may or may not have different chromosome numbers. The zygote resulting from this
hybridization event contains a complete 1n set of chromosomes from each parent,
and while the organism produced may survive and even thrive, it will be sterile due
to the fact that its chromosomes do not have true homologs with which to interact at
meiosis (e.g., a mule, the cross between a donkey and a horse, is viable yet sterile).
However, on rare occasion, the hybrid cell may replicate its chromosomes during S
phase and start a mitotic division only to have the division stall out and fail prior to
cytokinesis. In such “restitution nuclei,” the sister chromatids come apart to form
sister chromosomes (which then function as homologous chromosomes), and the
cell chromosome number doubles. If the cell or one of its daughter cells enters
meiosis, each chromosome will now have a homolog with which to pair, synapse,
and exchange DNA via crossing over, which may give this polyploidy the ability to
produce viable gametes. If the two parental species are closely related, there may be
issues in meiosis where homologs and their homeologs may form structures other
than bivalents, leading to full or partial infertility. Allopolyploidy is common in the
evolutionary histories of most plants.
2.10 Autopolyploidy
Sometimes a plant cell that is functionally diploid will have a failure in its mitotic
division and produce a restitution nucleus. The resulting cell will have four sets of
homologous chromosomes. If the cell or one of its daughter cells enters meiosis, it is
likely that a given group of four homologs won’t form bivalents but rather quadrivalents (four homologs that synapse together in full or part) or a trivalent (three
synapsed homologs) and a univalent (an unpaired chromosome). The result is
unbalanced “aneuploid” gametes (aneuploidy is the state of having more or less
than a full complement of chromosomes). For this reason, autopolyploids are
generally sterile, at least at first.
2.11 Diploidization
Autopolyploids are almost always sterile upon formation, and allopolyploids may
have issues that prevent them from producing balanced gametes. However, over
time, selective pressure on infertile polyploids may result in evolution of genetic
mechanisms that facilitate production of balanced gametes. The phenomenon where
a polyploid organism starts behaving like a diploid during meiosis is known as
diploidization. There are a number of ways in which a polyploid can develop
diploid-like behavior and regain fertility including divergence of homoeologous
genomes, development of genes that facilitate bivalent formation, and development
of mechanisms that limit crossing over to termini of multivalents. Dodsworth et al.
122
D. G. Peterson and M. Arick
hybridization event contains a complete 1n set of chromosomes from each parent,
and while the organism produced may survive and even thrive, it will be sterile due
to the fact that its chromosomes do not have true homologs with which to interact at
meiosis (e.g., a mule, the cross between a donkey and a horse, is viable yet sterile).
However, on rare occasion, the hybrid cell may replicate its chromosomes during S
phase and start a mitotic division only to have the division stall out and fail prior to
cytokinesis. In such “restitution nuclei,” the sister chromatids come apart to form
sister chromosomes (which then function as homologous chromosomes), and the
cell chromosome number doubles. If the cell or one of its daughter cells enters
meiosis, each chromosome will now have a homolog with which to pair, synapse,
and exchange DNA via crossing over, which may give this polyploidy the ability to
produce viable gametes. If the two parental species are closely related, there may be
issues in meiosis where homologs and their homeologs may form structures other
than bivalents, leading to full or partial infertility. Allopolyploidy is common in the
evolutionary histories of most plants.
2.10 Autopolyploidy
Sometimes a plant cell that is functionally diploid will have a failure in its mitotic
division and produce a restitution nucleus. The resulting cell will have four sets of
homologous chromosomes. If the cell or one of its daughter cells enters meiosis, it is
likely that a given group of four homologs won’t form bivalents but rather quadrivalents (four homologs that synapse together in full or part) or a trivalent (three
synapsed homologs) and a univalent (an unpaired chromosome). The result is
unbalanced “aneuploid” gametes (aneuploidy is the state of having more or less
than a full complement of chromosomes). For this reason, autopolyploids are
generally sterile, at least at first.
2.11 Diploidization
Autopolyploids are almost always sterile upon formation, and allopolyploids may
have issues that prevent them from producing balanced gametes. However, over
time, selective pressure on infertile polyploids may result in evolution of genetic
mechanisms that facilitate production of balanced gametes. The phenomenon where
a polyploid organism starts behaving like a diploid during meiosis is known as
diploidization. There are a number of ways in which a polyploid can develop
diploid-like behavior and regain fertility including divergence of homoeologous
genomes, development of genes that facilitate bivalent formation, and development
of mechanisms that limit crossing over to termini of multivalents. Dodsworth et al.
122
D. G. Peterson and M. Arick