Winter climate determines the success of the two main reproductive strategies employed by aphids. Permanent parthenogens survive as parthenogenetic females in mild winters, but are regularly eliminated by low temperatures; while cyclical parthenogens, which switch to sexual reproduction by the end of summer, produce every year fertilised diapausing eggs resistant to frost.
We have studied the variation in sexual morph production of several clones of the cereal aphid Rhopalosiphum padi (L.) showing both strategies. Twenty clones of this species differing by their geographic origin and their mode of reproduction were placed in two laboratory environments mimicking the changes of photoperiod and thermoperiod occurring naturally from the end of summer and during the autumn in oceanic and continental conditions. The analysis of clonal responses in both climatic conditions showed (i) a wide variation in investment of clones in sexual reproduction with, in particular, evidence for a mixed strategy employed by clones producing both sexuals without ceasing parthenogenetic reproduction, (ii) no geographic adaptation among clones belonging to cyclical parthenogenetic populations, (iii) an earlier production of sexuals in continental conditions and a higher production of males in oceanic conditions.
Furthermore, we have compared the dates of first appearance of sexuals in our experiments with those occurring in the field based on a suction trap database and found that sexuals were caught in nature at least four weeks earlier than in the lab. These results underline the need for a better understanding of the influence of the whole array of environmental factors inducing the transition from parthenogenetic to sexual reproduction in aphids., Maurice Hullé, Damien Maurice, Claude Rispe, Jean-Christophe Simon, and Lit
istorically speaking divorce is a relatively recent phenomenon. Divorce has become more important over the last century and has undoubtedly had an impact on the social reproduction strategies typically linked to marriage. In this paper use is made of the concept of Assortative Mating to explore if initial and subsequent marriage partner choices differ on the basis of heterogamy. This research question is formulated in terms of two contrasting hypotheses: (1) the learning and (2) the marriage market hypotheses (Gelissen 2004). These two perspectives form the basis of the theoretical framework used in the empirical analyses reported. Here logistic and linear regression and log-linear modelling techniques are used. The data used to test the learning and marriage market hypotheses contains information on all officially recorded marriages in the Czech Republic over a ten year period (1994-2004) gathered by the Czech Statistical Office. The results presented reveal that divorce changes the socially reproductive aspects of marriage choice strategies, and these changes vary systematically on the basis of gender. Whereas first and subsequent marriage choices are very similar for men, repeated marriage choices for women are on the whole more diverse. A number of explanations explaining this gender based difference are presented. These explanations centre on dissimilarities in the context of repeated choices for men and women., Petr Fučík., 5 grafů, 2 tabulky, Obsahuje bibliografii, and Anglické resumé