Obligate Apomixis Research Articles

High fruit setting rate without male participation: A case study of obligate apomixis in Rhomboda tokioi (Orchidaceae)

Seed production is the key factor for maintaining plant population renewal and development. Because of pollination limitations and other factors, orchids, especially rewardless ones, generally have low fruit set rates. In Rhomboda tokioi, however, the natural fruit setting rate is very high, more than 90%. Common reasons for such a high natural fruit setting rate include high pollinator efficiency, autonomous selfing, seed formation through apomixis and so on. To test these possibilities, we carried out field observations, morphological examinations, and breeding system experiments. The results showed no migration of pollinia into or out of flowers and suggested that the structure of the floral organs hinder autonomous selfing. Interestingly, the fruit setting rate of this orchid was still higher than 90% when the gynostemium was removed prior to flowering. Additionally, after hand pollination, the pollen tube stopped growing at a late stage and could not enter the ovule, indicating the seeds were produced by obligate apomixis. Morphological experiments showed that a megasporocyte first forms a chain tetrad and then develops into a mature embryo sac, while the embryo of the offspring developed from unfertilized egg cells. More interestingly, the offspring had the same number of chromosomes as the female parent. According to the existing theory of apomixis, we can infer that the reproductive model of R. tokioi is Allium-type apomixis.

A drought-driven model for the evolution of obligate apomixis in ferns: evidence from pellaeids (Pteridaceae).

Xeric environments impose major constraints on the fern life cycle, yet many lineages overcome these limitations by evolving apomixis. Here, we synthesize studies of apomixis in ferns and present an evidence-based model for the evolution and establishment of this reproductive strategy, focusing on genetic and environmental factors associated with its two defining traits: the production of "unreduced" spores (n = 2n) and the initiation of sporophytes from gametophyte tissue (i.e., diplospory and apogamy, respectively). We evaluated existing literature in light of the hypothesis that abiotic characteristics of desert environments (e.g., extreme diurnal temperature fluctuations, high light intensity, and water limitation) drive the evolution of obligate apomixis. Pellaeid ferns (Cheilanthoideae: Pteridaceae) were examined in detail, as an illustrative example. We reconstructed a plastid (rbcL, trnG-trnR, atpA) phylogeny for the clade and mapped reproductive mode (sexual versus apomictic) and ploidy across the resulting tree. Our six-stage model for the evolution of obligate apomixis in ferns emphasizes the role played by drought and associated abiotic conditions in the establishment of this reproductive approach. Furthermore, our updated phylogeny of pellaeid ferns reveals repeated origins of obligate apomixis and shows an increase in the frequency of apomixis, and rarity of sexual reproduction, among taxa inhabiting increasingly dry North American deserts. Our findings reinforce aspects of other evolutionary, physiological, developmental, and omics-based studies, indicating a strong association between abiotic factors and the establishment of obligate apomixis in ferns. Water limitation, in particular, appears critical to establishment of this reproductive mode.

Can obligate apomixis and more stable reproductive assurance explain the distributional successes of asexual triploids in Hieracium alpinum (Asteraceae)?

Although reproductive assurance has been suggested to be one of the most important factors shaping the differential distributional patterns between sexuals and asexuals (geographic parthenogenesis), it has only rarely been studied in natural populations of vascular plants with autonomous apomixis. Moreover, there are almost no data concerning the putative relationship between the level of apomictic versus sexual plant reproduction on one hand, and reproductive assurance on the other. We assessed the level of sexual versus apomictic reproduction in diploid and triploid plants of Hieracium alpinum across its distributional range using flow cytometric analyses of seeds, and compared the level of potential and realized seed set, i.e. reproductive assurance, between the two cytotypes under field and greenhouse conditions. Flow cytometric screening of embryos and endosperms of more than 4,100 seeds showed that diploids produced solely diploid progeny sexually, while triploids produced triploid progeny by obligate apomixis. Potential fruit set was much the same in diploids and triploids from the field and the greenhouse experiment. While in the pollination-limited environment in the greenhouse apomictic triploids had considerably higher realized fruit set than sexual diploids, there was no significant difference between cytotypes under natural conditions. In addition, sexuals varied to a significantly larger extent in realized fruit set than asexuals under both natural and greenhouse conditions. Our results indicate that triploid plants reproduce by obligate apomixis, assuring more stable and predictable fruit reproduction when compared to sexual diploids. This advantage could provide apomictic triploids with a superior colonisation ability, mirrored in a strong geographic parthenogenesis pattern observed in this species.

The classification of asexual organisms: Old myths, new facts, and a novel pluralistic approach

AbstractOrganisms reproducing via asexuality harbor a great diversity of lineages, morphotypes and ecotypes. However, classification of asexual taxa does not fit into contemporary species concepts, and hence the diversity of apomictic plant complexes is not well reflected in taxonomy. Plants reproducing via apomixis (i.e., asexual seed formation = agamospermy) exemplify the theoretical and practical problems of classification. Obligately asexual organisms do not form reproductive communities, but they do constitute ancestor‐descendant lineages. From the conceptual side, evolutionary lineage concepts would fit best for species delimitation. Recent research showed that these lineages are not necessarily threatened by rapid extinction and do have persistence in time and space. Facultative sexuality and low levels of residual recombination counteract the accumulation of deleterious mutations due to the lack of recombination (Muller’s ratchet). Apomictic lineages do have adaptive potential, which is demonstrated by the ability to occupy large distribution areas and to experience ecological niche shifts. The challenge for classification of asexual lineages, however, is to find operational criteria for species delimitation. Current practices of species delimitation can be grouped into four main principles: (1) the sexuals‐first principle means that obligate sexual progenitor species are classified separately from their apomictic derivatives. (2) The all‐in‐one principle merges sexual progenitors and highly facultative apomictic derivatives into one species, whereby the apomicts often represent autopolyploids without differentiated phenotypes. (3) The cluster concept applies to allopolyploid complexes with facultative apomixis and a huge diversity of genotypes and morphotypes; here the main genetic clusters are treated as species. (4) Almost obligate apomictic lineages are being classified as agamospecies. The four principles reflect quite well evolutionary traits and diversity of lineages. Finally, a recommendation for a workflow is given, following this gradient from obligate sexuality to obligate apomixis.

Harnessing apomictic reproduction in grasses: what we have learned from Paspalum

Apomixis is an alternative route of plant reproduction that produces individuals genetically identical to the mother plant through seeds. Apomixis is desirable in agriculture, because it guarantees the perpetuation of superior genotypes (i.e. heterotic hybrid seeds) by self-seeding without loss of hybrid vigour. The Paspalum genus, an archetypal model system for mining apomixis gene(s), is composed of about 370 species that have extremely diverse reproductive systems, including self-incompatibility, self-fertility, full sexual reproduction, and facultative or obligate apomixis. Barriers to interspecific hybridization are relaxed in this genus, allowing the production of new hybrids from many different parental combinations. Paspalum is also tolerant to various parental genome contributions to the endosperm, allowing analyses of how sexually reproducing crop species might escape from dosage effects in the endosperm. In this article, the available literature characterizing apomixis in Paspalum spp. and its use in breeding is critically reviewed. In particular, a comparison is made across species of the structure and function of the genomic region controlling apomixis in order to identify a common core region shared by all apomictic Paspalum species and where apomixis genes are likely to be localized. Candidate genes are discussed, either as possible genetic determinants (including homologs to signal transduction and RNA methylation genes) or as downstream factors (such as cell-to-cell signalling and auxin response genes) depending, respectively, on their co-segregation with apomixis or less. Strategies to validate the role of candidate genes in apomictic process are also discussed, with special emphasis on plant transformation in natural apomictic species.

INDUCTION OF APOMIXIS BY OUTCROSSING BETWEEN GENETICALLY DIVERGENT ENTITIES OF CALOGLOSSA LEPRIEURII (CERAMIALES, RHODOPHYTA) AND EVIDENCE OF HYBRID APOMICTS IN NATURE(1).

Our previous study revealed that apomixis, recycling of tetrasporophytes, can be generated through outcrossing between genetically divergent entities of Caloglossa monosticha M. Kamiya, though such apomicts have never been found in nature. In the case of C. leprieurii (Mont.) G. Martens, the most widespread species in this genus, many apomictic strains have been isolated worldwide, but it is unknown whether these apomicts evolved through an outcrossing process similar to that in C. monosticha. In this study, heterogeneity of the apomicts and their sexual relatives as well as their evolutionary relationships was examined using the nuclear-encoded actin gene and plastid-encoded RUBISCO spacer region. Thirteen out of 18 apomictic strains were heterogeneous and contained divergent actin alleles, whereas only two out of 23 sexual strains were heterogeneous. The five homogeneous apomicts were genetically identical, or quite similar, to the sexual strains isolated from adjacent sites. Furthermore, three of the five homogeneous apomicts frequently produced tetraspores that grew into gametophytes, while all the heterogeneous apomicts never generated gametophytes. Apomictic strains from Florida were allotriploid, and each of the three actin sequences was closely related to those of sexual strains from Florida, Peru, and Mexico/Guatemala. In crossing tests, obligate apomixis was generated through the outcrossing between the male from Madagascar and the female from the northwestern Atlantic. These results suggest that outcrossing between genetically divergent sexual entities is one factor that induces apomixis in C. leprieurii.

Ploidy levels and reproductive behaviour in natural populations of five Paspalum species

Knowledge of variation in ploidy levels and reproductive behaviour in natural populations is essential in order to understand the functioning of agamic complexes. The aim of this study was to analyse the ploidy level and mode of reproduction in several wild Paspalum populations. A total of 19 populations representing five different species (P. alcalinum, P. denticulatum, P. lividum, P. nicorae, and P. rufum) were collected. Ploidy level was determined in 1,187 individuals by using flow cytometry. Among these individuals, 2x, 3x, 4x, 5x, 6x, and 7x chromosome constitutions were observed. Diploid sexual cytotypes of P. denticulatum were detected for the first time; this will allow the development of future breeding strategies for this particular species. Flow cytometry seed screen (FCSS) in bulked and single seeds revealed the reproductive diversity of these species, ranging from complete sexuality in diploids and varying levels of facultative apomixis in most tetraploids, to obligate apomixis in pentaploids and hexaploids. A fully sexual tetraploid plant was never detected. Nevertheless, most tetraploid genotypes produced both maternal (by apomixis) and non-maternal (by sexuality) progeny. This residual sexuality is very interesting from an evolutionary point of view, since it would allow the creation of new genotypic combinations in natural populations. In addition, the residual sexuality found in some apomictic tetraploid populations can be used as a source of variability for genetic improvement.

ORIGIN OF APOMICTIC RED ALGAE: OUTCROSSING STUDIES OF DIFFERENT STRAINS IN CALOGLOSSA MONOSTICHA (CERAMIALES, RHODOPHYTA)(1).

Various red algae lack sexual reproduction and propagate by spore recycling, but it is still unknown how apomixis originates. In previous crossing experiments, we obtained an unusual hybrid of Caloglossa monosticha M. Kamiya through the outcrossing between a male from Australia and a female from Indonesia. This hybrid was morphologically identical to a normal tetrasporophyte, but its tetraspores grew into tetrasporophytes and repeated tetraspore recycling. During 5 years of culture, no sexual reproductive structures have formed on the tetrasporelings from this hybrid or its progenies. Further hybridization experiments revealed that all the five female strains from Indonesia successfully crossed with the male strain from the East Alligator River, Australia, and most of the F1 sporophytes demonstrated tetraspore recycling, though the germination rates of these tetraspores were quite low. The ploidy level of the hybrid tetrasporophyte was similar to the normal tetrasporophyte, rather than the parental gametophyte, based on the comparison of relative DNA contents of their nuclei. Single strand conformation polymorphism (SSCP) and sequence analyses of the internal transcribed spacer 1 (ITS1) region indicated that the alleles from both parents were present in all the hybrid tetrasporophytes examined. These results suggest that this hybrid does not carry out meiosis during sporogenesis, and heterozygous diploid sporophytes arose from tetraspores. Therefore, we believe that obligate apomixis was generated through outcrossing between genetically different entities of C.monosticha.

Apomixis in Eulaliopsis binata: characterization of reproductive mode and endosperm development

Apomixis represents an alteration of classical sexual plant reproduction to produce seeds with essentially clonal embryos, stimulating wide interest from biologists and plant breeders for its ability to fix heterosis. Eulaliopsis binata (Poaceae), is identified here as a new apomictic species. Embryological investigation indicates that the developmental pattern of embryo sac formation in E. binata represents gametophytic apospory, the embryo originating from an unreduced cell, without fertilization and the mode of endosperm development was autonomous. Sexual embryo sacs were found with a frequency of 1–4% depending on the biotype. The DNA content of nuclei (C-value) in mature seeds was screened by flow cytometry (FCSS) and demonstrated that the endosperm was derived autonomously without fertilization and the three biotypes of E. binata showed varying degrees of apomixis. The Wide-leaf type showed obligate apomixis whereas the Slender-leaf and the Red-haulm type displayed facultative apomixis. In addition, adventitious embryos were observed on the wall of ovary, integument and nucellus cells, indicating that E. binata produces embryos via a mixture of apospory and adventitious embryony.

First Report and Epidemics of Buffelgrass Blight Caused by Pyricularia grisea in South Texas.

A blight on buffelgrass, Cenchrus ciliaris L., has been observed for several years in south Texas and Mexico. The disease did not reach epidemic proportions until 1996. The causal agent, identified as Pyricularia grisea (Cooke) Sacc., is a common pathogen of grasses and other cultivated crops. Several Pennisetum spp. have been reported as hosts of Pyricularia spp.; this is the first report of buffelgrass as a host of this pathogen (1,2). Pathogenicity of P. grisea on buffelgrass was confirmed by greenhouse inoculations of 2-month-old buffelgrass plants with conidia washed with distilled water from monoconidial isolations of the pathogen, grown on potato dextrose agar, from infected leaves collected in several locations in south Texas and Mexico. Plants were placed for 8 h every night inside a plastic enclosure with a humidifier, simulating the high relative humidity conditions prevalent during the epidemic. Typical lesions developed after 7 days. The pathogen was re-isolated from the lesions after 10 days, fulfilling Koch's postulates. Conidia harvested from the sporulating samples were hyaline, transversely septate, with one to three septa, most of them having two. Conidia were obpyriform, with hylum often protuberant, measuring 20.6 to 26.3 μm in length and 8.5 to 10.1 μm wide. These measurements are consistent with those given for Pyricularia spp. by Ellis (1). Conidiophores were hyaline, single, slender, and unbranched. Initial symptoms were dark, discolored spots on the leaf that developed into tan, round to elliptical, necrotic lesions with a dark red border and a yellow, chlorotic halo. With increasing severity, lesions can coalesce, killing the entire leaf blade. Under heat and moisture stress, leaves with few lesions and yellow discoloration will wilt completely. Except for the presence of distinct lesions, wilted plants appear to be suffering from severe drought stress or herbicide injury. Losses vary from a few lesions to wilted whole plants and entire pastures. The pathogen also reduces the quantity and quality of seed by infecting involucres of the head. In the absence of the disease, even under severe moisture or drought stress, buffelgrass is able to thrive. Common T-4464 buffelgrass, which is highly susceptible to P. grisea, was introduced into south Texas in the late 1940s and is currently grown on 8 to 10 million acres in south Texas and Mexico. Buffelgrass reproduces by obligate apomixis, in which seeds are formed without sexual fertilization. Consequently, the progeny are genetically identical to the maternal parent. The monoculture of this grass with its unique type of reproduction encompasses millions of acres with genetically identical plants. Interaction of inoculum with weather conditions (nights with 8 to 10 h of more than 75% relative humidity) in 1996, 1997, and the late summer of 1998 produced epidemics of buffelgrass blight throughout south Texas and northern Mexico. P. grisea was also isolated from lesions on grassburr Cenchrus incertus M. A. Curtis collected throughout the area.

Inheritance of parental genomes in progenies of Poa pratensis L. from sexual and apomictic genotypes as assessed by RAPD markers and flow cytometry

Moving gene(s) responsible for the apomictic trait into crop plants that naturally reproduce through a sexual process would open up new areas in plant breeding and agricultural systems. Kentucky bluegrass (Poa pratensis L.) is one of the most important forage and turf grasses in temperate climates. It reproduces through facultative aposporous parthenogenesis, but the reproductive behaviour ranges naturally from nearly obligate apomixis to complete sexuality. In addition to apomictic reproduction, sexual hybridization may take place. Selfing may also occur, and occasionally reduced egg cells may develop through parthenogenesis generating (poly)haploids. The inheritance of parental genomes was assessed in Kentucky bluegrass progenies by employing RAPD markers in combination with flow cytometry (FCM). Nine progenies from different crosses carried out between completely sexual and highly apomictic genotypes were evaluated in order to probe the reproductive behaviour of the mother plants and to distinguish the different classes of aberrant plants. Not only were maternals and balanced BII hybrids recorded, but so were (poly)triploid BIII hybrids, selfs, and (poly)haploids. The application of these techniques demonstrated that FCM analysis accurately distinguishes the n, 2n, and 3n ploidy levels of progenies, and that RAPD markers unequivocally recognize progenies of apomictic and hybrid origin. The occurrence of aneusomaty was documented in one of the selected sexual genotypes, whose crossed progeny plants manifested two distinct classes of ploidy. The nomenclature BI was adopted to refer to hybrids with a hypodiploid nuclear condition. On the whole, the FCM analysis confirmed most of the RAPD data. The combined evaluation of DNA markers and DNA contents proved to be an efficient screening tool for scoring maternal plants, assessing the genetic origin of aberrant plants, and quantifying the inheritance of parental genomes in Kentucky bluegrass. Hybrid populations from sexual×apomictic matings that segregate for the mode of reproduction represent a valuable basis for attempting to identify molecular markers linked to the apomixis gene(s).

Cytoembryology of Paspalum chaseanum and Sexual Diploid Biotypes of Two Apomictic Paspalum Species

This study was undertaken to determine the cytology, method of reproduction, and flowering behaviour of three Paspalum species. Paspalum plicatulum has long been considered a segmental allotetraploid that reproduces by obligate apomixis with the type being apospory followed by pseudogamy. Paspalum simplex is an apomictic autotetraploid species, while P. chaseanum is a rare species for which no information regarding cytology and reproduction is available. This investigation concerns diploid cytotypes (2n = 2x = 20) of P. plicatulum and P. simplex that were recently collected in subtropical South America. In addition, two accessions of P. chaseanum were also recorded and both had 2n = 2x = 20 chromosomes. Meiosis showed regular bivalent chromosome pairing. Embryological observations indicated that the three species reproduce sexually at the diploid level. Pollen–pistil interaction following self-pollination suggested the presence of a self-incompatibility system responsible for allogamy. The results indicate that P. plicatulum and P. simplex complexes consist of diploid sexual allogamous cytotypes in addition to the known tetraploid apomictic cytotypes. Diploid strains constitute a source of sexuality for plant improvement. Chromosome doubling will likely produce sexual tetraploids to be used as females in crosses with natural apomictic tetraploid biotypes. Since diploid self-incompatible sexual Paspalum plants usually have apomictic tetraploid co-specific counterparts, the self-incompatible diploid P. chaseanum described in this study warrants further exploration for its apomictic counterpart.

Embryo sac development in some representatives of the tribe Cynodonteae (Poaceae)

Chloris virgata Sw., Cynodon dactylon (L.) Pers., Harpochloa falx (L. f.) Kuntze, and Tragus berteronianus Schult. have a Polygonum type of embryo sac development. Unreduced embryo sacs were found in Eustachys paspaloides (Vahl) Lanza Mattei, Harpochloa falx, and Rendlia altera (Rendle) Chiov. Both facultative and obligate apomixis were observed. The Hieracium type of embryo sac development was observed in the aposporic specimens.

Cytogenetics and Reproductive Behavior of 48‐Chromosome Pearl Millet ✕ Pennisetum squamulatum Derivatives

Pennisetum squamulatum Fresen is a perennial, hexaploid (2n = 6x = 54) species belonging to the tertiary gene pool of cultivated pearl millet [P. glaucum (L.) R. Br.]. Pennisetum squamulatum is a source of genes controlling apomixis, and useful genetic variability can be released in hybrids between sexual pearl millet and apomictic species. New genome combinations were produced between tetraploid pearl millet (2n = 4x = 28) and P. squamulatum by pollinating a sexual interspecific F1 hybrid (2n = 41) with an apomictic 56‐chromosome derivative from another F1 interspecific hybrid (2n = 41). Fifteen hybrids with 2n = 47 or 48 chromosomes resulted from the crosses. All were perennial and morphologically similar to or taller than their parents, showed irregular metaphase I chromosome behavior, and were partially male and female fertile. Nine plants reproduced sexually, two by facultative apomixis, and four by obligate apomixis. This cross allowed the increase of the pearl millet genome in the new hybrids while introducing apomictic reproduction into several genotypes.

Reproductive patterns of selected understorey trees in the Malaysian rain forest: the apomictic species

Garcinia is the predominant representative of the Clusiaceae in the understorey of the lowland rain forest of Peninsular Malaysia. In Pasoh Forest Reserve the dioecious species G. paruifolia was investigated since no male trees, but only trees with structurally hermaphodite flowers which later set fruit, were found there. Pollination and embryological studies indicated a condition of non-pseudogamous agamospermy, with no viable pollen grains being formed in the staminodes, and the unfertilized egg cell giving rise to the embryo. Supportive studies were made of microspore development in G. forbesi, and of embryogenesis and seed development in G. malaccensis and G. scortechinii which also appear to be agamospermous. The role of either facultative or obligate apomixis in gene fixation in these dioecious species is discussed, and the possible adaptive significance of reproductive versatility among certain rain forest trees is examined.

An apomictic polyhaploid obtained from a pearl millet x Pennisetum squamulatum apomictic interspecific hybrid

In a research program to transfer apomixis from Pennisetum squamulatum Fresen to pearl millet, P. americanum L. Leeke, a polyhaploid plant (2n=21) was discovered in the uniform open-pollinated progeny of an apomictic interspecific hybrid (2n = 41) between pearl millet and P. squamulatum. The polyhaploid was shorter, less vigorous and was smaller morphologically than its maternal parent. It probably originated by parthenogenetic development of a reduced gametophyte in the apomictic interspecific hybrid. The most common metaphase I chromosome association in the polyhaploid was 4 bivalents plus 13 univalents. Irregular chromosome distribution, tripolar spindles, bridges and fragments were observed at anaphase I and telophase I. The polyhaploid was male-sterile and partially female- fertile having multiple aposporous embryo sacs in 95% of the ovules. Seed set was 3% when open-pollinated and 33% when pollinated with pearl millet pollen. Low seed set was due to competition among multiple embryos developing in the same ovule. Seventeen progeny from seed produced under open-pollination on the polyhaploid each had 2n=21 chromosomes and were morphologically uniform and identical to the female parent. The expression of obligate apomixis in the polyhaploid conditioned by the P. squamulatum genome between the simplex and duplex condition indicates that apomictic reproduction is possible in nonpolyploid plants.

Microsporogenesis, reproductive behavior, and fertility in five Pennisetum species

Microsporogenesis, reproductive behavior, pollen fertility and seed set were studied in Pennisetum basedowii Summerhayes and C. E. Hubbard, 2n = 54; P. macrostachyum (Brough.) Trin., 2n = 54; P. macrourum Trin., 2n = 36; P. polystachion (L.) Schult, 2n = 54; and P. squamulatum Fresen 2n = 54. Meiosis was regular in P. basedowii with primarily bivalent pairing. As many as 54 univalents were observed at metaphase I in P. macrostachyum. A high frequency of univalents at metaphase I in P. macrourum resulted in lagging chromosomes and micronuclei at anaphase I and telophase I, respectively. Pennisetum polystachion and P. squamulatum showed frequent multivalent chromosome associations. Studies of megasporogenesis and embryo sac development in P. basedowii showed sexual reproduction. Pennisetum macrostachyum was highly male sterile with predominantly aposporous apomictic embryo sac development. Pennisetum macrourum, P. polystachion, and P. squamulatum had only aposporous embryo sac development. Seed propagated progenies of these latter three species were uniform and matromorphic, confirming the obligate apomixis nature.

Facultative Apomixis in Eragrostis curvula1

Previous reports indicate that weeping lovegrass, [Eragrostis curvula (Schrad.) Nees], reproduces either sexually or by obligate apomixis (diplospory). Recent studies have shown that certain plants of this species are facultative apomicts. Our research objective was to determine the cytological basis for facultative apomixis in weeping lovegrass and to relate reproductive cytology to performance of progeny from lovegrass parents and hybrids. We observed enlarged or late megaspore mother cells (MMC), as determined by integument size, in apomictic strains. Development from these MMCs occurred in either of two ways: 1) direct formation of the embryo sac (diplospory), or 2) delayed meiosis resulting in the formation of a linear tetrad. Progeny tests of plants with both types of reproductive cytology suggests that these patterns indicate facultative apomixis. We did not observe late, greatly enlarged megaspore mother cells or delayed meiosis in sexual plants. Hybrids between sexual plants and highly apomictic common weeping lovegrass were observed to reproduce either sexually or by facultative apomixis. Sexual plants, when self‐pollinated, produced no apomictic offspring. Therefore, we concluded that the sexual plants studied probably lack all the necessary genes for expression of apomixis. The uniformity observed in most experimental lines, the low level of sexuality in some apomictic hybrids, and the reduced vigor of many of the aberrant plants suggest that varietal development and maintenance in E. curvula should not be hindered too greatly by facultative apomixis.

Inheritance and Control of Obligate Apomixis in Breeding Buffelgrass, Pennisetum ciliare1

We studied the inheritance of obligate apomixis, using both selfed and F1 progenies. In both cases, the female parent was a completely sexual plant heterozygous for genes controlling method of reproduction, and the male parents in the crosses were obligate apomicts. Classification of S1 and F1 plants for mode of reproduction was achieved by embryo‐sac analyses, or study of progeny variability, or both. Four hundred and sixty‐nine S1 progeny, involving two populations grown in the years 1962 and 1965, respectively, segregated for sexual to apomictic types at ratios not significantly different from a 13:3. One hundred and forty‐five S1 progeny grown in 1963 gave a sexual to apomictic ratio of 8.66:l. Pooled data for these 614 S1 progeny of the sexual plant fit the 13:3 ratio. Seven hundred and sixty‐six F1 hybrids between the sexual plant and two obligate apomictic ecotypes, blue and common, gave sexual to apomictic ratios not significantly different from a 5:3. Interpretation of the 13:3 S1 and 5:3 F1 sexual to apomictic ratios is as follows. The genetic constitution of the sexual plant is hypothesized to be AaBb, where gene B conditions sexuality, and it is epistatic to gene A, which controls apomixis. The genetic constitution of the apomictic male parents is hypothesized to be Aabb. Results obtained support the hypothesis and suggest that the sexual plant originated by mutation at the b locus. The manipulation and use of obligate apomixis in a buffelgrass breeding program is discussed.

Megasporogenesis, Embryo Sac Development and Embryogenesis in Dallisgrass, Paspalum Dilatatum, Poir1

SynopsisA yellow‐anthered Dallisgrass biotype introduced from Uruguay has normal female development and apparently reproduces sexually. Common Dallisgrass reproduces apomictically. The occurrence of nucellar embryo sacs, autonomous embryo development and the necessity of pollination for endosperm development indicate apospory and pseudogamy. Combined evidence from cytological and breeding behavior observations suggests obligate apomixis.