La partenogenesi negli Afidi

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Abstract
A comparative study of the parthienogenetic and amphigonic egg development of Aphids, has shown & clear resemblance between some aspects of the two maturative processes. In the amphigonie egg bivalents become indistinet soon after the stage of highest contraction. In the parthenogenetie egg however the bivalents the stae of highest contraction. In the parthenogenetie egg however the bivalents separate and the diploid number of chromosomes is restored within the nuelear membrane. The second maturative division originates, in the partheogenetic egg a single diploid polar body. In the amphigoni egg the emission of two aploid polar bodies occurs in the normal way. In both ovogenetic processes the bivalents reach threfore the highest contraction stage before the «resting» stage. A delay in the beginning of the resting stage of the nucleus is shown therefore in the egg maturation of Aphids, which is not shown in most other Inscets orders. Such peeculiar behaiour of the chromosomes of the amphigonic egg of Aphids has probably permitted the establishment of endomeiosis in the parthenogenetie infividuals and consequently has brought about the genetie variability found in single parthenogenetic lines of Aphids. 30 parthenogenetic femals have been bred in two thermostats: 15 of them at a temperature of 18°C and 15 at 28°C, Light intensity. moisture and food supplies were kept constantly identieal in both thermostats. The eytological study of the embryos demostrated that endomeiosis occurs in the female individuals kept at 18°C and at 28°C normally ameiotie parthenogenesis occurs. Temperature influences therefore endomeiosis. This is the reason why meiotic stages in the maturation of Aphid's parthenogenetic eggs have been only irregularly observed by previous Authors. The presence of endomeiosis in a few embryos in individuals bred at 28°C shows on the other hand the existence of genotypes with favour endomeiosis in spite of the high temperature. Sclection for winged individuals and for apterous individuals respectively in lines originated from a single female, was obtaind in environmental constant conditions. Moreover selection for parthenogenetic individuals has been obtained in environment conditions which are favourable for the appearance of the sexual forms. The different results of selection demonstrte the existence of lines which are less able than others to produce sexual individuals. It is possible to select in the former lines for the appearance of parthenogenetic individuals even in environmental conditions which are not favourable for their development. Selection experiments in controlled environment have thus demonstrated the functionality of endomeiosis as a source of genetie varibility and the existence of multifactorial mechanisms for the determination of some characters. The importance of environmental factors influence populations having different multiple genotypes sven within single parthenogenetic lines. Therefore the individuals of such lines react differently in accordance with their different genotypes.

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