Aposporous Apomixis Research Articles

English

Paspalum notatum Flüggé (bahiagrass) is the most important forage grass in the native grasslands of southern Brazil; the native germplasm is tetraploid and apomictic. Breeding to produce cultivars has been possible due to chromosome doubling of sexual diploid accessions, allowing hybridizations with apomictic genotypes, generating progenies with variability for agronomic traits and segregating for reproduction mode. The aim of this study was to determine the reproduction mode of selected hybrids derived from intraspecific crosses through cytoembryological analysis and to evaluate the level of apospory expres¬sivity. Eighty one tetraploid hybrids corresponding to 21 families of sexual and apomictic parents were generated and 28 most productive plants based on their superior agronomic performance in the field were evaluated. Sixteen hybrids were reproduced sexually and 12 of them were highly apomictic or facultative apomictic. Sexual hybrids may be used for further crosses in the breeding program while highly apomictic plants produce uniform progeny and become candidates of new cultivars to promote pasture diversification. Key words: Aposporous apomixis, bahiagrass, intraspecific crosses, plant breeding.

The correlation of phylogenetics, elevation and ploidy on the incidence of apomixis in Asteraceae in the European Alps

Abstract Asexual reproduction has often been regarded as an evolutionary dead end, but asexual lineages (most notably those that are apomictic) are present in several angiosperm families and often comprise a large number of taxa, both widespread and endemic. Investigating correlations between genetic, environmental and taxonomic factors and the incidence of apomixis has represented a challenge for many years, with previous analyses frequently omitting one or more of these variables. Here, flow cytometric seed screening, cytological data and ecological variables have been integrated in a phylogenetic framework to create a comprehensive dataset for 229 of Asteraceae from the European Alps. Data were analysed using phylogenetically informed generalized linear mixed models (pMCMCglmm) where elevation, ploidy and phenology were assessed for their potential correlation with asexual reproduction and apomixis type. Although apomixis is not dominant among the species studied, our results confirm that an odd ploidy (e.g. 3x) and, to a lesser extent, an even polyploid level (i.e. 4x) significantly increase its probability, most probably due to chromosome misalignments during meiosis. The distribution of apomictic species does not correlate with elevation, and there is a weak correlation between early flowering initiation and aposporous apomixis. Although current and future changes in climate may severely impact the survival of the flora of the European Alps, asexual reproduction and polyploidization may prove to be, at least temporarily, lifelines for the survival of a species under the novel climatic conditions. Therefore, uncovering how apomicts and polyploids evolve and persist will be essential for understanding the ecology of the European Alps and hence informing future conservation strategies.

Chromosomal Evolution and Apomixis in the Cruciferous Tribe Boechereae.

The mustard family (Brassicaceae) comprises several dozen monophyletic clades usually ranked as tribes. The tribe Boechereae plays a prominent role in plant research due to the incidence of apomixis and its close relationship to Arabidopsis. This tribe, largely confined to western North America, harbors nine genera and c. 130 species, with >90% of species belonging to the genus Boechera. Hundreds of apomictic diploid and triploid Boechera hybrids have spurred interest in this genus, but the remaining Boechereae genomes remain virtually unstudied. Here we report on comparative genome structure of six genera (Borodinia, Cusickiella, Phoenicaulis, Polyctenium, Nevada, and Sandbergia) and three Boechera species as revealed by comparative chromosome painting (CCP). All analyzed taxa shared the same seven-chromosome genome structure. Comparisons with the sister Halimolobeae tribe (n = 8) showed that the ancestral Boechereae genome (n = 7) was derived from an older n = 8 genome by descending dysploidy followed by the divergence of extant Boechereae taxa. As tribal divergence post-dated the origin of four tribe-specific chromosomes, it is proposed that these chromosomal rearrangements were a key evolutionary innovation underlaying the origin and diversification of the Boechereae in North America. Although most Boechereae genera exhibit genomic conservatism, intra-tribal cladogenesis has occasionally been accompanied by chromosomal rearrangements (particularly inversions). Recently, apomixis was reported in the Boechereae genera Borodinia and Phoenicaulis. Here, we report sexual reproduction in diploid Nevada, diploid Sandbergia, and tetraploid Cusickiella and aposporous apomixis in tetraploids of Polyctenium and Sandbergia. In sum, apomixis is now known to occur in five of the nine Boechereae genera.

High-Resolution Linkage Map With Allele Dosage Allows the Identification of Regions Governing Complex Traits and Apospory in Guinea Grass (Megathyrsus maximus).

Forage grasses are mainly used in animal feed to fatten cattle and dairy herds, and guinea grass (Megathyrsus maximus) is considered one of the most productive of the tropical forage crops that reproduce by seeds. Due to the recent process of domestication, this species has several genomic complexities, such as autotetraploidy and aposporous apomixis. Consequently, approaches that relate phenotypic and genotypic data are incipient. In this context, we built a linkage map with allele dosage and generated novel information of the genetic architecture of traits that are important for the breeding of M. maximus. From a full-sib progeny, a linkage map containing 858 single nucleotide polymorphism (SNP) markers with allele dosage information expected for an autotetraploid was obtained. The high genetic variability of the progeny allowed us to map 10 quantitative trait loci (QTLs) related to agronomic traits, such as regrowth capacity and total dry matter, and 36 QTLs related to nutritional quality, which were distributed among all homology groups (HGs). Various overlapping regions associated with the quantitative traits suggested QTL hotspots. In addition, we were able to map one locus that controls apospory (apo-locus) in HG II. A total of 55 different gene families involved in cellular metabolism and plant growth were identified from markers adjacent to the QTLs and APOSPORY locus using the Panicum virgatum genome as a reference in comparisons with the genomes of Arabidopsis thaliana and Oryza sativa. Our results provide a better understanding of the genetic basis of reproduction by apomixis and traits important for breeding programs that considerably influence animal productivity as well as the quality of meat and milk.

Ovule Gene Expression Analysis in Sexual and Aposporous Apomictic Hypericum perforatum L. (Hypericaceae) Accessions.

Hypericum perforatum L. (2n = 4x = 32) is an attractive model system for the study of aposporous apomixis. The earliest phenotypic features of aposporous apomixis in this species are the mitotic formation of unreduced embryo sacs from a somatic cell of the ovule nucellus and the avoidance of meiosis. In this research we addressed gene expression variation in sexual and apomictic plants, by focusing on the ovule nucellus, which is the cellular domain primarily involved into the differentiation of meiocyte precursors and aposporous embryo sacs, at a pre-meiotic developmental stage. Gene expression analyses performed by RNAseq identified 396 differentially expressed genes and 1834 transcripts displaying phenotype-specific expression. Furthermore, the sequencing and assembly of the genome from a diploid sexual accession allowed the annotation of a 50 kb sequence portion located upstream the HAPPY locus and to address the extent to which single transcripts were assembled in multiple variants and their co-expression levels. About one third of identified DEGs and phenotype-specific transcripts were associated to transcript variants with alternative expression patterns. Additionally, considering DEGs and phenotype-specific transcript, the co-expression level was estimated in about two transcripts per locus. Our gene expression study shows massive differences in the expression of several genes encoding for transposable elements. Transcriptional differences in the ovule nucellus and pistil terminal developmental stages were also found for subset of genes encoding for potentially interacting proteins involved in pre-mRNA splicing. Furthermore, the sexual and aposporous ovule transcriptomes were characterized by differential expression in genes operating in RNA silencing, RNA-mediated DNA methylation (RdDM) and histone and chromatin modifications. These findings are consistent with a role of these processes in regulating cell fate determination in the ovule, as indicated by forward genetic studies in sexual model species. The association between aposporous apomixis, pre-mRNA splicing and DNA methylation mediated by sRNAs, which is supported by expression data and by the enrichment in GO terms related to these processes, is consistent with the massive differential expression of multiple transposon-related sequences observed in ovules collected from both sexual and aposporous apomictic accessions. Overall, our data suggest that phenotypic expression of aposporous apomixis is concomitant with the modulation of key genes involved in the two interconnected processes: RNA splicing and RNA-directed DNA methylation.

New apomictic pathway in Myrtaceae inferred from Psidium cattleyanum female gametophyte ontogeny

Adventitious embryony and aposporous apomixis have been described in Myrtaceae. Psidium cattleyanum Sabine is a South American species with commercial potential as a fruit tree in natural areas. Interestingly the species is listed among the 100 world’s worst invasive alien species. Morphological and molecular uniformity of the offspring obtained from seeds suggests an apomictic pathway of reproduction. The aim of this study was to describe the ovule ontogeny and embryology of selected materials of Psidium cattleyanum for two infraspecific entities Psidium cattleyanum f. cattleyanum and Psidium cattleyanum f. lucidum O. Deg. and to identify possible ontogenetic evidence of apomictic embryo sac development. We collected and analyzed anatomical sections of flowers from very early stages of development until fruit formation started. Both taxa showed the typical ontogenetic pattern of Myrtaceae. Ovules are anatropous, bitegmic and crassinucelate and they show a zig-zag micropyle. As the megaspore mother cell enlarges in the nucellus no evidence of meiosis was registered. A megaspore-like cell was observed in the following stages of development. The embryo sac is Polygonum type with persistent antipodals. Immature seeds have nuclear endosperm. Development patterns of embryo sac and non-meiotic origin of the megaspore lead us to reject the adventitious embryony and apospory cited for other taxa in the family and, instead, a possible disploporic origin of the embryo sac is proposed for the first time for Myrtaceae.

Pistil Transcriptome Analysis to Disclose Genes and Gene Products Related to Aposporous Apomixis in Hypericum perforatum L.

Unlike sexual reproduction, apomixis encompasses a number of reproductive strategies, which permit maternal genome inheritance without genetic recombination and syngamy. The key biological features of apomixis are the circumvention of meiosis (i.e., apomeiosis), the differentiation of unreduced embryo sacs and egg cells, and their autonomous development in functional embryos through parthenogenesis, and the formation of viable endosperm either via fertilization-independent means or following fertilization with a sperm cell. Despite the importance of apomixis for breeding of crop plants and although much research has been conducted to study this process, the genetic control of apomixis is still not well understood. Hypericum perforatum is becoming an attractive model system for the study of aposporous apomixis. Here we report results from a global gene expression analysis of H. perforatum pistils collected from sexual and aposporous plant accessions for the purpose of identifying genes, biological processes and molecular functions associated with the aposporous apomixis pathway. Across two developmental stages corresponding to the expression of aposporous apomeiosis and parthenogenesis in ovules, a total of 224 and 973 unigenes were found to be significantly up- and down-regulated with a fold change ≥ 2 in at least one comparison, respectively. Differentially expressed genes were enriched for multiple gene ontology (GO) terms, including cell cycle, DNA metabolic process, and single-organism cellular process. For molecular functions, the highest scores were recorded for GO terms associated with DNA binding, DNA (cytosine-5-)-methyltransferase activity and heterocyclic compound binding. As deregulation of single components of the sexual developmental pathway is believed to be a trigger of the apomictic reproductive program, all genes involved in sporogenesis, gametogenesis and response to hormonal stimuli were analyzed in great detail. Overall, our data suggest that phenotypic expression of apospory is concomitant with the modulation of key genes involved in the sexual reproductive pathway. Furthermore, based on gene annotation and co-expression, we underline a putative role of hormones and key actors playing in the RNA-directed DNA methylation pathway in regulating the developmental changes occurring during aposporous apomixis in H. perforatum.

Sequence Characterized Amplified Regions Linked with Apomictic Mode of Reproduction in Four Different Apomictic <i>Cenchrus</i> Species

The genus Cenchrus comprises tropical and sub-tropical forage grasses, many of which reproduce by aposporous apomixis. This mode of reproduction hampers genetic improvement of the species through conventional breeding; however, it may facilitate the fixing of heterosis in a hybrid. Apomixis can be deployed in plant breeding to maintain hybrid vigour over the generations, thereby accelerating the breeding process. Markers for apospory have been identified in grasses, but we have identified conserved genomic regions and apomixis-specific AFLP markers in eight Cenchrus spp. and converted them into sequence characterised amplified regions (SCARs). Seventeen of 94 identified polymorphic AFLP markers were located in the apospory-specific genomic region and were converted into SCAR markers. Only four (23.5%) were successfully converted into SCARs linked with an apomictic mode of reproduction in Cenchrus spp. The SCAR markers were validated in an F 2 population of C. ciliaris consisting of 48 apomictic and 38 sexual individuals. The SCARs were conserved across the four apomictic Cenchrus spp. with >98% sequence homology. In silico mapping of the SCARs based on sequence homology indicated synteny with segments of chromosome 12 of rice and chromosome 5 of maize. These markers would be very useful for genetic/molecular analyses of apomixis, comparative mapping studies and marker-assisted breeding of Cenchrus .

Development of an ISSR-derived SCAR marker linked to apospory in buffel grass (Cenchrus ciliarisL.)

Cenchrus ciliaris (buffel grass) is a valuable forage grass, reproducing through aposporous apomixis. Inter Simple Sequence Repeat (ISSR) markers converted to SCAR marker linked to apospory in this crop is reported. Out of twenty-five ISSR primers tested in DNA bulks from apomictic and sexual progenies, five primers detected polymorphism. The 1.1 kb amplicon derived from the primer ISSR-856 showed co-segregation with apomictic mode of reproduction. Accordingly, a pair of SCAR (Sequence Characterized Amplified Region) primers were designed which also generated a 1.1kb amplicon (Apo-856) only in the apomictic plants. Importance of such molecular markers in genetics and breeding of C. ciliaris is discussed.

The polyploid nature of Cenchrus ciliaris L. (Poaceae) has been overlooked: new insights for the conservation and invasion biology of this species – a review

Climate change, associated with increased aridity, and high grazing pressure by livestock results in the scarcity and loss of perennial Poaceae in arid ecosystems. The species threatened by this include Cenchrus ciliaris L., a native perennial grass of the tropical and sub-tropical arid rangelands of Africa and Western Asia and now introduced in Central and South America, and Australia. This species reproduces predominantly through aposporous apomixis although sexual individuals have been occasionally identified. Cenchrus ciliaris is characterised by a significant, heritable, phenotypic polymorphism and three ploidy levels including tetraploids (2n = 4x = 36), pentaploids (2n = 5x = 45) and hexaploids (2n = 6x = 54). Under water-deficit conditions, C. ciliaris shows plasticity in growth characteristics and aboveground biomass. This phenotypic plasticity has led to the identification of genotypic-associated responses conferring more productivity. This underlines the importance of conserving the genetic diversity of C. ciliaris in order to ensure the persistence of the vegetation cover in the arid ecosystems in which it occurs. Observations from cytogenetic and molecular data converge to underline the possibility of sexual reproduction, recombination and gene flow within and between populations of C. ciliaris. Genetic mechanisms, such as polyploidy, hybridisation between ploidy levels and apomixes, are generating and then maintaining the diversity of C. ciliaris. This review emphasises the role of polyploidy in the evolutionary development of C. ciliaris and how it may be a crucial factor for its conservation in some countries and its weedy nature in others.

An apomictic tetraploid Paspalum chaseanum cytotype and its cytogenetic relationship with P. plicatulum (Poaceae): taxonomic and genetic implications

Paspalum chaseanum Parodi (Poaceae) is a rare species seldom found in the vast phytogeographic Chaco region of South America. It occurs in some localities as a diploid with 20 somatic chromosomes, reproduces sexually and is self-incompatible. A tetraploid cytotype was recently collected in this geographic region. This accession was determined to reproduce of aposporous apomixis and was crossed, as pollen donor, onto a sexual autotetraploid plant of P. plicatulum Michx. The meiotic chromosome pairing behaviour of both parents and their hybrids was primarily as bivalents and quadrivalents, indicating that tetraploid P. chaseanum is likely to have an autoploid origin, and that both species share basically the same genome. Although some controversies exist regarding the subgeneric taxonomic classification of P. chaseanum, these results support its inclusion in the informal Plicatula group of Paspalum. The P. plicatulum × P. chaseanum hybrids segregated for apomixis. The amount of seed set in some hybrids (up to 17%) and the presence of sexual as well as facultative apomictic individuals in the progeny suggest that gene transfer through hybridisation is a feasible tool in genetic-improvement programs concerning these forage grass species.

Cytology and embryology of the pompom weed, Campuloclinium macrocephalum (Eupatorieae, Asteraceae)

Campuloclinium macrocephalum (Less.) DC. is a perennial herb widely distributed in the New World, but introduced to South Africa, where it is commonly called “pompom weed”. This species is considered one of the most important weeds from Brazil and it has been included among the plant invaders of South Africa. Results of the meiotic and embryological analyses of six populations of C. macrocephalum are reported in this paper. The microsporogenesis analysis revealed five triploid (2 n = 3 x = 30) and one diploid (2 n = 2 x = 20) populations with a basic chromosome number x = 10. The diploid specimens showed regular meiotic behavior, but the triploid plants presented irregular chromosome pairing which result in the formation of univalents, bivalents and trivalents at diakinesis. In consequence, laggard chromosomes, unbalanced nuclei and micronuclei were observed in subsequent phases of meiosis. The embryological analysis showed that the triploid specimens of C. macrocephalum have embryo sac development from a nucellar cell (apospory), which indicates that these specimens are apomictic. Almost all cases of apomixis found in tribe Eupatorieae are diplosporous apomixis. Campuloclinium macrocephalum constitutes the second species of the tribe and the first of the genus with apospory as reproductive system. The aposporous apomixis combined with the presence of xylopodium would be two important factors responsible for the invasiveness of C. macrocephalum.

Validation of molecular markers linked to apospory in tetraploid races of bahiagrass, Paspalum notatum Flüggé

Tetraploid (2n = 4x = 40) races of Paspalum notatum Flugge are important natural forage grasses for the tropical and subtropical areas of the Americas. Almost all natural accessions reproduce by obligate aposporous apomixis. Previous work on the species allowed the identification of several molecular markers completely linked to apospory, one component of apomictic reproduction. Moreover, after a fingerprinting characterization of a germplasm collection, 11 amplified fragment length polymorphism (AFLP) markers exclusive to apomictic accessions were detected. The objectives of this work were (1) to validate the presence of molecular markers linked to apospory in tetraploid races of different geographic origins, (2) to determine if markers specific to apomictic accessions were associated with the mode of reproduction, and (3) to develop single-locus markers of apospory that can be used for marker-assisted selection. Thirteen natural apomictic accessions were analyzed. Moreover, the parental plants Q4188 (non-aposporous) and Q4117 (aposporous) and 44 F1 progenies (36 non-aposporous, 8 aposporous) derived from them were used as a validation population. Nine markers [two random amplification of polymorphic DNA (RAPD) and seven AFLP] 100% linked to apospory in Q4117 were tested. Amplification reactions with the corresponding primers showed that all markers were present in the 13 aposporous (apomictic) accessions, but were absent in the non-aposporous controls. On the other hand, linkage analysis of the 11 AFLP markers specific to the apomictic accessions showed that all of them were linked in coupling to apospory (r = 0.00, LOD 13.245). Based on one AFLP (E36M37c), two sequence characterized amplification region (SCAR) markers (SPNA1 and SPNA2) co-segregating with the trait and present in the 13 apomictic accessions were developed. The presence of markers associated with apospory was conserved among tetraploid accessions of different geographic origins. Moreover, the single-locus markers SPNA1 and SPNA2 could be used for routine marker-assisted selection in hybrid populations segregating for apospory and to facilitate the isolation of apospory-related genes.

Analysis of expressed sequence tags in apomictic guineagrass ( Panicum maximum)

Apomixis is an intriguing asexual mode of reproduction, because it produces maternal clones that permit vegetative reproduction through seeds. Guineagrass (Panicum maximum) has both facultative aposporous apomixis and obligate sexual modes of reproduction. Despite the importance of apomixis in guineagrass, expressed sequence tags (ESTs) for this condition have not been studied in this species. We constructed a guineagrass cDNA library from two aposporous strains, Ku5954 and GM64-3A, and utilized them as microarray probes. To find genes uniquely expressed in the immature pistils of apomicts, we performed a microarray analysis using target RNA from another apomict, OKI64. Of the 4608 probes in the microarray, only 394 showed clear gene expression in the immature pistils. Of the 394 expressed probes, 196 were successfully sequenced. Of these, 181 had significant homology with other species, including 10 ESTs with matches in a pistil cDNA library from another aposporous species, Cenchrus ciliaris. Of the remaining ESTs, three showed significant homology only with animal database sequences and the other 12 ESTs showed no homology with any previously registered sequence. In reverse-transcriptase PCR and real-time quantitative PCR, nine ESTs reliably detected ovary-specific gene expression. Of these, three revealed aposporous ovary-specific genes expressed in the early developmental stage, suggesting that these could be apomixis-related genes.

Gene expression analysis at the onset of aposporous apomixis in Paspalum notatum

Apomixis is a route of asexual reproduction through seeds, that progresses in the absence of meiosis and fertilization to generate maternal clonal progenies. Gametophytic apomicts are usually polyploid and probably arose from sexual ancestors through a limited number of mutations in the female reproductive pathway. A differential display analysis was carried out on immature inflorescences of sexual and apomictic tetraploid genotypes of Paspalum notatum, in order to identify genes associated with the emergence of apospory. Analysis of approximately 10,000 transcripts led to the identification of 94 high-quality differentially expressed sequences. Assembling analysis, plus validation, rendered 65 candidate unigenes, organized as 14 contigs and 51 singletons. Thirty-four unigenes were isolated from apomictic plants and 31 from sexual ones. A total of 45 (69.2%) unigenes were functionally categorized. While several of the differentially expressed sequences appeared to be components of an extracellular receptor kinase (ERK) signal transduction cascade, others seemed to participate in a variety of central cellular processes like cell-cycle control, protein turnover, intercellular signalling, transposon activity, transcriptional regulation and endoplasmic reticulum-mediated biosynthesis. In silico mapping revealed that a particular group of five genes silenced in apomictic plants clustered in a rice genomic area syntenic with the region governing apospory in Paspalum notatum and Brachiaria brizantha. Two of these genes mapped within the set of apo-homologues in P. notatum. Four genes previously reported to be controlled by ploidy were identified among those expressed differentially between apomictic and sexual plants. In situ hybridization experiments were performed for selected clones.

A genetic map of tetraploid Paspalum notatum Flügge (bahiagrass) based on single-dose molecular markers

Paspalum notatum Flügge is a warm-season forage grass with mainly diploid (2n = 20) and autotetraploid (2n = 40) representatives. Diploid races reproduce sexually and require crosspollination due to a self-incompatible mating system, while autotetraploids reproduce by aposporous apomixis. The objectives of this work were to develop a genetic linkage map of Paspalum notatum Flügge at the tetraploid level, identify the linkage/s group/s associated with apomixis and carry out a general characterization of its mode of inheritance. A pseudo test-cross F1 family of 113 individuals segregating for the mode of reproduction was obtained by crossing a synthetic completely sexual tetraploid plant (Q4188) as female parent with a natural aposporous individual (Q4117) as pollen donor. Map construction was based on single-dose markers (SDAFs) segregating from both parents. Two linkage maps (female and male) were constructed. Within each map, homologous groups were assembled by detecting repulsion-phase linked SDAFs. Putative Q4188 and Q4117 homolog groups were identified by mapping shared single dose markers (BSDF). The Q4188 map consisted of 263 markers distributed on 26 co-segregation groups over a total genetic distance of 1.590.6 cM, while the Q4117 map contained 216 loci dispersed on 39 co-segregation groups along 2.265.7 cM, giving an estimated genome coverage of 88% and 83%, respectively. Seven and 12 putative homologous chromosomes were detected within Q4188 and Q4117 maps, respectively. Afterward, ten female and male homologous chromosomes were identified by mapping BSDFs. In the Q4117 map, a single linkage group was associated with apospory. It was characterized by restriction in recombination and preferential chromosome pairing. A BPSD marker mapping within this group allowed the detection of the female homolog and the putative four male groups of the set carrying apospory.

Inheritance of apospory in bahiagrass, Paspalum notatum.

Previous studies on the inheritance of aposporous apomixis in bahiagrass showed a wide range of segregation ratios in crosses involving sexual and aposporous apomictic plants. The F1 progenies were classified through a visual progeny test carried out on few F2 plants. The number of sexual F1s highly exceeded the apomictics leading to the conclusion that apomixis was controlled by a few recessive genes. The present study examines the inheritance of apospory in bahiagrass. A sexual plant was self-pollinated and crossed with an aposporous apomictic plant as pollen donor. Backcross and F2 progenies were obtained in several combinations. All self-pollinated sexual plants or sexual x sexual crosses produced progenies free of apospory. All crosses involving a sexual and an apomictic plant produced approximately three times more apospory-free plants than plants with apospory. Bahiagrass is of autotetraploid origin and hence is expected to display tetrasomic inheritance. The most widely accepted genetic model for inheritance of apospory in tropical grasses is a single dominant gene with tetrasomic inheritance. In the present experiments none of the apospory-free F1s segregated for the apospory trait indicating that it is most likely a dominant character. However, the observed results fit better a modified model: tetrasomic inheritance of a single dominant gene with pleiotropic effect and incomplete penetrance. The excess of apospory-free plants in the F1 progeny could be ascribed to some distortion in the segregation pattern due to a pleiotropic lethal effect of the dominant A allele with incomplete penetrance. Alternatively, partial lethality of factors linked to aposporous gene may account for segregation distortion against apospory.

Genetic diversity in sexual diploid and apomictic tetraploid populations of Paspalum notatum situated in sympatry or allopatry

Paspalum notatum is a subtropical grass widely distributed in the temperate areas of America. Diploids are sexual while polyploids give rise to clonal seeds through aposporous apomixis. RAPD markers were used to analyze the genetic structure of three natural populations: i) diploids reproducing sexually (R2X); ii) sympatric apomictic tetraploids collected in the vicinity of the diploids (R4X); iii) allopatric apomictic tetraploids growing in isolation (C4X). The apomictic reproduction rate was evaluated by the use of molecular markers in progeny tests, while chromosome-counting allowed the verification of ploidy levels. Data revealed that the R4X group presented a variation considerably higher than that observed for C4X. Jaccard’s coefficients were used to produce a cluster diagram using the UPGMA method. All but one tetraploid genotypes grouped together and were associated to diploid genotype A21. The possibility of occasional generation of novel tetraploid clones from the interaction between tetraploid and diploid individuals is discussed.

A rise of ploidy level induces the expression of apomixis in Paspalum notatum

The common races of bahiagrass, Paspalum notatum, are tetraploid (2n=4x=40) and reproduce by aposporous apomixis. Paspalum notatum var. saurae is the corresponding diploid (2n=2x=20) sexual race that outbreeds due to self-incompatibility. Chromosome doubling was induced by colchicine treatments in three individual plants from a natural diploid population. Embryological studies demonstrated that one of the induced autotetraploid plants reproduced sexually. The other two autotetraploids were facultative apomicts. These results indicate that an unexpressed gene(s) for apomixis exists at the diploid level. The expression of the trait is ploidy-dependent. The ploidy dependency may act either on the locus controlling apomixis through some transcription factors or via a secondary locus which requires a higher allele dosage to affect the expression of the main locus.

Non-Mendelian transmission of an apospory-specific genomic region in a reciprocal cross between sexual pearl millet ( Pennisetum glaucum ) and an apomictic F1 ( P. glaucum × P. squamulatum )

We recently showed that aposporous apomixis, a form of gametophytic apomixis, is controlled by a single apospory-specific genomic region (ASGR) in both Pennisetum squamulatum and Cenchrus ciliaris. We present evidence that in a reciprocal cross between sexual pearl millet (P. glaucum) and an apomictic F1 (P. glaucum× P. squamulatum) the ASGR is not transmitted at the same rate. When pearl millet was used as the female parent and the apomictic genotype as the pollen donor, the ASGR was transmitted at a rate of 0.41 in a progeny of 57 plants, indicating a slight transmission ratio distortion. However, in a population of 52 rare sexual progenies characterized among a large progeny of a quasi-obligate apomict (an F1 hybrid of P. glaucum×P. squamulatum), the transmission rate of ASGR was only 0.12. This strong segregation distortion may have occurred at four different levels: (1) female meiosis, (2) during female gametophyte maturation, (3) upon fertilization with differential survival of embryos being a consequence of differential gene expression controlled by parent-of-origin specific effects (imprinting) and (4) at a later developmental stage of the embryo through an embryo/endosperm genetic incompatibility system.