Aposporous Apomixis Research Articles

Genetic Control of Apospory in Apomictic Paspalum Species.

The genetic control of aposporous apomixis in Paspalum was investigated by determination of the reproductive mode of the progeny from a cross of the sexual, self-incompatible tetraploid, P. ionanthum by the facultative apomict, P. cromyorrhizon. One hundred and sixty-nine progeny were assessed using embryological analysis, of these, 127 were shown to produce aposporous embryo sacs whilst 42 were completely free giving a ratio of 3.02/1. Fourteen plants of the aposporous group showed a high level of female sterility and may be considered as being members of the non-aposporous group which would alter the ratio to 2.02/1. Selfing of the female parent produced 120 caryopses of which only 25 gave rise to flowering plants. Six of the latter showed occasional aposporous embryo sac formation. Cytological analyses of meiosis indicates that any interpretation of the genetic ratios must take into account disomic or tetrasomic inheritance in P. ionanthum and tetrasomic with random chromatid assortment in P. cromyorrhizon. On this basis the observed phenotypic ratios can be accommodated within a genetic model of the sexual female parent being of the genotype AAAa if tetrasomy applies or Aa if disomy, whilst the apomitic male is Aaaa. In this situation apospory requires the presence of at least two doses of the recessive allele together with a single copy of the wild type allele, homozygous recessive genotypes will be non-viable. Alternatively, a model of one dominant gene controlling apospory together with lethality of gametes carrying two copies of the gene may account for the results.

Diplosporous and aposporous apomixis in a pentaploid race of Paspalum minus

Paspalum minus is a small grass widely distributed from North America and the Greater Antilles to Paraguay and Bolivia in South America. Previous studies indicated that diploid, tetraploid, and pentaploid races exist. Cytoembryological studies in pentaploid plants indicate irregular meiotic behavior with mostly unpaired chromosomes. Melosis is interrupted after the stage of metaphase I and the nucleus is restored enabling mitosis to occur. Each male meiocyte results in two pollen grains with unreduced nuclei, 2 n=5 x=50. In the ovule, two unreduced megaspores arise from the archesporial cell. The micropilar one degenerated and the other forms a diplosporous embryo sac. The cytologicaly unreduced egg cell forms the embryo by parthenogenesis. In addition, 1–3 aposporous embryo sacs develope from nucelar cells beside the diplosporous sac in more than one half of the ovules. Thus, diplospory ( Taraxacum type) and apospory occur in the same species and in the same ovule. This is the first report of diplospory and apospory occurring simultaneously in the grass family. The genetic specificity for embryo sac development is restricted to the archespore in diplosporous apomicts. In P. minus this specificity may express in both the archespore and in the surrounding nucelar cells.

Apospory‐Linked Molecular Markers in Buffelgrass

Isolation of a gene controlling apomixis would be useful to plant breeders for fixing hybrid vigor. A single gene encodes for aposporous apomixis in buffelgrass (Pennisetum ciliare L. Link), which is inherited in a autotetraploid fashion. Only two molecular markers linked to the gene are known: C04‐600, a RAPD (random amplified polymorphic DNA) and UGT197‐144, an STS (sequence‐tagged site). The objective of this study was to identify new RAPD markers linked to the apospory locus in butfelgrass and to prepare a linkage map. Sexual clone B‐2s was crossed to aposporous butfelgrass cultivar Higgins and to aposporous clone B‐12‐9. Progeny of each cross were screened for RAPD markers by bulked segregant analysis. Seven preparations of bulked genomic DNA (equal amounts DNA from each of 10 individuals) from sexual or aposporons progeny were examined for RAPD markers. Using 500 decanucleotide primers, we found five new markers potentially linked to the apospory gene. Analysis by MAPMAKER/EXP 3.01 indicated markers J16‐800 and M02‐680 were linked to the apospory gene, as was previously established marker UGT197‐144, and the three were placed on the map. Marker A20‐730 was loosely linked in both crosses, marker B14‐550 was unlinked in the Higgins cross, and marker N15‐370 was unlinked in both crosses, and they were not placed on the map. These data provide the first map for the bulfelgrass chromosome bearing the apospory gene.

Isozyme, Protein, and RAPD Markers within a Half‐Sib Family of Buffelgrass Segregating for Apospory

Isolation of genes controlling apomixis would be useful to plant breeders for fixing hybrid vigor. A single gene codes for aposporous apomixis in buffelgrass [Pennisetum ciliare (L.) Link]. This study was undertaken to assess the feasibility of using isozyme, protein, and random amplified polymorphic DNA (RAPD) markers to detect apospory‐linked sequences within a segregating half‐sib population. Sexual plant B‐2s, five sexual and three aposporous progeny of sexual B‐2s, and cultivar Higgins were studied. Floret and leaf proteins were separated by starch gel electrophoresis, and enzymes in the gel were stained to detect isozyme polymorphisms. Of 22 isozyme systems tested, 12 showed polymorphisms but none cosegregated with apomixis. Two‐dimensional polyacrylamide gel electrophoresis was used to separate steady state proteins of pistils at meiotic and post‐meiotic stages. This technique revealed ≈12% polymorphism within 308 spots, but none of the spots cosegregated with reproductive mode. Genomic DNA was screened for RAPD markers with 111 10‐mer random primers and polymerase chain reaction. Of 569 markers identified, 87% were polymorphic. One marker cosegregated with sexual lines, but none cosegregated with aposporous lines. Analysis of molecular variance examination of the B‐2s parent and the eight half‐sib progeny (Higgins excluded) showed that on the basis of 404 RAPD markers, the aposporous and sexual groups were not significantly different. No RAPD markers were tightly linked with apospory. Additional screening of new primers will allow identification of markers for the gene in full‐sib families.

Cytology and reproductive behavior of diploid, tetraploid and hexaploid germplasm accessions of a wild forage grass: Paspalum compressifolium

Fourteen germplasm accessions of Paspalum compressifolium native from southern Brazil were cytologically and embryologically analysed. The study revealed that one accession was diploid (2n=20), twelve were tetraploid (2n=40) and one was hexaploid (2n=60). This is the first report of diploid and hexaploid cytotypes for this species. Studies on microsporogenesis, megasporogenesis, and embryo sac development indicated that the diploid cytotype had regular meiotic behavior and reproduces sexually. Tetraploid cytotype usually had an important proportion of chromosomes that associated as quadrivalents during meiosis and reproduced by mean of aposporous apomixis. The hexaploid cytotype showed irregular meiotic behavior with about one third of the chromosomes associated as multivalents and reproduced by aposporous apomixis. Thus, P. compressifolium could be an agamic complex. Breeding in this complex is possible due to the presence of diploid sexuals which can be treated by colchicine and pollinated by apomicts.

Regeneration and somaclonal variation in apomictic Paspalum dilatatum Poir

In an attempt to incorporate variation into a uniform obligate apomict, plants of apomictic common dallisgrass, Paspalum dilatatum Poir., were regenerated from callus derived from immature inflorescences. Plants developed through both organogenesis and embryogenesis. A total of 682 regenerants were produced and more than 400 were transplanted into a field nursery and screened for somaclonal variation. Eventually 20 regenerants were selected, increased, and planted into a replicated nursery along with normal common dallisgrass. The characteristics examined were maturity date, plant height, number of racemes per inflorescence, number of spikelets per raceme, pubescence, stigma and anther color, ergot resistance, seed germination, seed set, pollen stainability, method of reproduction, and chromosome number. There were differences among the regenerants and between them and common dallisgrass for all traits except chromosome number, stigma and anther color, and ergot resistance. One of the more important regenerants had significantly higher seed set than common dallisgrass. All regenerants reproduced by aposporous apomixis but some exhibited a high degree of abortion while others had more aposporous embryo sacs per ovule than common dallisgrass. These findings demonstrate that common dallisgrass can be regenerated through tissue culture and that somaclonal variation is expressed in some of the regenerants, even though some of the altered traits are deleterious.

Cytology of Sexual and Apomictic Paspalum Species.

The cytology, reproductive behavior and fertility of seven different Paspalum species were studied to determine how they might be used in an interspecific hybridization program. Paspalum palustre Mez and P. bertonii Hack. were diploids with 2n=2x=20 chromosomes. Paspalum dedeccae Quarin; P. denticulatum Trin., and one P. distichum L. accession were tetraploids with 2n=4x=40 chromosomes. Another P. distichum accession, P. inaequivalve Raddi, and P. ramboi Barreto were hexaploids with 60 chromosomes. Meiosis was regular with primarily bivalent pairing in P. palustre, P. bertonii, tetraploid P. distichum, P. inaequivalve and P. ramboi. Paspalum dedeccae, two P. denticulatum accessions, and hexaploid P. distichum were meiotically irregular with univalent, bivalent and multivalent (primarily quadrivalent) associations at metaphase I. Megasporogenesis and embryo sac development in P. palustre, P. bertonii, and P. inaequivalve indicated sexual reproduction. Paspalum palustre and P. bertonii were self-incompatible and did not produce self-pollinated seed but P. inaequivalve was self-compatible. Paspalum dedeccae, P. denticulatum, P. distichum, and P. ramboi all reproduced by aposporous apomixis.

Cytology of Paspalum chacoence and P. durifolium and Their Relationship to P. dilatatum

Paspalum chacoence Parodi is a sexual diploid with 20 chromosomes that pair as 10 II at meiosis. Paspalum durifolium Mez. has 60 chromosomes with essentially 30 II during meiosis and reproduces by aposporous apomixis. A yellow-anthered sexual biotype of Paspalum dilatatum, 2n = 4x = 40, was crossed with both species. Meiosis for six P. dilatatum x P. chacoence hybrids, 2n = 3x = 30, was irregular; a mean association of 27.32 I and 1.34 II per cell indicated that these species do not have homologous genomes. Two P. dilatatum x P. durifolium hybrids, 2n = 5x = 50, had irregular meiosis and mean chromosome associations of 20.4 I, 14.61 II, 0.1 III, and 0.01 IV, indicating one homologous genome and a partially homologous genome. On the basis of chromosome pairing in the species and the hybrids, P. durifolium was given the genome formula II J2J2 XX. This is the first indication that a form of the J genome is present in a species of the Quadrifaria group. Implications that P. durifolium may be the hexaploid progenitor of the common apomictic P. dilatatum biotype, 2n = 5x = 50, are discussed.

Cleared-pistil and thick-sectioning techniques for detecting aposporous apomixis in grasses

Two techniques were evaluated for detecting sexual and aposporous embryo sacs in an apomictic grass. Pistils of Cenchrus ciliaris L. were fixed in formalin – acetic acid – alcohol (FAA), cleared in methyl salicylate, and examined in toto by interference contrast microscopy. The same pistils were then embedded in plastic, sectioned, and studied by conventional microscopy. The two methods yielded identical interpretations of embryo sac morphology. The cleared-pistil technique was simple and required about 1/10 the time of the sectioning method.

Cytology of Some Apomictic Paspalum Species1

Paspalum almum Chase had a chromosome number of 2n=4x=24. The number for P. brunneum Trin., P. coryphaeum Trin., P. haumanii Parodi, and P. rufum Nees. was 2n−4x−40; P. proliferum Arech. was 2n=6x−60. This is the first report of the chromosome number and cytological behavior for P. brunneum. The chromosome numbers found in P. coryphaeum, P. haumanii, P. rufum, and P. proliferum were different from any previously reported for these species. During metaphase I, the chromosomes in P. almum, P. brunneum, P. coryphaeum, P. haumanii, and P. rufum paired primarily as bivalents with occasional quadrivalents and a limited number of univalents were observed. P. almum, previously considered a diploid, had as many as five quadrivalents, which indicates it is an autotetraploid. Meiosis in P. proliferum was irregular with pairing primarily as 50 univalents and 5 bivalents at metaphase I and numerous laggards and micronuclei were observed.Megasporogenesis and embryo sac development of all six species showed aposporous apomixis followed by pseudogamy. This limits their use in an interspecific hybridization program. The possible origin of the new chromosome races, their relationships to other reported chromosome races of the same species, the role of apomixis in their evolution, and changes in the basic chromosome number of the genus were discussed.