We would also like to thank Nick Carter for his advice and support, and two anonymous referees for their constructive comments. The analyses revealed strong geographical partitioning of breeding systems, with obligate asexuals mostly restricted to the south of France, while lineages producing sexual forms were more common in the north. How frequent is clonal inbreeding and how is its disadvantage balanced by other factors such as mortality during migration? Clone names were related to origin and sometimes colour, such that S2 and S4 were the second and fourth Scottish clones collected and RB was from Rothamsted and was brown in colour. This has consequences for population models, such as that of Rispe and Pierre (1998), who assumed fecundity to be independent of the proportion of males produced. Mortality during dispersal and the cost of host‐specificity in parasites: how many aphids find hosts? Aphids provide the first example of a specific endocrine influence on chromosome behaviour in sex determination. m 8. Significant differences were found between the production of males by winged and wingless females in androcyclic, intermediate, and holocyclic clones (Table 5) . How does the possible difficulty in mate finding affect the fitness of androcyclic clones that cannot mate with their siblings? Both fecundity and total offspring biomass were found to show significant negative correlations with the proportion of offspring that was male (r = −0.74, P < 0.001; r = −0.65, P < 0.01 respectively; n = 19 in both cases) (Fig. We tested the hypothesis that these life cycle variants are selected in space and time by climatic factors, mainly winter severity, due to an ecological link between sexual reproduction and the production of a cold-resistant form, the egg. Unfortunately, data on males are generally lacking. In this way, males help to maintain the balanced polymorphism shown between overwintering strategies, even in areas of harsh winters where it would otherwise not exist (Rispe et al., 1998a). No clear east-west pattern of decreasing sexuality was found, and annualvariation in the relative proportions of life cycle variants was not clearly influenced by the severity of the previous winter. padi onPrunus padus were observed through the winter. Similarly, assuming that S2 and RB had genotype Mm, the S2 × RB cross would produce a holocyclic : intermediate/androcyclic/anholocyclic ratio of 3:1, but the results (Table 2) again showed 5:14. 2. In aphids and cladocera, sex is generally the only way to produce a resistant form; this gives a potential ecological advantage to sex. Overwintering in S. avenae can be either by eggs or parthenogenetically. The gene flow from barley to wheat clones appeared to be more likely than that in the reverse direction. Specialization and host plant use of the common clones of Sitobion avenae (Homoptera: Aphididae). The numbers of intermediate and holocyclic clones were combined to allow comparison of the proportion of clones producing mating females, which were found to be significantly more frequent among the Scottish clones than in those collected from Hertfordshire (Fisher's exact test, P = 0.018, P < 0.05). Similar results have been found before in S. avenae (Newton & Dixon, 1987), Acyrthosiphon pisum (Harris) (Lamb & Pointing, 1975), Myzus persicae (Sulz.) Of course, males are also important in ensuring the overwintering survival of holocyclic clones as well as a means of generating genetic variation in insects that reproduce asexually throughout most of the year. Patterns of male and mating female production by the different types of clone may be related to different advantages and disadvantages of dispersal and inbreeding.
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