absence polymorphism can result in unpaired
(i.e., univalent) accessory chromosomes during
meiosis if the two parental strains have different chromosome complements (Fig. 2.2). How
are univalent accessory chromosomes transmitted during meiotic divisions? In animals
and plants, the widespread occurrence of accessory chromosomes is generally attributed to
their selfish mode of transmission (Houben
2017; Jones 2012), which increases their freFig. 2.2 Potential drive mechanisms for accessory chromosomes in plants and fungi. (a) Suggested mechanism
for a post-meiotic drive affecting the B chromosome
(chr.) in rye. The centromeres of the B chromosome are
not separating during the first pollen mitosis. Due to the
non-symmetric cell division the B chromosome sisterchromatids are more likely to become part of the generative nucleus of which the two sperm cells are generated in the second pollen mitosis. This mechanism leads
to an increase in the B chromosome frequency in the
progeny without additional DNA replication. (b & c)
Two possible mechanisms for a meiotic chromosome
drive in Z. tritici involving additional replication of
accessory chromosomes by the example of a paired
(i.e., homologs in both parental strains) accessory chromosome 14 and an unpaired chromosome 19. (b)
Unpaired accessory chromosomes undergo an additional round of DNA replication initiated after the pairing of homologs within the zygote suggesting an
additional feedback mechanism between pairing of
homologs and DNA replication. (c) Alternatively, additional DNA replication of unpaired chromosomes takes
place prior to meiosis affecting all accessory chromosomes. Since no additional copies of the paired accessory chromosomes are found, these must be lost during
the zygote stage where pairing occurs. Please note that
for the sake of clarity, the subsequent mitotic cell division resulting in eight ascospores is not displayed, and
recombination events were omitted. b & c adapted from
(Habig et al. 2018)
36
M. Habig and E. H. Stukenbrock
(i.e., univalent) accessory chromosomes during
meiosis if the two parental strains have different chromosome complements (Fig. 2.2). How
are univalent accessory chromosomes transmitted during meiotic divisions? In animals
and plants, the widespread occurrence of accessory chromosomes is generally attributed to
their selfish mode of transmission (Houben
2017; Jones 2012), which increases their freFig. 2.2 Potential drive mechanisms for accessory chromosomes in plants and fungi. (a) Suggested mechanism
for a post-meiotic drive affecting the B chromosome
(chr.) in rye. The centromeres of the B chromosome are
not separating during the first pollen mitosis. Due to the
non-symmetric cell division the B chromosome sisterchromatids are more likely to become part of the generative nucleus of which the two sperm cells are generated in the second pollen mitosis. This mechanism leads
to an increase in the B chromosome frequency in the
progeny without additional DNA replication. (b & c)
Two possible mechanisms for a meiotic chromosome
drive in Z. tritici involving additional replication of
accessory chromosomes by the example of a paired
(i.e., homologs in both parental strains) accessory chromosome 14 and an unpaired chromosome 19. (b)
Unpaired accessory chromosomes undergo an additional round of DNA replication initiated after the pairing of homologs within the zygote suggesting an
additional feedback mechanism between pairing of
homologs and DNA replication. (c) Alternatively, additional DNA replication of unpaired chromosomes takes
place prior to meiosis affecting all accessory chromosomes. Since no additional copies of the paired accessory chromosomes are found, these must be lost during
the zygote stage where pairing occurs. Please note that
for the sake of clarity, the subsequent mitotic cell division resulting in eight ascospores is not displayed, and
recombination events were omitted. b & c adapted from
(Habig et al. 2018)
36
M. Habig and E. H. Stukenbrock