Coexistence between cyclical parthenogens, obligate parthenogens, and intermediates in a fluctuating environment.

We evaluate the cost of sex in cyclical parthenogens coexisting with obligate parthenogens that produce a few sexuals, a situation found in numerous species. In aphids and cladocera, sex is generally the only way to produce a resistant form; this gives a potential ecological advantage to sex. We focus on the selective factor constituted by variations of the environment, that may be alternatively advantageous to cyclical parthenogens (cold winters in aphids, drought in daphnia) or to obligate parthenogens. Our calculations show that the production of a few sexuals by lineages that are essentially obligate parthenogens has a critical effect on the balance between the different life-cycles, especially when this production is limited to males. This male production by intermediates confers additional fitness to obligate parthenogenesis genes and then sharply increases the cost of sex. Copyright 1998 Academic Press