DNA-containing organelles (plastids and mitochondria) are inherited in a non-Mendelian fashion in all eukaryotes. In most organisms, organellar genomes are inherited from only one parent, with maternal inheritance being much more widespread than paternal inheritance. Plastids and their DNA can be inherited maternally, paternally or biparentally (reviewed, e.g., in Mogensen 1996; Birky Jr 1995; Hagemann 2002). At least in higher plants, plastid genomes do not normally undergo sexual recombination, even when they are inherited biparentally. This means that, except in very rare cases (which may be considered accidents; Medgyesy et al. 1985; Thanh and Medgyesy 1989; Baldev et al. 1998), chloroplast fusion and genetic recombination do not occur (an exception is the green alga Chlamydomonas reinhardtii; see 4.1.4).
Uniparentally maternal plastid inheritance has long been considered the rule, although the first exception (Pelargonium as a species with biparental inheritance; Baur 1909) was discovered simultaneously with the rule (maternal inheritance of plastids in several angiosperm species; Correns 1909; Baur 1910). Although it is still true that the majority of flowering plants transmit their plastids uniparentally from the female parent to the progeny, exceptions are found in nearly all major lineages of plant evolution (Mogensen 1996; Birky Jr 1995). This suggests that maternal inheritance, as the presumably ancestral mode of plastid transmission has been broken many times independently in plant evolution (Birky Jr 1995).
While the different modes of plastid inheritance (maternal, paternal, biparental) are cytologically reasonably well described (see below), the characterization of the molecular mechanisms underlying plastid inheritance is still in its infancy. While in Chlamydomonas, a model alga in which plastid inheritance is genetically tractable (but mechanistically very different from higher plants; see 4.1.4), some of the molecular components involved in maternal inheritance have been identified, next to nothing is known about the factors involved in the various modes of plastid inheritance existing in flowering plants. Plastid transmission is very likely controlled by nuclear genes (Tilney-Bassett 1984, 1994), but to date, not a single gene involved in plastid inheritance has been identified in any higher plant.
Plastid exclusion during fertilization
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