Agamic Complex Research Articles

Mexican species of the genus Stevia (Eupatorieae, Asteraceae): Chromosome numbers and geographical distribution

AbstractChromosome numbers from 244 accessions representing 73 species, five varieties and two putative species hybrids in Mexican Stevia are reported with comments on agamospermy, cytogeography and chromosomal evolution. The chromosome numbers of 47 species and two varieties were counted for the first time. Counts of 12 species and two varieties were new cytotypes, or differed from all previously reported numbers for the same species. B chromosomes were found in 16 species and two varieties. All species of shrubs or shrublets were sexual diploids (2n = 24). Perennial herbs were diploids or polyploids based on x = 11 or 12 and the widespread prevalence of agamospermous polyploids was revealed. Chromosome numbers of annual herbs varied from 2n = 24 to 2n = 8. The cytogeographical survey revealed five distributional patterns of agamic complexes based on the relative abundance of polyploids compared to their diploid ancestors. The wider distributional range (extending to northern and inland areas) of some agamospermous polyploids have been achieved by recent colonization accompanying climatic warming after the last glaciation. This has occurred even in lower latitudes such as Mexico, due to more successful founder population reproduction and the acceleration of seed dispersal, as well as the creation of new and open habitats by human activities.

Apomixis and sexuality in diploid and tetraploid accessions of Brachiaria decumbens

Meiotic and aposporous embryo sacs and the initial steps of parthenogenetic embryogenesis and endosperm formation were investigated in diploid and tetraploid accessions of Brachiaria decumbens in two environments, differing mainly in day length: early summer and late autumn. Both diploid and tetraploid accessions were facultative apomicts. Di(ha)ploids showed a much lower level of apomixis (10% to15%) than tetraploids (80% to 95%). No obligate sexual diploids were found; thus, their occurrence in natural populations is obscure. It is suggested that reproduction in B. decumbens, as in other agamic complexes of the Paniceae tribe, in general, approximates a diploid-tetraploid-(di)haploid reproductive cycle which does not involve triploids. The dihaploids were fertile and survived in nature. Development of the reproductive structures depended on the environment. In autumn, in contrast to early summer, many meiotic and aposporous embryo sacs degenerated during development, leading to a significant reduction in the proportion of parthenogenetic embryos. Whether this effect can be attributed to day length or simply to age remains to be investigated. The ratio of aposporous to sexual embryo sacs was relatively stable over the two seasons.

Inheritance, distribution and biology of andromonoecy in the agamic complex of the Maximae (Panicoideae)

Panicum maximum belongs to the agamic complex of the Maximae Panicoideae) which includes two other species, P. infestum Anders., P. trichocladum K. Schum., and several morphologically intermediate types. Andromonoecy and hermaphroditism have been recorded in this complex, but their distribution depends on the species, the level of ploidy, the mode of reproduction and the geographical origin. In particular, hermaphroditism appears to exist only in polyploid apomicts of the species P. maximum and was more frequent in maritime regions. Andromonoecy showed a monogenic and recessive inheritance. In andromonoecious plants, flowering of the male flower occurred later than that of the hermaphrodite flower within the spikelet. This flowering discrepancy varied between clones in respect of both mean and variance. All results are discussed in terms of the evolutionary process involved.

Taxonomy of agamic complexes in plants: A role for metapopulation thinking

The argument is made that species should be comparable with respect to the numbers of genotypes they include, regardless of whether their reproduction is exclusively sexual or not. Specifically, this means recognizing species at a level comparable to that of the section or series, as these are used in genera likeCrataegus L. orTaraxacumWeber ex F.H.Wigg. Anticipated advantages of this approach for the taxonomy of agamic complexes include the following: (1) a common basis for communication between taxonomists and population biologists, hence (2) the opportunity to find possible explanations for patterns of variation in theoretical models of population behavior. In addition, (3) revising the hierarchical level at which species are recognized will make it easier to avoid describing species that are paraphyletic, i.e. that include some but not all related genotypes sharing a common ancestor. As an example, revising the hierarchical level at which species are recognized makes it possible to see many agamic complexes as being made up of metapopulations of genotypes that arise and go extinct as habitats become available and then disappear for whatever reason (succession, disturbance, etc.) and at whatever scale (local, regional, continental).

Species concepts in agamic complexes: Applications in theRanunculus auricomus complex and general perspectives

The application of different current species concepts to the predominantly apomicticR. auricomus complex (goldilocks) is discussed. As with other uniparental reproducing organisms, biological species concepts are hardly applicable in apomictic groups. Information on reproductive systems, phenetic and ecological differentation, and evolutionary traits favour an “agamospecies” concept. It is argued that agamic lineages in goldilocks can be treated neither as subspecific taxa, nor as hybrids. A general viewpoint is proposed that species are stable phases within a continuous process of diversification of ancestral-descendent lineages. Constancy of progeny, similarity of phenotype, and ecogeographical niches of organisms are regarded as the most important operational criteria for grouping and ranking of species. Mode of reproduction is seen as a feature of a species — not as a criterion for its definition. Internal stability of features is regarded as more important for species definition than the features themselves.

Species concept in agamic and autogamic complexes

Species concept in agamic and autogamic complexes

A note on the taxonomy of agamic complexes. A reply to Tim Dickinson

The paper is a reply to Dickinson (see this volume). Criteria of easy recognizability and multiclonality for an agamospermous species are shown to be very dependent on the individual ability of a student and on the particular case of an apomict. Examples are given of extreme cases of uniclonal and multiclonal agamospermous species inTaraxacum. Use of a section as a category comparable with a sexual species is discussed; blurred limits of sections in hybridogenous groups are pointed out. Acceptance of agamospermous species is advocated as a means how to maximize the information content of the classification.

Non-Mendelian transmission of apomixis in maize-Tripsacum hybrids caused by a transmission ratio distortion.

Apomixis is a mode of asexual reproduction through seeds. The apomictic process bypasses both meiosis and egg cell fertilization, producing offspring that are exact genetic replicas of the mother plant. In the Tripsacum agamic complex, all polyploids reproduce through the diplosporous type of apomixis, and diploids are sexual. In this paper, molecular markers linked with diplospory were used to analyse various generations of maize-Tripsacum hybrids and backcross derivatives and to derive a model for the inheritance of diplosporous reproduction. The results suggest that the gene or genes controlling apomixis in Tripsacum are linked with a segregation distorter-type system promoting the elimination of the apomixis alleles when transmitted through haploid gametes. Hence, this model offers an explanation of the relationship between apomixis and polyploidy. The evolutionary importance of this mechanism, which protects the diploid level from being invaded by apomixis, is discussed.

Non-Mendelian transmission of apomixis in maize–Tripsacum hybrids caused by a transmission ratio distortion

Apomixis is a mode of asexual reproduction through seeds. The apomictic process bypasses both meiosis and egg cell fertilization, producing offspring that are exact genetic replicas of the mother plant. In the Tripsacum agamic complex, all polyploids reproduce through the diplosporous type of apomixis, and diploids are sexual. In this paper, molecular markers linked with diplospory were used to analyse various generations of maize–Tripsacum hybrids and backcross derivatives and to derive a model for the inheritance of diplosporous reproduction. The results suggest that the gene or genes controlling apomixis in Tripsacum are linked with a segregation distorter-type system promoting the elimination of the apomixis alleles when transmitted through haploid gametes. Hence, this model offers an explanation of the relationship between apomixis and polyploidy. The evolutionary importance of this mechanism, which protects the diploid level from being invaded by apomixis, is discussed.

Review of Pennisetum section Brevivalvula (Poaceae)

Section Brevivalvula is one of five sections in the large tropical grass genus Pennisetum. It belongs to the tertiary genepool of P. glaucum (L.) R. Br., pearl millet, and consists of six morphological taxa: P. atrichum Stapf & Hubb., P. hordeoides (Lam.) Steud., P. pedicellatum Trin., P. polystachion (L.) Schult., P. setosum (Swartz) L. Rich. and P. subangustum (Schum.) Stapf & Hubb., which together form a polyploid and agamic complex. Four euploid (x = 9) and twelve aneuploid chromosome levels have been found till now; the polyploids are apomictic, while diploid populations of P. polystachion and P. subangustum are considered sexual.

Genetic variation in the agamic species complex of Pennisetum section Brevivalvula (Poaceae) from West Africa: ploidy levels and isozyme polymorphism.

Some characteristics of the species complex Pennisetum section Brevivalvula are polyploidy and apomixis. Four euploidy levels (x = 9) were assessed by DAPI-flow cytometry for 304 plants of the section, distributed among five species: P. hordeoides (2n = 36, 54), P. pedicellatum (2n = 36, 45, 54), P. polystachion (2n = 18, 36, 45, 54), P. setosum (2n = 54), and P. subangustum (2n = 18, 36, 54). The geographical distribution of the ploidy levels seems to be related to major ecological zones of West Africa. The hilly regions displayed a higher ploidy diversity than the others; diploid populations of the annual species P. polystachion and P. subangustum were found. Genotypic variation expressed by isozyme polymorphism did not show any significant difference between the diploid, sexual populations and the polyploid, apomictic populations of these two species.

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.

Megasporogenesis and megagametogenesis in several Tripsacum species (Poaceae)

The Tripsacum agamic complex (x = 18) will provide valuable characters for maize breeding, provided that apomixis can be manipulated. Apomixis in Tripsacum was first reported 40 years ago, but its prevalence in the genus has not been established. Reproductive development was determined for eight Mexican and two South American Tripsacum species by microscopic analysis of ovaries cleared in a benzyl benzoate‐dibutyl phthalate solution using interference contrast optics. The occurrence and distribution of callose deposition during megasporogenesis were determined by fluorescence microscopy of ovaries optically cleared in an aqueous sucrose solution containing aniline blue. Diploid genotypes were sexual. Polyploid forms reproduced apomictically following the Antennaria type (complete meiosis abortion) of diplospory. The Taraxacum type (unreduced megaspore production through meiotic restitution nuclei) of diplospory also occurred but rarely. The walls of diplosporic megasporocytes lacked callose whereas the walls of sexual megasporocytes contained a normal complement of callose. The absence of callose suggests that the diplosporic forms of reproduction result from mutations affecting the normal meiotic process. Apomixis in the Tripsacum genus is facultative, and the production of new polyploid genotypes through genetic exchanges involving both apomictic and sexual genotypes is possible.

Allelic ratios and sterility in the agamic complex of the Maximae (Panicoideae): evolutionary role of the residual sexuality

AbstractThe agamic complex of the tribe Maximae is constituted by two pools characterized by their own breeding system (sexuality on the one hand, facultative apomixis on the other hand). Facultative apomicts shows two types of embryo‐sacs (aposporic or meiotic). Presence of aposporic embryo‐sacs is genetically controlled by the dominant allele A (all apomictic plants are Aaaa). Meiotic embryo‐sacs, constituting residual sexuality, can allow crosses between apomicts and theoretically the generation of other genotypes than Aaaa. Additionally, some cases of sterile apomicts appears in a breeding system known to bypass it.This led us to study these two paradoxes. In fact, the sterility would exist when the ratio A/a is greater than 0.25. This would prevent the total overrunning of an apomictic population by AAAA genotypes. The role of residual sexuality is then to allow generation of true sexuals (aaaa) from apomicts (Aaaa).

Inheritance and genetic diversity of some enzymes in the sexual and diploid pool of the agamic complex of Maximae (Panicum maximum Jacq., P. infestum Anders. and P. trichocladum K.Schum.)

The tribe Maximae (Panicum maximum Jacp., P. infestum Anders., P. trichocladum K.Schum.) includes two sympatric pools with different modes of reproduction and ploidy levels: an apomictic and tetraploid pool on the one hand, and a smaller, sexual and diploid pool on the other.

Hybridization (N + N and 2N + N) of Facultative Apomictic Species in the Pennisetum Agamic Complex

It has been hypothesized that within agamic complexes, apomixis acts to preserve fertility in wide crosses. Through hybridization studies with two facultative (partially sexual) Pennisetum species (P. flaccidum and P. mezianum), n + n (B_II) and 2n + n (B_III) sexual, facultative, and obligate apomictic hybrids were obtained. The objectives of this study were to determine the frequency for wide-cross B_III hybrids in an agamic complex and to compare fertility of B_II and B_III hybrids. Sexual all hybrids derived from hybridization of P. flaccidum with P. mezianum were always highly sterile because of male and female abortion, while obligate apomictic B_II hybrids obtained through pseudogamy were fertile when supplied with viable pollen. All B_III hybrids from fertilization of 2n apomictic eggs were highly fertile regardless of mode of reproduction. The ability to recover obligate apomictic B_III hybrids allowed for transfer of whole genomes to the female chromosome complement and to synthesize new, fertile, true breeding 2n + n genotypes.

Fertility of Brachiaria ruziziensis in interspecific crosses with Brachiaria decumbens and Brachiaria brizantha: meiotic behaviour, pollen viability and seed set

Colchicine induced tetraploid (2n=4x=36) Brachiaria ruziziensis were used as female parent in crosses with apomictic tetraploid species (2n=4x=36) Brachiaria decumbens and Brachiaria brizantha. Tetraploid B. ruziziensis pollinated with B. decumbens set significantly more seed than selfed or crossed with B. brizantha. The crossability between B. ruziziensis and B. decumbens is also better than between B. ruziziensis and B. brizantha. In addition, hybrid seedlings obtained in crosses involving B. brizantha are more frequently lethal. All the viable F1 hybrids are tetraploid with 36 chromosomes. Meiotic chromosome behaviour suggest that the three species belong to the same genomic group and therefore the same agamic complex. Chromosome associations at metaphase I do not allow to identify fertile and sterile hybrids. The interspecific hybrids averaged a lower fertility than their female parent, but some hybrids were more fertile than their apomictic male parent.

PATTERNS OF CLONAL DIVERSITY IN THE ANTENNARIA ROSEA (ASTERACEAE) POLYPLOID AGAMIC COMPLEX

The perennial herbaceous species, Antennaria rosea, is a large, morphologically diverse, polyploid agamic complex that is widespread in the cordillera of western North America. The species consists of triploid and tetraploid, nonpseudogamous, gametophytic apomicts. Populations of A. rosea are gynoecious, consisting almost entirely of pistillate clones. Clonal diversity among 63 populations of A. rosea was studied over a large portion of its range. Isozyme electrophoresis utilizing four polymorphic enzyme systems detected 192 multilocus genotypes among the populations. Populations of A. rosea tend to be composed of one or a few genotypes (range 1–11; mean 3.5), and these genotypes usually occur in only one or a few localized populations. Geographic patterns of clonal diversity may be a result of frequent genesis of new clones in populations that occur in areas where sexual relatives of A. rosea donate compatible pollen to facultatively sexual apomicts. Populations from previously glaciated regions tend to have fewer clones per population than those from unglaciated portions of the range.

Patterns of Clonal Diversity in the Antennaria rosea (Asteraceae) Polyploid Agamic Complex

The perennial herbaceous species, Antennaria rosea, is a large, morphologically diverse, polyploid agamic complex that is widespread in the cordillera of westem North America. The species consists of triploid and tetraploid, nonpseudogamous, gametophytic apomicts. Populations of A. rosea are gynoecious, consisting almost entirely of pistillate clones. Clonal diversity among 63 populations of A. rosea was studied over a large portion of its range. Isozyme electrophoresis utilizing four polymorphic enzyme systems detected 192 multilocus genotypes among the populations. Populations of A. rosea tend to be composed of one or a few genotypes (range 1-11; mean 3.5), and these genotypes usually occur in only one or a few localized populations. Geographic pattems of clonal diversity may be a result of frequent genesis of new clones in populations that occur in areas where sexual relatives of A. rosea donate compatible pollen to facultatively sexual apomicts. Populations from previously glaciated regions tend to have fewer clones per population than those from unglaciated portions of the range.

Cytotaxonomic studies in Themeda triandra Forssk.: Part III: Sexual and apomictic embryo sac development in 53 collections

The embryo sac development of 53 collections of Themeda triandra Forssk. was studied. All the diploids and two of the tetraploids had sexual embryo sacs of the Polygonum -type, whereas the remaining polyploids possess 4-nucleate aposporic embryo sacs. Some of the collections are facultative apomicts, while others (in particular the penta- and hexaploids) are essentially obligate apomicts. Interesting variations in the type of apospory was encountered in the different apomicts. This variation might be due to environmental influences. On the whole it would appear from the results that Themeda triandra is a young and therefore still actively developing agamic complex in southern Africa.