Advances in Morphogenesis

A.Eastward. Demaggio , V. Raghavan , in Advances in Morphogenesis, 1973

Publisher Summary

Sporophytes, which are recognizable fern plants, take been utilized in morphogenetic studies and have yielded important results bearing on apical arrangement, cellular totipotency, phyllotactic pattern, stelar morphology, vascular tissue initiation, leaf and bud development, and embryogenesis. Gametophytes (prothalli) are morphologically insignificant in comparing to the sporophyte, they have not been utilized for experimental studies. However, the simplicity of prothallial development and organisation make these plants ideal for studying sure aspects of cellular differentiation. The transition from a elementary filamentous prothallus to a gametophyte with biplanar morphology provides a useful experimental system for investigating mechanisms controlling the planes of cell division, which contribute to the development of class. The affiliate reviews the literature on the control of growth in the filamentous prothalli and the causal factors promoting the transition to the two-dimensional gametophyte. Information technology focuses on the effects of calorie-free quality and nucleic acid metabolism; the survey includes non only contempo accomplishments in these areas but also provides a disquisitional assessment of the outstanding issues.

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Polar Auxin Send and Plant Sporophyte Trunk Plans

Alexandru Grand.F. Tomescu , Kelly K.S. Matsunaga , in Reference Module in Life Sciences, 2019

Introduction

The sporophytes of country plants (or embryophytes) comprehend a wide range of levels of complexity. Their organization, particularly with reference to their differentiation into specialized organs, has been referred to as organography or torso program. The organography of embryophyte sporophytes records an evolutionary transition from simple torso plans, consisting of undifferentiated axes bearing sporangia, to more than complex body plans featuring differentiated vegetative organs alongside sporangia.

The simplest sporophyte organography, seen in bryophytes, consists of a single sporophyte axis (the seta) that does not branch (Fig. 1(a)), thus bearing a single sporangium at its tip, at maturity. This simple organization is thought to have arisen from a bones sporophyte consisting solely of a sporangium (Mishler and Churchill, 1984), by intercalation of a brief phase of apical meristematic growth, which produced the seta, prior to the switch to a reproductive growth programme that generates the sporangium (Tomescu et al., 2014). The early vascular plants (tracheophytes) of the Silurian and Early Devonian, usually classified as rhyniophytes, zosterophylls, and trimerophytes (Kenrick and Crane, 1997), take sporophyte torso plans very similar to that of bryophytes. These differ from the bryophytes only in their ability to branch (Fig. 1(b)) and, thus, can acquit multiple sporangia (hence their designation every bit polysporangiophytes). Otherwise, these early on tracheophyte sporophytes are little more than branched bryophyte setae (Tomescu et al., 2014) and, equally in the bryophytes, some of them may have been nutritionally dependent on the gametophytes (Boyce, 2008; Libertín et al., 2018). More circuitous body plans involving differentiated specialized organs – some referred to as stem-leaf-root organography (Fig. 1(c, d)) – evolved during the Devonian explosion of tracheophyte variety. Multiple lines of evidence point to several independent origins of leaves and roots in different lineages (Boyce, 2005; Boyce, 2010; Tomescu, 2009, 2011; Harrison, 2017), but their evolutionary trajectories are however incompletely understood.

Fig. 1

Fig. 1. Sporophyte organography in embryophytes. (a) Bryophyte sporophyte with unipolar development and simple organography, consisting of an unbranched axis with ephemeral apical growth. (b) Early vascular plant sporophyte with unipolar development and simple organography, consisting of an undifferentiated branching axis. (c) Tracheophyte sporophyte with unipolar development and complex organography, differentiated into stems, leaves, and roots (stalk-leaf-root organography); unidirectional growth from the shoot pole produces all constitute organs, including roots (homorhizic condition): Roots are exclusively accidental and initiation of each root requires de novo specification of root identity within shoot tissues. (d) Tracheophyte sporophyte with bipolar development and complex organography, differentiated into stems, leaves, and roots; bidirectional growth produces the shoot and root systems that are topologically and ontogenetically distinct components of the body plan: Root identity specified independently of shoot development, in the embryo radicle, is perpetuated throughout ontogeny in the mature sporophyte (allorhizic condition). Color fundamental: Black – embryo foot; green – aboveground or positively gravitropic undifferentiated axes or shoots; orange – roots; purple – upmost meristems.

Modern treatments of the diverseness and evolution of sporophyte body plans have emphasized attributes of growth and organographic differentiation. In ane of these, Rothwell (1995) used the degree of dependence of sporophyte on gametophyte, the polarity and determinacy of sporophyte growth, and the developmental origin of branching, to ascertain six basic models of growth amid embryophytes (bryophyte, cooksonioid, psilotioid, selaginelloid, isoetoid, and cotyledonoid; Tabular array i). In another treatment, Tomescu (2011) used organographic diversity, vascular architecture (stele blazon), and apical meristem structure, along with the polarity of growth and developmental origin of branching, to differentiate seven types of sporophyte torso programme among tracheophytes (lycopodialean, selaginellalean, isoetalean, psilotopsid, sphenopsid, pteropsid, and spermatophyte; Tabular array 2).

Table ane. Rothwell's vi models of growth

Model of growth Bryophyte Cooksonioid Psilotioid b Selaginelloid Isoetoid Cotyledonoid
Gametophyte long-lived a + + + − − −
Sporophyte growth Determinate Determinate Indeterminate Indeterminate Indeterminate Indeterminate
Growth polarity of sporophyte Unipolar Unipolar Unipolar Unipolar Bipolar Bipolar
Sporophyte branching (aboveground axes) Absent Apical Upmost Apical Apical Lateral (axillary)
Systematic occurrence Bryophytes Cooksonioids c Psilotopsids, rhyniophytes d , zosterophylls d , trimerophytes d , Lycopodiales, some ferns (east.g., Ophioglossales) Selaginellales, sphenopsids due east , some ferns (eastward.g., Filicales) Isoetales f Spermatophytes

Note: Diagrams modified from Rothwell, 1000.W., 1995. The fossil history of branching: Implications for the phylogeny of country plants. In: Hoch, P.C., Stephenson, A.G. (Eds.), Experimental and Molecular Approaches to Establish Biosystematics. St. Louis, MO: Missouri Botanical Garden, pp. 5–12.

a
Gametophyte long-lived, usually producing more than than one sporophyte; sporophyte nutritionally dependent on the gametophyte entirely or, at to the lowest degree, throughout the early growth stages.
b
The psilotioid model includes two subtypes: One in which the gametophytes are hypogeal (and relatively modest), as in extant psilotopsids, Lycopodiales, and Ophioglossales; and ane in which the gametophytes are epigeal, as in several early polysporangiophytes and tracheophytes.
c
The overall growth habit of the earliest polysporangiophytes (e.g., Cooksonia) has notwithstanding to be demonstrated with direct fossil evidence, but indirect testify suggests that it consisted of branched sporophytes that were entirely dependent nutritionally on the gametophytes (Boyce, 2008; Strother, 2010; Gerrienne and Gonez, 2011; Tomescu et al., 2014).
d
The overall growth habit of the rhyniophytes, zosterophylls, and trimerophytes, including epigeal axial gametophytes is inferred here based on the few gametophytes documented for these groups (e.g., Sciadophyton, Lyonophyton; Remy et al., 1993; Taylor et al., 2005).
e
Although included hither, sphenopsids accept lateral non-axillary branching of the shoots, instead of apical branching.
f
Bipolar growth in isoetalean lycopsids is the result of early-ontogenetic branching of the growth pole of the embryo, which produces a negatively gravitropic axial organisation (the aboveground shoot system) and a positively gravitropic axial system (the belowground rhizomorph, interpreted as a shoot homolog; Rothwell and Erwin, 1985; Rothwell and Tomescu, 2018). Color central: Yellow – gametophyte; brown – sporophyte; sporophyte apices with sporangia (circles) or indeterminate growth from upmost meristems (arrowheads).

Tabular array 2. Summary of the seven types of vascular plant sporophyte trunk plans based on Tomescu

Notation: Tomescu, A.M.F., 2011. The sporophytes of seed-free vascular plants – Major vegetative developmental features and molecular genetic pathways. In: H. Fernandez, H., Kumar, A., Revilla, M.A. (Eds.), Working with Ferns: Problems and Applications. New York, NY: Springer, pp. 67–94.

Stem-leaf-root organography: Nowadays in Ly, Se, Sph, Pt, Spe; Se body plan includes a rhizophore(greyness); Is body plan includes a rhizomorph interpreted every bit a shoot modified for rooting (run into Rothwell and Tomescu, 2018); stems, leaves, and aboveground axes green; roots orangish. Growth polarity: Unipolar axis in Ps; unipolar homorhizic in Ly, Se, Sph, Pt; bipolar in Se; secondarily bipolar in Is. Aboveground axis/shoot branching: Upmost in Ly, Se, Is (including belowground rhizomorph), Ps, Pt; lateral not-axillary in Sph; lateral axillary in Spe. Root branching: Upmost in Ly, Se; lateral in Sph, Pt, Spe; roots absent in Is, Ps. Shoot upmost meristem organization: Unmarried apical cell in Se, Is, Ps, Sph, Pt; ii upmost cells in Se; multiple apical initials in Ly, Se, Is, Pt; layered with multiple apical initials in Spe. Root upmost meristem organization: Unmarried upmost cell in Sph, Pt; multiple upmost initials in Ly, Se, Pt, Spe; roots absent in Is, Ps. Aboveground centrality/shoot xylem architecture: Exarch actinostele (protostele) in Ly, Is, Ps, extinct Sph (Sphenophyllales; regal); plectostele in Ly, Se; equisetostele in Sph; Pt – mesarch eustele and mesarch siphonostele, extinct representatives (purple) with mesarch moniliform actinosteles (iridopterids, cladoxylopsids, stenokolealeans, etc.), mesarch radiate actinosteles (aneurophyte progymnosperms) and mesarch eusteles (archaeopterid progymnosperms); Spe – endarch eustele, extinct representatives (purple) with mesarch radiate actinosteles.

Irrespective of these classifications, axial organisation is, more than anything, the hallmark of the embryophyte sporophyte. This organization is a reflection of longitudinal polarity along an apical-basal axis, which underlies all other attributes of the trunk program. Ane of the main factors underlying this polarity and the associated features of the body plan is auxin, specifically its polar transport. The multiple roles of auxin in the establishment of the trunk plan and its developmental stability have been well documented in seed plants, particularly angiosperms.

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Awarding of Constitute Biotechnology

Saurabh Bhatia , in Mod Applications of Plant Biotechnology in Pharmaceutical Sciences, 2015

five.3.1.14 Consecration of Haploidy

Haploids are sporophytes of higher plants with a gametophytic chromosomal constitution, that is, they possess a single set of chromosomes that forms the alternation of generation (haploid and diploid every bit mentioned in Fig. v.nine). These types of plants are obtained from androgenesis, gynogenesis, parthenogenesis, semigamy, and polyembryony. In 1922, Belling and Blakeslee identified the first haploids in flowering plants (D. stramonium) [96,97]. The starting time culture of haploid plants (young anthers of D. innoxia) was successfully established by Guha and Maheswari in Delhi University, which attracted worldwide attention in the awarding of tissue culture to synthesize haploid plants [98]. Ordinarily somatic cells of higher plants have a diploid chromosome number while reproductive cells are haploid. They are succeeded in raising haploid embryoids and plantlets from adult culture (from microspores inside the anthers). This opens the field of androgenesis. In the following year Bourgin and Nitsch confirmed the totipotency of pollen grains raising total haploid plants of tobacco, rice, and wheat [99]. Haploids past anther culture are now reported to have been raised in 247 species belonging to 34 families. Induction of haploid plants from pollinated ovaries and ovules (gynogenesis) is some other recent advancement in institute tissue culture and experimental embryology. San Noeum reported her first result on in vitro civilisation of ovary isolated from Hordeum vulgare [100]. This demonstrates that not just the microspores but also the megaspores or female gametophytes of angiosperms tin be triggered in vitro to saprophytic development, thus making way for an alternative arroyo to haploid establish convenance. It may exist noted that anther culture has given rise to diploid, polyploid, and aneuploid plants. Thus, haploid consecration or haploid represents meaning applications for plant breeders and genetics in agronomics and plant tissue culture science.

Effigy 5.9. Establish life cycle alternation of generations.

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Maternal Outcome Genes in Evolution

Allison R. Phillips , Matthew G.South. Evans , in Current Topics in Developmental Biology, 2020

5.ane Control of seed size and composition past maternal transfer tissues

The maternal sporophyte controls seed size via transfer tissues such as the integuments, which classify nutrients to the endosperm and deliver molecules or signals that mediate patterning of the embryo ( Chen et al., 2015; Ingouff et al., 2006; Schruff et al., 2006). SWEET11, 12 and fifteen are sucrose transporters in Arabidopsis that likely coordinate sucrose efflux from the inner integuments through the micropylar region of the endosperm, to the suspensor, and finally fuel growth of the embryo proper (Chen et al., 2015). When disrupted in the maternal sporophytic sweet11;12;15 triple mutant, seeds have reduced seed weight and underdeveloped embryos (Chen et al., 2015). A fix of maternal sporophyte issue mutants in barley besides have a shrunken endosperm reminiscent of defects in nutrient transfer from the mother found (Felker, Peterson, & Nelson, 1985).

Many mutants bear on integument growth, which in plow has effects on seed size and shape (reviewed in Li & Li, 2015). Increased or decreased integument length tin can either change the size of the seed cavity allowing for more than or less endosperm expansion or change nutrient transfer into the developing seed (Schruff et al., 2006). Mutants of this form include disruptions in transcription factors, the ubiquitin pathway, cytochrome P450s, One thousand-protein signaling, and hormone signaling. Many have not been rigorously tested for sporophytic or gametophytic maternal effects. Still, since they disrupt integument development, a sporophytic maternal effect is assumed.

Transcription factors of the WRKY, AP2/EREBP (ethylene responsive chemical element binding protein), MADS-box, and C2H2 blazon zinc finger families, as well as the found hormones auxin, brassinosteroid (BR), and cytokinin control seed coat and seed development (Colombo et al., 1997; Gaiser, Robinson-Beers, & Gasser, 1995; Li & Li, 2015). Transparent Testa Glabra ii (TTG2) encodes a transcription gene of the WRKY family that is expressed in the integument (Johnson, Kolevski, & Smyth, 2002). Loss of TTG2 causes a reduction in integument length and consequent reduction in endosperm size (Garcia, Fitz Gerald, & Berger, 2005). The maternal event of ttg2 also modulates the phenotype of crosses between diploid and tetraploid plants (Dilkes et al., 2008). Other testa mutants of Arabidopsis, such as aberrant testa shape (ats) and apetela2 (ap2), brandish sporophytic maternal effects on seed development (Leon-Kloosterziel, Keijzer, & Koornneef, 1994; Ohto, Fischer, Goldberg, Nakamura, & Harada, 2005). Wild-type AP2 restricts seed size and regulates the aggregating of proteins, oils, and sugars in the seed by limiting the development of the outer integuments/seed coat (Ohto et al., 2005). Potential targets of AP2 include signaling by the hormones, gibberellin (Jofuku, Omidyar, Gee, & Okamuro, 2005) and brassinosteroid (Li & Li, 2015), and enzymes involved in sugar metabolism, such as cell-wall-bound invertases (Ohto et al., 2005). Reduced office of the MADS-box Floral Binding Protein7 (FBP7) and FBP11 genes in petunia results in sporophytic maternal furnishings on seed coat development and endosperm development (Colombo et al., 1997). Specifically the endothelial cells of the seed coat, derived from the integuments, completely degenerate late in evolution, with negative impacts on transport into the developing endosperm (Colombo et al., 1997).

DA1 and DA1-Related1 (DAR1), two ubiquitin receptors, and DA2 and Enhancer Of Da1 (EOD1)/Big Blood brother (BB), 2 E3 ubiquitin ligases, restrict cell proliferation in the integuments (Disch et al., 2006; Li, Zheng, Corke, Smith, & Bevan, 2008; Xia et al., 2013). Both DA2 and EOD1 human action synergistically with DA1 but independent of each other (Xia et al., 2013). Finally, UBP15, a ubiquitin specific protease, has been identified every bit a (likely sporophytic) maternal effect gene that functions with DA1 to regulate seed growth, but acts to promote rather than restrict cell proliferation (Du et al., 2014). Members of the CYP78A cytochrome P450 family act to promote integument growth and consequently affect seed size in Arabidopsis and rice, and CYP78A6 specifically enhances the furnishings of the ubiquitin pathway genes above (Adamski, Anastasiou, Eriksson, O'Neill, & Lenhard, 2009; Fang, Wang, Cui, Li, & Li, 2012; Nagasawa et al., 2013). AGG3, a heterotrimeric Chiliad-poly peptide complex subunit, has a positive maternal issue on seed growth (Chakravorty et al., 2011; Li et al., 2012), while two homologs in rice, GS3 and DEP1 negatively regulate seed size (Huang et al., 2009; Li & Li, 2015; Mao et al., 2010).

Several major developmental hormones in plants, including auxin, brassinosteroids and cytokinin, take maternal furnishings on seed size through effects on integument development. In auxin response factor2 (arf2) mutants, seeds are larger and have an abnormal shape, due to increased cells in both the inner and outer integuments (Schruff et al., 2006). Analysis of several BR-insensitive mutants (de-etiolated2, dwarf4 and brassinosteroid insensitive1) revealed decreased integument growth resulting in reduced elongation of seeds presumably via BR-responsive genes, such as KLUH, AINTEGUMENTA, APETELA2, ARF2, ARABIDOPSIS HIS KINASE3 (AHK3), and AHK4 (Jiang et al., 2013). The triple cytokinin receptor mutant, ahk2;ahk3;akh4/cre1, has larger seeds than wild type although the nature of the maternal result is less articulate (Riefler, Novak, Strnad, & Schmulling, 2006).

The altered meristem program1 (amp1) mutant produces seeds with two embryos (because there are multiple egg cells) and oft no endosperm, similar to some of the gametophytic maternal effectmutants mentioned above (Kong et al., 2015). Notwithstanding, amp1 is required in the surrounding maternal sporophytic tissue rather than in the embryo sac itself (Kong et al., 2015). This is an example of a sporophytic maternal effect on embryo sac development which so leads to a maternal effect on seed evolution.

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Comparative Reproduction

Bharti Sharma , Elena 1000. Kramer , in Encyclopedia of Reproduction (Second Edition), 2018

Overview of Alternation of Generations in Seed Plants

1.

Mature diploid sporophyte undergoes meiosis to produce haploid unicellular microspores and megaspores.

two.

The microspore undergoes mitotic divisions to produce the male gametophyte, which is equanimous of a haploid vegetative jail cell and haploid generative cell.

iii.

The haploid generative cell will split mitotically to form 2 haploid sperm nuclei.

4.

The haploid megaspore undergoes 3 rounds of mitotic divisions that consequence in eight haploid nuclei contained in 7 cells: Two synergids, one egg prison cell, a central jail cell with two nuclei, and three antipodals.

5.

Double fertilization results in a diploid zygote and a triploid endosperm.

half dozen.

The diploid zygote grows mitotically into a next diploid sporophyte generation.

Summary of Sexual Reproduction in Plants

With their tremendous variety in form, and equally the dominant inhabitants of the land ecosystem, land plants have undergone many genetic, developmental and structural changes in reproductive structures over the course of their diversification. The transition from a water ecosystem for dark-green algae to a state ecosystem posed many challenges, including desiccation of spores, secure transfer of male gametes to the female gametophyte, and nourishment and protection of embryo until it matures. Plants evolved the following characteristics to successfully reproduce.

1.

A protective roofing of sporopollenin around the spores to prevent desiccation.

2.

Retentiveness of the zygote in the female parent provided added protection and nutritional back up to the young embryo.

3.

Differentiation of the gametophyte into male or female. The multiple transitions from homospory to heterospory in early land plants to strictly heterospory in seed plants is associated with both reduced inbreeding and reduction of the gametophyte.

four.

Development of the seed, which includes both the loss of swimming sperm in favor of a mobile male person gametophyte and the evolution of indehiscent megasporangia, which protect the enclosed, reduced megagametophytes.

v.

With the evolution of flower, we see the evolution of double fertilization besides as the carpel.

Diverse reproductive strategies let plants to inhabit ecosystems where other organisms might not survive.

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Plant Development and Evolution

Péter Szövényi Manuel Waller Alexander Kirbis , in Current Topics in Developmental Biological science, 2019

5.6 Evolution of indeterminacy

The fossil record suggests that all indeterminate sporophyte axes take laterally arranged sporangia ( Boyce & Kevin Boyce, 2010; Tomescu et al., 2014). Therefore, the evolution of indeterminacy of sporophytic axes and the lateral displacement of sporangia are linked, indicating spatially and temporally separated action of reproductive and vegetative functions (Kenrick, 2018). Data on the molecular mechanisms underlying indeterminacy is exclusively coming from investigations on the determinant sporophyte of P. patens. It was shown that 2 components of the moss PRC2, encoded by PpFIE and PpCLF, are necessary to repress the meristematic activeness of the sporophyte upmost cell (Kenrick, 2018; Mosquna et al., 2009; Pereman et al., 2016). In mutants disrupting PRC2, the sporophytes produced branched structures by continuous proliferation. Grade I KNOX genes are known to be responsible for the maintenance of the meristematic activity of the sporophyte apical cell (Sakakibara et al., 2008). That is, the interaction of PRC2 and Class I KNOX gene activity is primal in the regulation of determinant and indeterminant growth. Class Two KNOX genes are also expressed in and necessary for the development of the sporophyte by repressing the gametophytic plan (Sakakibara et al., 2013). These observations imply that part of the PRC2 in repressing pluripotent sporophytic cells is conserved across country plants. Whether the antagonistic result of Course I and II KNOX genes seen in flowering plants is likewise conserved in the development of the sporophyte of mosses is unknown (Furumizu et al., 2015). However, it is possible that development of indeterminacy is partially accomplished by the proper coregulation of Class I and Class Two KNOX activities, which may connect reproductive and vegetative proliferative activities.

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The Maternal-to-Zygotic Transition

Peng Zhao , Meng-Xiang Dominicus , in Current Topics in Developmental Biology, 2015

Abstract

Fertilization marks the turnover from the gametophyte to sporophyte generation in college plants. Later fertilization, sporophytic development undergoes genetic turnover from maternal to zygotic control: the maternal-to-zygotic transition (MZT). The MZT is thought to be critical for early embryogenesis; still, footling is known about the time grade or developmental impact of the MZT in higher plants. Here, we discuss what is known in the field and focus on techniques used in relevant studies and their limitations. Some significant questions and technical requirements for farther investigations are also discussed.

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Plant Evolution and Evolution

Dieter Hackenberg , David Twell , in Current Topics in Developmental Biological science, 2019

i.3 Heterospory and the origin of microgametophytes

The increase in size and complexity of the sporophytes of vascular plants is matched past corresponding reductions in their gametophytes ( Fig. 1). Basal extant lycophytes and monilophytes produce free-living exosporic gametophytes (prothallia) with flattened thalloid anatomy, or subterranean tuber-like structures (Ligrone et al., 2012). These taxa retain an oogamous life manner and prothallia produce gametes in carve up male and female gametangia, similar to those of bryophytes.

The spores of extant bryophytes and basal vascular plants are uniform in size, a condition known as homospory. Heterospory, defined equally the production of smaller male microspores and larger female megaspores, arose independently in all major tracheophyte lineages (lycophytes, monilophytes and spermatophytes) and is considered a key innovation for terrestrial colonization (Bateman & DiMichele, 1994; Petersen & Burd, 2016). Endospory, the development of gametophytes within the spore wall, is tightly coupled to unisexuality and is proposed every bit an innovation that allowed the evolution of heterospory (Bateman & DiMichele, 1994). The transition to endospory increased the influence of the diploid parent on gametogenesis, and allowed spore production and release to be synchronized with favorable environmental conditions.

The reduction of the gametophyte and transition to endospory is exemplified by comparing of the male gametophytes of the dioicous liverwort M. polymorpha and the heterosporous water fern Marsilea vestita. In M. polymorpha, antheridial initial cells arise from epidermal cells of the receptacle on a reproductive branch called the antheridiophore (Durand, 1908). Initial cells divide several times prior to a formative asymmetric division, which gives rise to a spermatogenous mother cell and a sterile jacket cell. Each mother cell undergoes multiple rounds of division to produce several grand spermatozoids within each antheridium (Fig. twoB). In contrast, microspores of Grand. vestita consummate one disproportionate and one symmetric sectionalisation before spermatogenous cells are produced by a formative asymmetric division. The spermatogenous cells undergo four rounds of symmetric division to produce a total of 32 spermatozoids within each microspore (Fig. 2C; Sharp, 1914).

This trend toward fewer antheridial divisions prior to spermatogenesis culminates in the highly reduced male (micro)gametophytes of seed plants (Fig. 2D–F). In gymnosperms, microspores undergo disproportionate prison cell divisions to produce i or more prothallial cells, prior to segregation of a spermatogenous jail cell lineage (Fig. 2D and East). Reduction of the microgametophyte is most extreme in angiosperms, which require only two jail cell divisions to produce two non-motile sperm cells from the microspore (Fig. 2F). This indicates that the disproportionate cell divisions which produce one or more prothallial cells in gymnosperms are absent, and the germline is directly segregated past disproportionate partition of the microspore (come across Rudall & Bateman, 2007).

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Constitute Evolution and Development

Gloria León-Martínez Jean-Philippe Vielle-Calzada , in Current Topics in Developmental Biology, 2019

eight Evolutionary considerations on the origins of apomixis

The prevalence of an independent gametophyte over the sporophyte in the life wheel of non-vascular plants and many tracheophytes complicates their developmental comparison to angiosperms in terms of the genetic basis and molecular mechanisms that control their reproductive habits. Asexual reproduction is rare in not-vascular plants such equally mosses ( Bryophyta), hornworts (Anthocerotophyta), and liverworts (Marchantiophyta), only frequent in ferns (Polypodiopsida), where it occurs in almost ten% of species (Burt, 2000; Mogie, 1992). Although production of unreduced gametophytes through diplospory has been observed in liverworts (Smith, 1979), parthenogenesis has never been observed in nature, the only reported case occurring under in vitro conditions (Lal, 1984). And although apospory is common in ferns, it is never found in association with parthenogenesis, which is extremely rare in more than 12,000 species (Mogie, 1992). Parthenogenesis also appears to accept minimal evolutionary consequences for the genetic construction of approximately 730 species of gymnosperms, being rare and in most cases unsuccessful (Droga, 1966; Durzan et al., 1994).

The origin of apomixis in flowering plants must exist related to the selective forces responsible for the development of sexual reproduction through meiosis and fertilization. Extant eukaryotes share many genes essential for meiosis, confirming it was already present in their last common ancestor, probably as a precondition already existing in bacteria (Bernstein & Bernstein, 2013; Cavalier-Smith, 2010). Although a grade of reductional segmentation could appear in asexual unicellular eukaryotes, e.g., in response to a need of alternate between haploidy and diploidy due to accidental replications of the nuclear genome in the absence of cytokinesis (Mable & Otto, 1998; Rescan, Lenormand, & Roze, 2016), the origin of meiosis remains unsolved. The evolutionary benefits and adaptive functions of meiosis in the context of sexual costs are all the same nether give-and-take (Lenormand, Engelstädter, Johnston, Wijnker, & Haag, 2016; Mirzaghaderi & Hörandl, 2018). As a effect, agreement the origin of unreduced gamete formation as a transgenerationally stable component of gametophytic apomixis is still at an early stage. An attractive hypothesis suggests that, from an evolutionary point of view, the principal office of meiosis could be related to Deoxyribonucleic acid repair (perhaps in response to oxidative damage) rather than crossing-over or recombination (Hörandl, 2009; Hörandl & Hadacek, 2013), implying that homologous chromosome pairing and double-strand break formation during prophase of meiosis I should be among the well-nigh ancestral and conserved components of a primitive reductional partition. In understanding with this hypothesis, facultative apomicts could maintain Dna repair functions and purifying selection through sexually reduced male and female gametes (Lovell, Williamson, Wright, McKay, & Sharbel, 2017; Mirzaghaderi & Hörandl, 2018). Theoretical considerations suggest that unreduced gamete formation should be detrimental to fitness, both through a reduction in the potential contribution to random fertilization, and through the generation of triploid (2n   +   n) embryos with low fertility and viability (Husband & Sabara, 2003; Ramsey & Schmeske, 1998). The mechanisms of unreduced gamete formation are likely to emerge under unstable climatic atmospheric condition or stressful environments, in which polyploidy could provide a selective advantage (Vanneste, Maere, & Van de Peer, 2014). Perhaps the merely selective forcefulness that can provide an evolutionary reward for the production of unreduced gametes is the acquisition of reproductive stability through gametophytic apomixis (Bicknell & Koltunow, 2004, Kotlunow & Grossniklaus, 2003; Kreiner et al., 2017a, 2017b). Genotyping-by-sequencing approaches to estimate the frequency of unreduced egg jail cell fertilization in gametophytic apomicts could provide important evidence to ostend or discard our current views on this evolutionary trouble.

Our current genetic, molecular, and phylogenetic evidence suggests that apomixis has emerged multiple independent times from a deregulation of the sexual developmental pathway (Grossniklaus, 2001; Grossniklaus, Moore, & Gagliano, 1998; Hojsgaard, Greilhuber, et al., 2014; Hojsgaard, Klatt, et al., 2014; Hörandl & Hojsgaard, 2012; Koltunow, 1993; Kotlunow & Grossniklaus, 2003; Sharbel et al., 2010; Vielle-Calzada, Crane, & Stelly, 1996). The phylogenetic distribution illustrated in Fig. one confirms that frequent evolutionary reversions occur from facultative apomixis to obligate sexuality (Ortiz et al., 2013; Taliaferro & Bashaw, 1966). Based on a significant correlation between the occurrence of sexual (polyspory) and asexual (apomixis) abnormalities during female person gametogenesis, Carman (1997) suggested that apomixis could result from the hybridization-derived confrontation of divergent alleles that cause a temporal deregulation of cell specification and fate during early ovule development. Hörandl and Hojsgaard (2012) extended this hypothesis by suggesting the existence of an ancestral predisposition in all flowering plants that could exist favored by polyploidization and hybridization. If such a predisposition is confirmed, triploid evolutionary transitions associated with genomic disparities caused past hybridization could forbid developmental alterations to sexuality that transiently induce apomixis equally a requirement to enhance the evolutionary institution of new polyploids (Hojsgaard, 2018). Their eventual reversion to sexuality would crusade the re-establishment of meiosis nether developmental circumstances that favor polyspory through precocious megaspore formation in the absence of cytokinesis (Hörandl & Hojsgaard, 2012).

Polyploidization and hybridization could likewise influence the natural sources of epigenetic variation that shape the developmental versatility exhibited by the ovule of flowering plants, probably as an adaptive response to changing environmental factors. Waddington proposed that epigenetic adaptation could be a dynamic machinery to stabilize phenotypic variation through "canalization," the natural tendency of a trait to resist mutation or ecology modifications (Waddington, 1942). Following his hypothesis, canalization of mechanisms leading to the survival of a unmarried meiotic production (monospory) could depend on the epigenetic landscape that prevails in the developing ovule through the action of non-coding RNA (ncRNA)-dependent regulatory pathways (Fig. three). Obligate monospory would require the establishment and maintenance of consistent methylation patterns and/or chromatin epigenetic marks through compatible recognition between ncRNAs and their targets. Following hybridization, a lack of sequence recognition involving ncRNAs in divergent genotypes could result in epigenetic changes through Dna methylation or chromatin modification, leading to a deregulation of female person meiosis and jail cell specification in the ovule. The epigenetic landmarks involved in the regulation of transcriptionally active or silent elements would not exist targeted by ncRNAs produced from divergent parental genomes, as sequence complementarity would tend to diverge in recently formed hybrids. Polyploidization could also pb to this type of deregulation through chromosome duplications and re-arrangements that quantitatively affect the epigenetic mechanisms decision-making genomic methylation through the activeness of ncRNAs. Environmental factors affecting female meiosis found the third evolutionary force shaping the epigenetic mechanisms that command the reproductive outcome. Following this model, the formation of unreduced female gametes could depend on sources of epigenetic variation that do not contribute to the stable range of phenotypes manifested under monospory. These sources of epigenetic variation would only appear after the occurrence of hybridization and polyploidy (Grossniklaus, 2001), nether the influence of a wide diversity of ecology perturbations.

Fig. 3

Fig. 3. Sexual reproduction and apomixis as reversible alternatives in the evolutionary epigenetic landscape that guides female gametogenesis. In this epigenetic landscape, adaptation is flexibly modulated by shaping forces that depend on ploidy, hybridization, and the influence of environmental factors.

Smith used to say that from an evolutionary perspective, sexual reproduction could be more than important than life itself (Smith, 1979). By incorporating apomixis as part of the epigenetic landscape required for versatile adaptation, flowering plants might have developed one of the nearly evolutionary robust alternatives to sexuality. The continuous investigation of natural forms of epigenetic variation in multicellular organisms should offer new and exciting insights into the mechanisms of reproductive innovation and development that prevail in the plant kingdom.

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Advances in the Study of Incompatibility

C.East. Townsend , in Pollen, 1971

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This chapter discusses the advances in the study of incompatibility. The genotype of the sporophyte, rather than that of the gametophyte, determines the incompatibility reaction of the pollen in the sporophytic homomorphic type of incompatibility. Temperature has been 1 of the variables in incompatibility studies for many years. The issue of temperature on the pollen tube growth of compatible and incompatible matings varies with the species. Controlled ecology chambers have assisted materially in studying the effect of temperature on the compatibility reaction. The biochemical nature of incompatibility has received considerable impetus in relatively recent years. Increased activity in this very important area has paralleled the advent of more than sophisticated instrumentation in the field of biochemistry.

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https://www.sciencedirect.com/scientific discipline/article/pii/B9780408701495500373