Download Meiosis in flowering plants and other green

Journal of Experimental Botany, Vol. 61, No. 11, pp. 2863–2875, 2010
doi:10.1093/jxb/erq191
FLOWERING NEWSLETTER REVIEW
Meiosis in flowering plants and other green organisms
C. Jill Harrison*, Elizabeth Alvey and Ian R. Henderson*
Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EA, UK
* To whom correspondence should be addressed: E-mail: [email protected] and [email protected]
Received 17 February 2010; Revised 21 May 2010; Accepted 25 May 2010
Abstract
Sexual eukaryotes generate gametes using a specialized cell division called meiosis that serves both to halve the
number of chromosomes and to reshuffle genetic variation present in the parent. The nature and mechanism of the
meiotic cell division in plants and its effect on genetic variation are reviewed here. As flowers are the site of meiosis
and fertilization in angiosperms, meiotic control will be considered within this developmental context. Finally, we
review what is known about the control of meiosis in green algae and non-flowering land plants and discuss
evolutionary transitions relating to meiosis that have occurred in the lineages giving rise to the angiosperms.
Key words: Meiosis, organogenesis, recombination, sex, sporangia.
Introduction
Man has selected from natural genetic variation to breed
crop species useful for agriculture. Considerable genetic
diversity remains to be fully exploited, including variation
capable of increasing yield and broadening crop range.
Sustainable increases in agricultural yield in the range of
50% will be required to feed the 9 billion people estimated to
exist by 2050 (The Royal Society, 2009). To increase the
efficiency of crop breeding, it is important to understand the
mechanism by which variation is generated and transmitted.
Variation is generated in a specialized, reductive cell division termed meiosis during which recombination between
homologous chromosomes, termed crossover (CO), occurs.
CO frequency varies within and between species and can be
limiting such that useful variation is not accessible for
breeding. Although the majority of crops are angiosperms
in which meiosis occurs within floral organs, non-flowering
plants have provided novel insights into the control of plant
meiosis. The nature of the meiotic cell division in plants, the
genetic mechanisms that promote variation, and the developmental context in which meiosis occurs in land plants
and their closest sister groups are reviewed here.
Meiotic cell divisions
Meiosis is a mode of cell division specific to eukaryotic
organisms whereby four haploid daughter cells are produced
from a single diploid parent cell (Villeneuve and Hillers,
2001; Hamant et al., 2006; Mezard et al., 2007). This
reductive division is achieved by a single round of DNA
replication followed by two rounds of chromosome segregation and cell division (meiosis-I and meiosis-II) (Fig. 1).
Meiosis appears to be an ancestral trait within eukaryotes
and is speculated to have arisen close to the group’s origin
(Villeneuve and Hillers, 2001; Cavalier-Smith, 2002). This
idea is strengthened by observations that many unicellular
eukaryotes, once presumed to be asexual, have since been
found to possess conserved meiosis-specific genes (Ramesh
et al., 2005; Malik et al., 2007). The first meiotic division
differs dramatically from mitosis as homologous chromosomes pair before segregation. During the second division
sister chromatids segregate to opposite poles in the same
manner as during mitosis. While homologues are paired
during meiosis-I they are tightly associated via the synaptonemal complex that forms along their length (Page and
Hawley, 2004). Meiosis-specific expression of an endonuclease, SPO11, causes a large number of double strand
breaks (DSBs) along the paired chromosomes (Villeneuve
and Hillers, 2001). A subset of these break sites are repaired
through recombination pathways that lead to physical
exchange between the paired chromosomes (CO) (Villeneuve
and Hillers, 2001; Hamant et al., 2006; Mezard et al., 2007).
As paired chromosomes segregate during meiosis-I, CO sites
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2864 | Harrison et al.
can be visualized cytologically as chiasmata (Armstrong
et al., 2009). Independent segregation of maternal and
paternal chromosome sets during meiosis in combination
with CO between chromosomes means that gametes are
likely to possess novel combinations of genetic variation
(Fig. 1).
Programmed DNA breakage and repair
Fig. 1. Chromosome inheritance during meiosis. (A) Maternally
(white) and paternally (black) inherited copies of two chromosome
pairs are shown in a diploid parental cell. (B) During S-phase of
meiosis-I each chromosome is replicated. (C) Maternally and
paternally inherited pairs of homologous chromosomes physically
associate during prophase of meiosis-I. Coincident with pairing,
a large number of DNA double-strand breaks (shown as short
vertical lines) are induced by the SPO11 endonuclease. (D) At the
end of meiosis-I spindle microtubules attach to the centromeres of
paired bivalents and recombination events are completed.
(E) Members of homologous chromosome pairs segregate to
opposite cell poles. (F) Meiosis-I is completed with equal numbers
of replicated, recombined chromosomes in daughter cells.
(G) During meiosis-II cohesion is lost between replicated chromatids, which segregate to opposite cell poles. (H) Four haploid
daughter cells are produced with novel combinations of maternally
and paternally inherited genetic information.
An essential first step in achieving CO is the generation of
DSBs throughout the genome by SPO11. SPO11 is related
to the A subunit of archaebacterial topioisomerase IV,
which acts to relieve torsion in DNA by generating
transient DSBs (Villeneuve and Hillers, 2001; CavalierSmith, 2002). Hence, components of the meiotic recombination machinery may have been recruited from prokaryotic
DNA repair mechanisms. Two related orthologues SPO111 and SPO11-2 are non-redundantly required for meiotic
DSBs in Arabidopsis thaliana (Grelon et al., 2001; Stacey
et al., 2006; Hartung et al., 2007). The spo11-1 and spo11-2
mutants lack meiotic DSBs and show an absence of
homologue pairing and synapsis, meaning that univalent
chromosomes segregate at meiosis-I (Grelon et al., 2001;
Stacey et al., 2006; Hartung et al., 2007). As univalent
chromosomes segregate independently from their homologous partner, spo11 mutants show a high incidence of
chromosomally unbalanced gametes (Grelon et al., 2001;
Stacey et al., 2006; Hartung et al., 2007). The PUTATIVE
RECOMBINATION INITIATION DEFECTS1 (PRD1),
PRD2 and PRD3 genes are also necessary for SPO11dependent DSB formation and cause univalent segregation
when mutated (De Muyt et al., 2007, 2009). PRD1 and
PRD3 share sequence similarity with known meiotic
proteins, mammalian Mei1 and rice PAIR1, respectively,
but perform unknown functions during DSB formation
(Libby et al., 2002; Nonomura et al., 2004; De Muyt et al.,
2007, 2009).
A second functional class of genes, including MND1,
AHP2, RAD50, RAD51C, XRCC3, MRE11, and COM1/
SAE2, share a mutant phenotype of meiotic chromosome
fragmentation without synapsis, which is suppressed when
combined with spo11 or prd3. (Schommer et al., 2003;
Bleuyard et al., 2004; Bleuyard and White, 2004; Puizina
et al., 2004; Li et al., 2004; Kerzendorfer et al., 2006; Panoli
et al., 2006; Uanschou et al., 2007; Vignard et al., 2007).
This indicates that these proteins act to process DSBs
during meiotic recombination, and unrepaired DSBs cause
chromosome fragmentation. Meiotic DSB processing proceeds via break site resection to form 3’ single-stranded
DNA, which is used to invade the intact duplex of a paired
homologous chromosome (Bhatt et al., 2001; Villeneuve
and Hillers, 2001; Hamant et al., 2006; Mezard et al., 2007).
Invasion of DNA duplexes by ssDNA requires the DMC1
and RAD51 recombinases, which are related to prokaryotic
RecA DNA strand exchange proteins (Couteau et al., 1999;
Masson and West, 2001; Villeneuve and Hillers, 2001;
Cavalier-Smith, 2002; Li et al., 2004). Though RAD51
Flowering and meiosis | 2865
and DMC1 are both thought to act at the step of
ssDNA invasion, rad51 mutants show SPO11-dependent
chromosome fragmentation, whereas dmc1 mutants show
univalent segregation (Couteau et al., 1999; Li et al., 2004).
One explanation for this difference may be that DSBs are
repaired using sister chromatids in dmc1, but not in rad51
(Couteau et al., 1999; Li et al., 2004).
DSBs lead to CO via formation of a double Holliday
junction (dHJ) between paired homologous DNA duplexes
(Villeneuve and Hillers, 2001). Subsequent to strand invasion, a series of events are required for dHJ formation,
including second end capture and DNA synthesis and
ligation (Villeneuve and Hillers, 2001). These junctions are
ultimately resolved into CO events via an unknown dHJ
resolvase in plants, although the structure-specific endonucleases GEN1/Yen1 and SLX4 serve this function in animals
(Svendsen and Harper, 2010). An excess of DSBs are
generated by SPO11 relative to the number of CO events
ultimately observed. The remaining DSBs are repaired via
a gene conversion (also known as a non-crossover, NCO)
pathway, which does not involve the exchange of flanking
genetic markers (Villeneuve and Hillers, 2001). The synthesis-dependent strand annealing pathway appears to be
a major mechanism for NCO formation in Saccharomyces
cerevisiae, which acts independently of dHJ formation
(McMahill et al., 2007). In S.cerevisiae the decision to
repair DSB sites as CO or NCO events is made early in
prophase-I, prior to dHJ formation, so it is unlikely that the
CO/NCO choice represents alternative processing of dHJs
(Allers and Lichten, 2001; Borner et al., 2004).
Control of meiotic crossover frequency
The position of COs can be influenced by other COs on the
same chromosome via a phenomenon known as interference. In interference one event inhibits the formation of
adjacent events in a distance-dependent manner meaning
that they are more widely distributed than expected at
random (Sturtevant, 1915; Muller, 1916; Copenhaver et al.,
2002). However, non-interfering COs that have a random
distribution also occur (Copenhaver et al., 2002). In A.
thaliana class I interfering COs are the majority (;75–85%)
and class II non-interfering COs the minority (;15–25%)
(Copenhaver et al., 2002; Higgins et al., 2004; Mercier et al.,
2005; Berchowitz et al., 2007; Higgins et al., 2008a). Class I
COs in A. thaliana require a set of genes including MSH4,
MSH5, MLH3, MER3/ROCK-N-ROLLERS, PARTING
DANCERS, ZIP4/SPO22, RPA1a and SHOC1 (Higgins
et al., 2004, 2008b; Chen et al., 2005; Mercier et al., 2005;
Jackson et al., 2006; Wijeratne et al., 2006; Chelysheva
et al., 2007; Macaisne et al., 2008; Osman et al., 2009).
Knockout of these genes results in a dramatic reduction in
CO frequency and the events that remain are randomly
distributed. The ancestry of meiotic proteins in prokaryotic
DNA repair is again evident as MSH4 and MSH5 proteins
are related to bacterial MutS mismatch repair proteins
(Villeneuve and Hillers, 2001; Cavalier-Smith, 2002). Class
II non-interfering COs require the MUS81 gene, which
encodes a protein similar to structure-specific endonucleases
(Berchowitz et al., 2007; Higgins et al., 2008a).
Paired chromosomes generally show at least one CO
event, the obligate CO, that is required for proper patterns
of chromosome segregation in A. thaliana (Grelon et al.,
2001). Each A. thaliana chromosome shows approximately
1.8 CO per meiosis and very few chromosomes show no CO
(Copenhaver et al., 1998, 2002; Higgins et al., 2004;
Drouaud et al., 2006, 2007). CO position is highly variable
and hot- and cold-spots of CO frequency exist along the
chromosomes (Copenhaver et al., 1998, 1999; Drouaud
et al., 2006, 2007). Pronounced increases in CO frequency
and gene density are observed towards the telomeres of
wheat and maize chromosomes (Liu et al., 2009; Saintenac
et al., 2009; Schnable et al., 2009). A recombination hotspot
at the maize Bronze (Bz) locus lies in a gene-rich region
close to the telomere. This region shows 40–80-fold higher
CO rates than the genome average and is flanked by large
stretches of nested retrotransposon insertions that infrequently crossover (Dooner and Martinez-Ferez, 1997; Fu
et al., 2001, 2002). Repetitive regions flanking centromeres
are also suppressed for CO, and centromere-proximal
events associate with chromosome mis-segregation (Koehler
et al., 1996; Lamb et al., 1997; Deng and Lin, 2002;
Rockmill et al., 2006).
Differences in crossover frequency may be accounted for
by epigenetic information. For example in mouse and S.
cerevisiae H3 K4 trimethylation marks DSB hotspots, and
disruption of this modification reduces DSBs (Borde et al.,
2009; Buard et al., 2009). Conversely, in A. thaliana the COcold repetitive sequences flanking the centromere are
transcriptionally silenced using epigenetic information including DNA cytosine methylation and targeted DNA
methylation in Ascobolous immersus is sufficient to repress
CO frequency several hundred fold (Maloisel and Rossignol, 1998; Zhang et al., 2006; Zilberman et al., 2007).
Repetitive insertions and inversions can also locally suppress CO, and play an important functional role in genome
organization (Dooner, 1986; Nacry et al., 1998). This is
illustrated by CO suppression at sex chromosomes, matingtype loci and self-incompatibility loci, where it is important
to maintain linkage between genes required for opposite
mating/incompatibility types (Ferris and Goodenough,
1994; Casselman et al., 2000; Ming and Moore, 2007;
Bergero and Charlesworth, 2009). Hence, CO frequency is
likely to be determined by a combination of local DNA
sequence, trans-factors, and epigenetic information.
Control of meiotic cell cycle progression
Meiosis requires the modification of mitotic cell cycle
control, such that a single S-phase is followed by two
sequential rounds of chromosome segregation. Progression
through the cell cycle is controlled by cyclins that interact
with and activate cyclin-dependent kinases (CDKs) which
mediate stepwise transitions through the cycle via phosphorylation (Huntley and Murray, 1999). Several genes
2866 | Harrison et al.
implicated in the regulation of meiotic progression have
been identified in A. thaliana. A novel, cyclin-like gene
SOLO DANCERS (SDS) is specifically expressed during
meiosis and sds mutants show defects in chromosome
pairing, segregation, and CO (Azumi et al., 2002). Recently,
sds mutants have been observed to form DSBs but repair
them efficiently, most likely via RAD51-mediated intersister repair (De Muyt et al., 2009). The novel gene
OMISSION OF SECOND DIVISION1 (OSD1) is required
for meiosis-II and osd1 produces diploid dyad products of
meiosis instead of haploid tetrads (d’Erfurth et al., 2009). A
temperature-sensitive substitution allele of CYCLINA1;2,
termed tardy asynchronous meiosis1 (tam1) causes a delay in
meiotic progression, which also leads to dyad formation
(Magnard et al., 2001; Wang et al., 2004). Regulation of
protein stability is critical for cell cycle control and SKP1like F-box proteins act to promote ubiquitination and
destruction of target proteins, including cyclins (Bai et al.,
1996). Consistently, mutations in the SKP1-related gene
ASK1 show defects in meiotic chromosome segregation
(Yang et al., 1999a; Zhao et al., 2006). In animals, cell cycle
checkpoints cause later events to depend upon the completion of earlier events (Murakami and Nurse, 1999). Meiotic
checkpoint mechanisms have not been genetically defined in
A. thaliana, although msh4 and asy1 show a significant
delay in meiosis, suggesting feedback on the regulation of
meiotic progression (Higgins et al., 2004; Sanchez-Moran
et al., 2007). Together these results demonstrate that defects
in cell cycle progression can disrupt meiosis.
Key early events in meiosis are the identification of
homologous chromosome partners, pairing, and formation
of the synaptonemal complex (SC) (Page and Hawley,
2004). Homologous partner identification occurs by unknown mechanisms during prophase-I. In polyploid species
pairing is complex as homologues must also avoid pairing
with related homeologues (Martinez-Perez et al., 1999). For
instance, in hexaploid wheat Ph1 affects the stringency of
homologue/homeologue discrimination by influencing chromatin remodelling associated with pairing and localization
of the SC component ASY1 (Martinez-Perez et al., 1999;
Prieto et al., 2004, 2005; Boden et al., 2009). Ph1 maps to
a repetitive locus containing cell cycle-dependent kinase
genes, which causes down-regulation of unlinked CDK
genes (Griffiths et al., 2006; Al-Kaff et al., 2008). As related
CDK genes in mammals influence meiotic progression,
trans-silencing of CDKs by Ph1 could lead to changes in
homologue pairing (Ortega et al., 2003). Pairing may
depend on DSB formation as in A. thaliana or may occur
via achiasmate mechanisms as in female Drosophila melanogaster (Grelon et al., 2001; Page and Hawley, 2004).
SWITCH1/DYAD encodes a novel protein expressed during
prophase-I, necessary for chromosome pairing, synapsis, and
recombination and in swi1/dyad mutants univalents segregate
at meiosis-I. Studies of a maize homologue, AMEIOTIC1
indicate that these functions are conserved within angiosperms (Golubovskaya et al., 1993; Mercier et al., 2001, 2003;
Agashe et al., 2002; Ravi et al., 2008; Pawlowski et al., 2009).
POOR HOMOLOGOUS SYNAPSIS1 encodes a second
novel protein required for chromosome pairing and synapsis,
which causes high levels of non-homologous pairing when
mutated (Pawlowski et al., 2004; Ronceret et al., 2009).
Co-incident with pairing, the SC forms between homologous chromosomes (Page and Hawley, 2004) and SC
components identified in A. thaliana include ASYNAPTIC1
(ASY1), ZYP1A, ZYP1B, SCC3, and REC8/DIF1/SYN1
(Bai et al., 1999; Bhatt et al., 1999; Caryl et al., 2000;
Armstrong et al., 2002; Cai et al., 2003; Chelysheva et al.,
2005; Higgins et al., 2005). ASY1 and ZYP1 show distant
identity with the animal HOP1 and ZIP1 SC proteins,
respectively (Caryl et al., 2000; Armstrong et al., 2002;
Higgins et al., 2005). Loss of SC proteins causes failures in
synapsis and CO formation, and can lead to univalent
segregation and chromosome fragmentation (Bai et al.,
1999; Bhatt et al., 1999; Caryl et al., 2000; Armstrong
et al., 2002; Cai et al., 2003; Chelysheva et al., 2005; Higgins
et al., 2005). Interestingly, increases in CO frequency are
associated with increases in total SC length via an unknown
mechanism (Lynn et al., 2002; Drouaud et al., 2007). These
genetic mechanisms provide novel insights into the interrelated processes of homologue recognition, pairing, and
synapsis in plants.
Correct patterns of chromosome segregation are required
to generate balanced gametes and depend on regulation of
the SC and chromosome cohesion. During mitosis the
cohesin complex holds sister chromatids together until the
SCC1 subunit is cleaved by SEPERASE1 at anaphase,
allowing chromosome segregation (Uhlmann et al., 1999).
REC8/DIF1/SYN1 is a meiosis-specific orthologue of SCC1
and the rec8/dif1/syn1 mutant disrupts normal SC localization of SCC3 (a cohesin subunit shared with mitosis) (Bhatt
et al., 1999; Cai et al., 2003; Chelysheva et al., 2005). As in
animal systems, A. thaliana REC8 is cleaved by the cysteine
protease SEPERASE1 (ESP1) (Liu and Makaroff, 2006).
Cohesion in the chromosome arms is released by ESP1 at
anaphase-I, but maintained at the centromeres until
anaphase-II (Liu and Makaroff, 2006). In maize, centromeric REC8/AFD1 is protected from ESP1 destruction by
the conserved Shugoshin protein (SGO1) during anaphase-I
(Hamant et al., 2005; Watanabe, 2005). SGO1 is then
removed and REC8 is destroyed at the centromere during
anaphase-II to allow chromatid segregation. Step-wise
formation and removal of connections between homologues
is thus required for correct patterns of meiotic chromosome
segregation and recombination.
The developmental context for meiosis
Successful completion of meiosis and sexual life cycles
depends on activation of the meiotic cell cycle at the correct
developmental time and place. In many animals separation
of a dedicated germ cell lineage from somatic cell types
occurs during embryogenesis, a distinction not observed in
some early diverging animal lineages and plants (Gilbert,
1994; Dickinson and Grant-Downton, 2009). In the algal
sister groups to land plants single-celled zygotes undergo
Flowering and meiosis | 2867
meiosis immediately following fertilization (Fig. 2A). By
contrast, in all land plants mitotic divisions intercede
fertilization and meiosis, and meiosis occurs after a period
of diploid development in specialized structures termed
sporangia that produce numerous spores (Bower, 1935;
Becker and Marin, 2009). The initiation of sporangium
development pathways often follows a switch in meristem
identity from a vegetative to a reproductive fate (Steeves
and Sussex, 1989). The location and structure of sporangia
vary by plant group and are associated with their secondary
functions, which are spore dispersal and nutrition. Heteromorphic sporangia and spores have evolved convergently in
vascular plants and associate with specialized functions
(Fig. 2). Female megasporangia have fewer, larger spores
(megaspores) that may be retained within the parent plant
after fertilization, whereas male microsporangia develop
numerous small spores (microspores) that have a dispersal
function (Bower, 1935). Gender can also influence patterns
of CO frequency (Drouaud et al., 2007). Thus the initiation
and progression of meiosis depend on the developmental
identity of the tissue in which it is activated, discussed by
plant group below.
Meiosis in chlorophyte and charophyte algae
In both chlorophyte and charophyte algal sister groups to
the land plants meiosis occurs immediately following
fertilization (Becker and Marin, 2009). In the single-celled
chlorophyte alga, Chlamydomonas reinhardtii two haploid
mating types, plus and minus, differentiate into gametes
which fuse during fertilization to form a single-celled zygote
Fig. 2. Phylogenetic distribution of characters associated with meiosis in plants. (A) Phylogenetic relationships between major plant groups
showing synapomorphies associated with meiosis. Heterospory has a polyphyletic origin in lycophytes, monilophytes, and spermatophytes,
indicated by asterisks. (B) Reproductive structures of representatives of clades illustrated in (A), and extinct protracheophyte fossil forms.
1. Electron micrograph of Chlamydomonas monoica zygospores (photograph courtesy of Professor Karen P Van Winkle-Swift and by kind
permission of John Wiley and Sons: see VanWinkle-Swift KP, Rickoll WL. 1997. The zygospore wall of Chlamydomonas monoica
(Chlorophyceae): Morphogenesis and evidence for the presence of sporopollenin1. Journal of Phycology 33, 655–665.) 2. Light micrograph
of the haploid plant and oogonium (inset) in the charophyte alga Chara sp. 3. The creeping haploid thallus and a diploid erect and
determinate sporophyte of Pellia epiphylla (photographs of bryophytes courtesy of Li Zhang). 4. The haploid leafy gametophyte of Atrichum
angustatum bearing diploid unbranched determinate sporophytes. 5. The haploid creeping thallus of Folioceros sp. with upright
indeterminate sporophytes. 6. A fossil sporophtye of Cooksonia sp. showing branching with terminal sporangia (photo courtesy of Jenny
Morris and Dianne Edwards). 7. A fossil sporophyte of Zosterophyllum showing lateral sporangia (photo courtesy of Jenny Morris and
Dianne Edwards). 8. A Selaginella kraussiana sporophyte showing vegetative branching habit, an unbranched reproductive strobilus, and
a mega- and microsporangium (inset). 9. Sporangia formed on the stem of Psilotum nudum. 10. A frond of Adiantum mairisi showing
marginal sori that contain sporangia. 11. The terminal cones of Cupressus sp. that contain the mega- and microsporangia. 12. The
stamens and carpels of a Magnolia sp. flower that contain the micro- and megasporangia, respectively.
2868 | Harrison et al.
(Fig. 2B) (Lee et al., 2008). Plus and minus gamete identity
is specified from the MATING-TYPE locus and requires
cytoplasmic accumulation of BEL [GAMETE-SPECIFIC
PLUS1 (GSP1)] and KNOX [GAMETE-SPECIFIC MINUS1 (GSM1)] class homeodomain proteins, respectively
(Ferris and Goodenough, 1994; Lee et al., 2008). Following
fertilization GSP1 and GSM1 heterodimerize, translocate to
the nucleus, and initiate zygotic gene expression patterns
(Lee et al., 2008). Constitutive expression of either GSP1 or
GSM1 in the opposite gamete type is sufficient to trigger
zygote development in the absence of fertilization (Zhao
et al., 2001; Lee et al., 2008). Stable C. reinhardtii diploids
that do not initiate meiosis can also be generated, and
constitutive expression of GSP1/GSM1 together in these
cells is sufficient to induce meiosis with normal patterns of
recombination (Lee et al., 2008). This indicates that GSP1/
GSM1 homeodomain proteins are potential triggers of
meiosis in a chlorophyte alga. In contrast to chlorophytes,
charophytes have a multicellular haploid body that generates free-swimming sperm in antheridia and egg cells that
are retained within an oogonium on the parent plant
(Fig. 2B). Egg retention (oogamy) is an innovation shared
with land plants thought to have been a key adaptation in
their evolution (McCourt et al., 2004). As there are
currently no charophyte genetic models, potential roles for
KNOX/BEL genes are unexplored, and the initiation of
meiosis in charophytes is via an unknown mechanism.
Sporophyte development and meiosis in
bryophytes
In contrast to their algal sisters, all land plants have a period
of multicellular diploid growth, the extent of which varies
by plant group (Lewis and McCourt, 2004; McCourt et al.,
2004; Becker and Marin, 2009). The bryophyte sister groups
to the vascular plants exhibit limited post-embryonic development with no indeterminate apical growth (Mishler
and Churchill, 1985; Shaw and Renzaglia, 2004; Donoghue,
2005). Sporophytes comprise a small single stem with
a terminal sporangium that represents the simplest basal
land plant body plan (Kenrick, 2002; Donoghue, 2005; Qiu
et al., 2006; Boyce, 2008) (Fig. 2). In liverworts and mosses
sporangium development arrests diploid growth, whereas
hornwort sporophytes contribute to their own nutrition and
have sporangia that grow indeterminately from a basal
meristem (Boyce, 2008; Kato and Akiyama, 2005). A subepidermal archesporial cell layer is specified during sporangium development and divides either by meiosis to generate
spores (mosses) or spore mother cells and interspersed elater
cells that perform nutritive or dispersal functions (liverworts
and hornworts). The tissues surrounding the archesporial
cell layer perform dispersal functions specific to each
bryophyte group (Bower, 1935). The genetic and developmental mechanisms that regulate bryophyte sporophyte
development are currently poorly understood, but interest
has recently accelerated due to the establishment of moss
(Physcomitrella patens) and liverwort (Marchantia polymor-
pha) models (Ishizaki et al., 2008; Rensing, 2008). Two gene
classes that affect sporangium development in P. patens are
homologues of A. thaliana LEAFY (LFY) and KNOX genes
(Champagne and Ashton, 2001; Tanahashi et al., 2005;
Sakakibara et al., 2008). A pair of LFY homologues
redundantly control the first zygotic division in P. patens
and double mutants exhibit arrested zygotic development
(Tanahashi et al., 2005). In mutants that do not arrest,
sporangium number, initiation, and development are perturbed. These defects may arise as a consequence of
abnormal sporophytic development, although spore number
and germination are also highly variable in the mutants,
suggesting meiotic defects (Tanahashi et al., 2005). Similarly
Class I KNOX mutants in P. patens have abnormal
sporangia and reduced spore numbers (Sano et al., 2005;
Sakakibara et al., 2008). Interestingly KNOX expression is
sporophyte specific in P. patens (Champagne and Ashton,
2001; Sakakibara et al., 2008), and a potential role of
KNOX and BEL genes in diploid development conserved
between algae and bryophytes remains to be explored.
Protracheophytes and seed plant sister
groups
Key features that distinguish vascular plants from bryophytes are the elaboration of an indeterminately growing
and branching diploid body (Mishler and Churchill, 1985;
Donoghue, 2005; Langdale and Harrison, 2008). Fossil
plants whose form is not represented in living plants, such
as Cooksonia, have low orders of branching and may have
amplified spore numbers by increasing numbers of terminal
sporangia (Fig. 2B) (Edwards and Feehan, 1980; Graham
et al., 2000; Donoghue, 2005; Gerrienne et al., 2006). These
fossils raise interesting questions about the developmental
nature of the association between axis development, sporangium development, and branching and, intriguingly, rare
bryophyte branching mutants strikingly resemble Cooksonia
sporophytes (Fig. 2B). Alternative lateral sporangial placements appear independent of branching and may have
served a similar purpose in other fossil groups (Fig. 2B).
This arrangement is exhibited in modern lycophytes, and
sporangia arise either at the base of leaves or from the stem
via one or two sub-epidermal archesporial cell layers. These
archesporial cells give rise to sporogenous tissue (Lycopodium) or sporogenous and tapetal tissues (Selaginella,
Isoetes) (Bower, 1935). Monilophyte sporangia are diverse
in terms of their size, the number of spores produced per
sporangium, and their number and position on the plant
(Fig. 2B). The eusporangiate basal monilophyte grade
comprising marattioid ferns, horsetails, ophioglossoid ferns,
and whisk ferns possess sporangia that develop from several
cells and produce thousands of spores (Bower, 1935;
Wagner, 1977; Parkinson, 1987; Pryer et al., 2004). By
contrast, the leptosporangiate ferns develop numerous,
small sporangia from single cells, which typically contain
tens of spores (Bower, 1935; Pryer et al., 2004). Sporangia
may show terminal, adaxial, abaxial, or marginal locations
Flowering and meiosis | 2869
on leaves (Fig. 2B). With the exception of the leptosporangiate and whisk ferns, nutritive tapetal tissues arise from
non-sporogenous tissue (Bower, 1935; Parkinson, 1987). As
in bryophytes, the genetic basis of diploid development is
poorly characterized in lycophytes and monilophytes.
Notably sporophytic KNOX expression is conserved, and
meristematic expression domains suggest likely roles in
indeterminate growth (Bharathan et al., 2002; Harrison
et al., 2005; Sano et al., 2005), although reproductive roles
have not yet been explored. Thus the structure and dispersal
functions of sporangia vary broadly across the land plants
and the developmental context for the initiation of meiosis
is lineage specific. An evolutionary trend towards the
amplification of spore numbers by alterations in body plan,
sporangium size, and the number of sporangia is apparent
(Bower, 1935).
Sporophyte development in seed plants
In seed plants (gymnosperms and angiosperms) a prolonged
period of vegetative growth is followed by the reproductive
transition. This transition involves a change in meristem
identity and leads to the development of cones or flowers
(Steeves and Sussex, 1989). Seeds develop in the context of
the ovule following fertilization of the female egg cell by
a male sperm cell transferred in pollen, thus dispersal
functions are provided both by haploid pollen and diploid
seed. Ovules are the site of megasporangium (nucellus)
development, which precedes meiosis. Whilst in gymnosperms one to several nucellar cells enter meiosis, in
angiosperms a single megaspore mother cell undergoes
meiosis to form a tetrad, three members of which degenerate to form a single functional megaspore, which
divides mitotically to form the embryo sac (Campbell,
1940; Colombo et al., 2008). Pollen sac (microsporangium)
development occurs from a microsporophyll or in the
anther in gymnosperms and angiosperms respectively. In
both, sub-epidermal cells are specified as archesporial cells
that divide periclinally to form a layer of parietal cells
surrounding the sporogenous cells (Campbell, 1940; Feng
and Dickinson, 2007). Sporogenous cells may then either
directly enter meiosis or continue to proliferate. Parietal
cells divide further to form a variable number of concentrically arranged cell layers, the innermost of which differentiates into the nutritive tapetum (Campbell, 1940;
Feng and Dickinson, 2007). During meiosis sporogenous
cells become encased in an impermeable callose ((1-3)b-D-glucan) wall, which later breaks down when members
of the microspore tetrads are released (Gifford and Foster,
1988). Callose appears to play an important role in
sporogenesis, as tapetal expression of callase causes male
sterility in tobacco (Worrall et al., 1992). Following meiosis,
the resulting haploid microspores undergo mitosis and
differentiate into pollen grains.
The genetic control of vegetative development, the reproductive transition, and sporangium formation are well
studied in the angiosperm A. thaliana. Activity of the class I
KNOX gene SHOOTMERISTEMLESS (STM) is necessary for the establishment of an indeterminate meristem
(Long et al., 1996), and STM and BREVIPEDICELLUS
(BP) act redundantly to maintain indeterminacy and repress
determinate leaf development (Byrne et al., 2002). Class I
KNOX proteins promote indeterminacy by dimerizing with
BEL transcription factors and triple bellringer, poundfoolish,
Arabidopsis thaliana homeobox1 BEL mutants phenocopy
stm mutants (Rutjens et al., 2009). Thus, in A. thaliana
KNOX and BEL genes play key roles in elaboration of the
diploid body, providing the context for later reproductive
development and meiosis. Flower development follows
conversion of indeterminate, vegetative shoot meristems to
reproductive fates. This switch is controlled by a large
network of genes that ensure reproduction is co-ordinated
with environmental and developmental conditions (Baurle
and Dean, 2006). These signalling pathways converge on
a key set of transcription factors required for floral
meristem identity, including LEAFY and APETALA1
(Baurle and Dean, 2006). Floral organ identity genes
encode three functional classes of MADS-box transcription
factors (A, B, and C) that are expressed in overlapping
domains to specify the four floral organ types (Coen and
Meyerowitz, 1991). Stamen identity requires overlapping
expression of the B and C class MADS genes PISTILLATA
and APETALA3, and AGAMOUS (AG) (Yanofsky et al.,
1990; Jack et al., 1992; Goto and Meyerowitz, 1994). Carpel
identity requires activity of the C class gene AG, which acts
in conjunction with three additional MADS proteins,
SEEDSTICK, SHATTERPROOF1, and SHATTERPROOF2 to specify ovule identity (Yanofsky et al., 1990;
Pinyopich et al., 2003). The KNOX genes STM and
KNAT2, and BEL1 genes also play roles in the specification
of carpel and ovule identity (Modrusan et al., 1994; Pautot
et al., 2001; Scofield et al., 2007). Thus genes involved in
meristem identity also play roles in the specification of
reproductive fate.
In both micro- and megasporangia archesporial cell
specification precedes sporangium formation (Gifford and
Foster, 1988). The mechanisms of archesporial cell specification are poorly understood, but one gene, SPOROCYTELESS/NOZZLE (SPL/NZZ), functions directly
downstream of the C class organ identity gene AG to
promote sporogenesis (Schiefthaler et al., 1999; Yang et al.,
1999b; Ito et al., 2004). SPL is a nuclear protein with distant
homology to MADS box transcription factors that performs an unknown function (Schiefthaler et al., 1999; Yang
et al., 1999b). spl mutants differentiate microsporangial
archesporial cells that divide once, but fail to form microsporocytes or tapetal cells, causing sterility (Schiefthaler
et al., 1999; Yang et al., 1999b). The spl/nzz mutants are
also female sterile due to nucellar defects, meaning that the
archesporial cells do not differentiate and meiosis fails to
initiate (Schiefthaler et al., 1999; Yang et al., 1999b).
Ectopic activation of SPL in agamous mutants is sufficient
to induce staminoid development and pollen formation (Ito
et al., 2004). Together this indicates that SPL/NZZ
performs an upstream meiotic function in both male and
2870 | Harrison et al.
female development. Male sporocyte identity in A. thaliana
is also regulated by the leucine-rich repeat (LRR) receptor
kinase EXTRA SPOROGENOUS CELLS/EXCESS
MICROSPOROCYTES1 (EXS/EMS1) in conjunction with
its small protein ligand TAPETUM DETERMINANT1
(TPD1) (Yang et al., 1999b, 2003; Canales et al., 2002; Zhao
et al., 2002). The first archesporial cell division normally
separates reproductive sporocyte fate from non-reproductive wall and tapetal fates. Microsporangial development is
altered in exs/ems1 and tpd1 mutants such that sporogenous
cells develop at the expense of tapetal cells (Yang et al.,
1999b, 2003; Canales et al., 2002; Zhao et al., 2002). This
implies that EXS/EMS1 kinase signalling is important either
to promote tapetal or to repress sporogenous cell identity.
Although exs/ems1 mutations in A. thaliana show normal
megasporangial development, mutations in the rice homologue MULTIPLE SPOROCYTES1 (MSP1) show supernumerary sporocytes in both the anther and ovule, as does
the multiple archesporial cells1 (mac1) mutant in maize
(Sheridan et al., 1996; Nonomura et al., 2003). Interestingly,
additional LRR receptor kinases have also been implicated
in proper differentiation of the anther cell layers (Albrecht
et al., 2005; Colcombet et al., 2005; Mizuno et al., 2007;
Hord et al., 2008). How these signalling processes are
organized between the cell types within the developing
anther is not yet clear.
Future perspectives
Developmental genetic studies in A. thaliana have significantly advanced our understanding of the context in which
meoisis arises in plants. Future goals will be to tie together
our understanding of the context, initiation, and progress of
meiosis in diverse plant groups so that potential variation
can be released to breeding. During plant diversification,
genes that may have originally been involved in reproductive development have been co-opted to vegetative development pathways. Whilst KNOX/BEL proteins may
trigger meiosis in a chlorophyte alga, their role is unknown
in charophytes and most bryophytes and thus the point of
functional diversification remains to be identified. The
mechanisms for archesporial cell development are not yet
fully characterized in flowering plants and are unknown in
non-flowering plants. The derivation of nutritive tapetal
tissues in different plant groups may or may not be
independent of archesporial lineages. It will be interesting
to test homology between archesporial and tapetal cell types
by testing the function of SPL, EXS, and TPD1 homologues in different lineages. A detailed picture is emerging of
the mechanisms that control plant meiotic chromosome
pairing, synapsis, recombination, and segregation in A.
thaliana. Understanding how these mechanisms integrate
during progression of the meiotic cell cycle will be a major
challenge. Equally, the pattern of CO hot- and cold-spots is
complex and the mechanisms that determine plant CO
frequency remain to be determined. Knowledge of these
mechanisms may allow CO to be targeted during crop
breeding and facilitate the generation of novel high-yielding
agricultural strains.
Acknowledgements
We thank the Royal Society and the Gatsby Charitable
Foundation for funding, Karen Van Winkle-Swift, Vasily
Kantsler, Li Zhang, Jenny Morris, and Dianne Edwards for
photographs, and two anonymous reviewers for helpful
comments on the manuscript.
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