Paternal inheritance

Thus far, only a single angiosperm species has been found to inherit its plastids uniparentally paternally: the kiwi plant Actinidia deliciosa (Testolin and Cipriani 1997). By contrast, in gymnosperms, paternal inheritance (or biparental inheritance with a strong predominance of paternal transmission) seems to be widespread (Szmidt et al. 1987; Neale et al. 1989; Mogensen 1996). Distinction between purely paternal inheritance and biparental inheritance with a strongly prevailing paternal component has been difficult, because most studies on plastid inheritance in gymnosperms suffer from statistically limited datasets. This is due to the lack of suitable phenotypic markers (i.e. plastome mutations resulting in pigment deficiencies and, thus, providing visible markers) in most species analyzed to date, which restricts the assay of progeny plants to RFLP analysis employing phenotypically neutral polymorphisms in the paternal and maternal ptDNAs. Naturally, this limits the number of progeny seedlings that can be analyzed and makes it difficult to exclude maternal plastid transmission below a certain level (Hagemann 2004).

Electron microscopic investigations confirmed the absence of plastids from egg cells (and the presence of them in sperm cells) in gymnosperm species displaying paternal plastid inheritance. In analogy to the diverse cytological mechanisms leading to maternal plastid inheritance (Fig. 2), at least two distinct mechanisms can contribute to paternal inheritance: plastid exclusion by unequal organelle distribution during female gametophyte development and/or plastid degradation in the egg cell (Mogensen 1996; Hagemann 2004).

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