva ntage to sex. We focus on the selective factor constituted by variatio ns of the environment, that may be alternatively advantageous to cycli cal parthenogens (cold winters in aphids, drought in daphnia) or to ob ligate parthenogens. Our calculations show that the production of a fe w sexuals by lineages that are essentially obligate parthenogens has a critical effect on the balance between the different life-cycles, esp ecially when this production is limited to males. This male production by intermediates confers additional fitness to obligate parthenogenes is genes and then sharply increases the cost of sex. (C) 1998 Academic Press.