Title イネの内乳における生殖隔離機構の解析 Author(s - 金沢大学

Title
イネの内乳における生殖隔離機構の解析
Author(s)
関根, 大輔
Citation
要旨以下に掲載：The Plant Journal 76(5) pp.792-799.共著者
：Daisuke Sekine, Takayuki Ohnishi, Hiroyasu Furuumi, Akemi Ono,
Toshihiro Yamada, Nori Kurata, Tetsu Kinoshita
Issue Date
2014-03-22
Type
Thesis or Dissertation
Text version
none
URL
http://hdl.handle.net/2297/38990
Right
学位授与機関
金沢大学
学位の種類
博士（理学）
学位授与年月日
2014年3月22日
学位授与番号
甲第4063号
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http://dspace.lib.kanazawa-u.ac.jp/dspace/
Post-zygotic hybridization barriers
in rice endosperm
(イネの内乳における生殖的隔離機構の解析) 金沢大学大学院自然科学研究科
生命科学専攻
生物多様性動態学講座
氏名関根大輔
Summary
Abnormal development of the endosperm in response to hybridization is widely
recognized as a major post-zygotic hybridization barrier, which might hold the key to
Darwin’s “abominable mystery” of the rapid diversification of angiosperms. In
Arabidopsis species, for example, hybrid endosperm from interspecific and interploidy
crosses share the common features of altered timing of cellularization and altered rate of
nuclear divisions. However, the hybridization barrier may be initiated by different
intrinsic cues in these two types of cross. The interspecific cross barrier arises after
hybridization of genomes with differences in DNA sequences, while the interploidy
cross barrier arises after hybridization of genomes with differences in ploidy levels. To
identify similarities and differences between the two types of cross, I focus on the
outcomes of interploidy crosses between rice (Oryza) species. In the cross
autotetraploid (4n) x diploid (2n) plants, I found precocious cellularization and a
decreased rate of nuclear division in the syncytial endosperm. By contrast, seeds from
the reciprocal cross showed delayed cellularization and an increased rate of nuclear
division. These effects on nuclear division rates contrast with the outcome of rice
interspecific crosses, which were previously shown to have altered timing of
cellularization without any change in nuclear division rates. Thus, I propose that the
posy-zygotic hybridization barrier in rice endosperm has two components, namely
control of the timing of cellularization and control of the nuclear division rates in the
endosperm development.
Back ground
Angiosperms are the largest and more diverse group of land plants. Although
angiosperms have evolved relatively ‘recent’ in the land plant groups, rapid
diversification and ecological radiation took place in short term. The sudden rise of
angiosperms was referred to Charles Darwin as ‘an abominable mystery’, as the
underlying mechanisms might have remained enigmatic. To address to this question, it
is required to understand the mechanism of speciation including establishment of
reproductive barriers in angiosperms. Abnormal development of the endosperm caused
by hybridization between different species or with different levels of ploidy is
recognized as a type of post-zygotic reproductive barrier. Since the endosperm is a
specific characteristic of angiosperms in the land plant groups, unraveling of the
endosperm hybridization barrier may enhance our understanding of mechanisms that
drove rapid diversification in angiosperms. During early endosperm development, the
primary endosperm nucleus initiates several rounds of synchronous division in the
absence of cell wall synthesis and cytokinesis to form a syncytium; this period of
synchronous nuclear division is followed by cellularization. In Arabidopsis, altered
timing of cellularization and change to mitotic activity were observed in interploidy and
interspecific crosses. Although the both of two types cross can share common outcomes,
the triggers that induce the hybridization barrier are not necessarily the same. The plants
used for interploidy crosses are isogenic and only differ with respect to ploidy levels,
while plants used for interspecific crosses inevitably show clear differences in their
genomic sequences. However, in the genus Arabidopsis, interspecific crosses usually
involve differences in both chromosome number and genome size. To avoid the
complication of multiple variables in the effects of ploidy levels compared to genomic
sequence differences, I focus on Oryza species that have the same chromosome number
and similar genome sizes. In this thesis, I investigated the effects of ploidy levels on
endosperm development using diploid and autotetraploid Oryza sativa to compare the
previous results of hybrid endosperm development in Oryza species.
Result
To investigate the effect of varying maternal or
same ploidy
paternal genome dosage, I analyzed the
endosperm development in the crossed seeds
syncytium
cell division
starch granules
between diploid (2n) and autotetraploid (4n)
rice. Seeds with a maternal genome excess (4n x
cellularization
protein bodies
maternal excess
2n) showed precocious cellularization and a
decreased rate of mitotic divisions. By contrast,
syncytium
cellularization
seeds with a paternal genome excess (2n x 4n),
showed a lack of cellularization and an
cell division
starch granules
protein bodies
paternal excess
increased rate of mitotic divisions during the
early
stages
of
endosperm
development.
Accumulation of storage compounds, such as
starch granules and protein bodies, were more
pronounced in 4n x 2n seeds than those from
selfed plants, although the beginning of
accumulation was similar in both groups. By
contrast, no storage compounds were detected in
seeds from the 2n x 4n cross. Therefore, these
results indicate that maternal and paternal
genome excess have opposite effects on
endosperm development.
syncytium
starch granules
protein bodies
Figure Parent-of-origin-effect in rice interploidy
crosses
Seeds with a maternal genome excess showed
precocious cellularization and a decreased rate of
mitotic divisions. Accumulation of storage
compounds was more pronounced in maternal excess
seeds than in seeds from same ploidy cross. By
contrast, seeds with a paternal genome excess,
showed a lack of cellularization and an increased rate
of mitotic divisions during the early stages of
endosperm
development.
Moreover,
storage
compound was not detectable in seeds from the
paternal excess cross.
Discussion
In this thesis, I revealed the similarities and differences of developmental
abnormality on endosperm between interploidy and interspecific crosses. The
effects on timing of cellularization in the reciprocal interploidy crosses are similar to
those found previously for interspecies crosses. By contrast, the effects on the mitotic
activity on syncytium endosperm in the reciprocal interploidy crosses differ from those
of interspecific crosses, which showed the similar rates of nuclear divisions in each
hybrid endosperm. In the results of Arabidopsis, both interploidy and interspecific
crosses display altered timing of cellularization with abnormal mitotic activity. Hence,
although the endosperm developmental characteristics in interploidy and interspecific
crosses are similar in A. thaliana, those in rice interploidy and interspecies crosses are
different. The interspecies difference may be related to the intrinsic differences in the
endosperm development between a eudicot, A. thaliana, and a monocot, O. sativa.
Alternatively, it is also possible that interspecies differences result from extra factors of
Arabidopsis species. In the genus Arabidopsis, interspecific crosses usually involve
differences in both chromosome number and genome size. By contrast, Oryza species
have the same chromosome number and similar genome sizes. Therefore, the
complication of multiple variables in Arabidopsis species may cause differences
compared to Oryza species.

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