A small number of angiosperms transmit their plastids biparentally. Working with Pelargonium, Mirabilis, Melandrium, Antirrhinum and Aquilegia, already Erwin Baur and Carl Correns noted in their first experiments on the inheritance of leaf variegations almost hundred years ago (Baur 1909, 1910; Correns 1909) that the mode of plastid inheritance may differ between species. While Melandrium, Antirrhinum and Aquilegia mutants transmitted their altered leaf color (which, as we now know, represented plastome mutations) purely maternally, similar traits could also be transmitted via pollen in Pelargonium (for review see Hagemann 2000). Baur concluded that the plastids (or the 'chromatophors', as they were called at that time) must be biparentally inherited in Pelargonium zonale. Later, other examples of species with biparental chloroplast inheritance were found (Table 2), including Oenothera (evening primrose), Hypericum (St. John's wort), and Medi-cago (alfalfa).
Extensive genetic work has determined the relative contributions of maternal and paternal plastids to the organelle population in the progeny in these species and revealed striking differences. In Oenothera and Hypericum, the rate of paternal transmission is relatively low, as evidenced by reciprocal crosses between white plastome mutants and green wild type plants. When the plastome mutant served as maternal parent, many white and variegated seedlings were obtained, but almost no green progeny. In contrast, when the plastome mutant was the paternal parent (i.e. the pollen donor), most F1 seedlings were uniformly green or variegated and only very few were white. In Pelargonium and Medicago, the paternal contributions are much greater. Whereas in Pelargonium, sperm and egg seem to make about equal plastid contributions to the zygote, paternal plastids are even predominantly inherited in alfalfa (Shi et al. 1991; Hagemann 2002).
Cytological investigations confirmed that, as expected, biparental plastid inheritance correlates with (i) the distribution of microspore plastids between vegetative cell and generative cell during the first pollen mitosis, (ii) the regular presence of viable plastids in sperm cells and (iii) their entry into the zygote.
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