Genetics - Theses

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End date: 1979 × Start date: 1966 × Type: PhD thesis × Subject: Chironomidae. ×

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    Parthenogenesis, chromosomal polymorphism and morphological variation in Chironomids
    Porter, David Laurence. (University of Melbourne, 1973)
    Parthenogenesis, in the forms of arrhenotoky, deuterotoky, or thelytoky, is a quite common phenomenon in the animal kingdom. Thelytoky, in which females produce exclusively female progeny in the absence of genetic fertilization, is the most widespread and most mechanistically diverse form of parthenogenesis. It does not follow that thelytoky is the most common ; arrhenotoky, which only occurs in one rotifer, one arachnid and four insect orders (Hartl 1971), is probably the most successful form of parthenogenesis in terms of numbers, due mainly to the presence of over 100,000 species of hymenopterans, the vast majority of which are arrhenotokous. Thelytoky itself is present in a wide variety of forms. The mechanism for the maintenance of thelytoky may be automictic, in which at least the first meiotic division is normal, the chromosomes pairing at prophase and forming bivalents. The zygoid phase is restored by the restitution of anaphase I or metaphase II chromosome plates, fusion of second division products or endomitosis in cleavage nuclei. Alternatively the mechanism may be apomictic, in which meiotic features may be partly or wholly absent, the one or two maturation divisions being equational. Thelytoky may be complete, it being the only manner of reproduction; or it may be cyclical, where it alternates either regularly, or under the influence of environmental factors, with amphimixis or arrhenotoky. Thelytoky may also be either facultative or obligatory. Facultative thelytoky is the situation whereby reproduction is normally bisexual, however a percentage of eggs may develop without fertilization. Obligatory thelytokous forms produce all their offspring without genetic fertilization, reproduction can therefore never be bisexual. There are many thelytokous forms in which the eggs require penetration by the sperm of the same or related species before they develop. In this case, gynogenesis, the sperm makes no chromosomal contribution to the embryo, although there is a cytoplasmic contribution which may have some effect. In this chapter there are two main areas to be examined. The first is to investigate the cytology of two members of the chironomid subfamily Chironominae, Lundstroemia parthenogenetica and Lauterbornia sp. , and to compare them to other thelytokous forms with analogous maturation mechanisms, especially the members of the subfamily Orthocladiinae described by Scholl (1956, 1960). The second area is the discussion of the evolution and properties of thelytokous organisms. Most review articles tend to be limited to a broad presentation of the magnitude of the phenomenon (Oliver 1971), but also order the paper from the point of view of mechanism or animal groupings. Here the discussion is ordered from the point of view of the importance of various phenomena in the evolution and maintenance of thelytokous forms, e.g. hybridization, genetic and ecological considerations. This may not be a more systematic mode of presentation, but it seems to me to be more logical.