Biology Reference
In-Depth Information
(
Autran et al., 2011
). In addition, it has been shown that loss of AGO9
function, which is expressed exclusively in the maternal tissues surrounding
the germ cells, results in the reactivation of transposable elements in the egg
(
Olmedo-Monfil et al., 2010
), suggesting that somatic tissues play an addi-
tional role in protecting the gametic genome.
4.4. Reproduction without sex
Sexual reproduction is crucial for the definition of embryonic fate. It is thus
all the more surprising that plants have also developed numerous alternatives
to biparental sexual reproduction, most of which allow the formation of
totipotent embryos with neither meiosis nor fertilization.
A particularly intriguing form of asexual reproduction is apomixis, which
refers to diverse mechanisms of reproduction resulting in clonal reproduc-
tion through seeds and that can be found in over 400 angiosperm species
(Reviewed in
Grimanelli, 2012; Rodriguez-Leal & Vielle-Calzada,
2012
). Apomictic plants bypass both meiotic reduction and egg fertilization,
developing parthenogenetic embryos that are genetic clones of their mother.
Interestingly, apomixis and sex are not mutually exclusive and can coexist in
the same species and even the same individual, which illustrates a high
degree of developmental plasticity. The mechanisms driving reproduction
toward either sexual or asexual reproduction remain unclear, but recent
results suggest a key role for chromatin in the process. In particular, loss-
of-function mutants of
AGO9
and of several RdDM components in
Ara-
bidopsis
phenocopy essential aspects of apomictic reproduction, producing
spores that skip meiosis altogether (
Olmedo-Monfil et al., 2010
). Similarly,
mutations in
AGO104
and
DMT103
, the maize homologue of
Arabidopsis
AGO9 and DRM2, respectively, produce ameiotic spores and gameto-
phytes, a key step toward apomictic development (
Singh et al., 2011
;
Garcia-Aguilar et al., 2010
). Interestingly, these mutations do not abolish
sexual reproduction, which coexists with alternative development in the
mutants. Rather, they seemingly endow the genome with added develop-
mental flexibility. Thus, establishment or maintenance of specific patterns of
non-CG methylation in the ovule is likely essential in defining reproductive
strategies in angiosperms. Because most of these genes are also involved in
reprogramming, these observations raise important questions regarding the
stability of the genome during asexual reproduction. Incidentally, an unsta-
ble epigenome during asexual reproduction might provide an interesting
explanation to the surprising ecological success of clonal reproduction in
plant, as an unexpected source of adaptive variation.
