Triploid Apomicts Research Articles

Nuclear-cytoplasmic male-sterility in diploid dandelions.

Male-sterility was found in diploid dandelions from two widely separated populations from France, and its inheritance was analysed by crossing a diploid male-sterile dandelion to diploid sexuals and triploid apomicts. Nuclear genetic variation, found in full-sib families, segregated for male-fertility, partial male-sterility, and full male-sterility, and also segregated for small-sized versus normally sized pollen. The crossing results are best explained by a cytoplasmic male-sterility factor in combination with two dominant restorer genes. Involvement of the cytoplasmic male-sterility factor was further investigated by chloroplast haplotyping. Male-sterility was exclusively associated with a rare chloroplast haplotype (designated 16b). This haplotype was found in seven male-sterile plants and one (apparently restored) male-fertile individual but does not occur in 110 co-existing male-fertile plants and not in several hundreds of individuals previously haplotyped. Apomicts with cytoplasmic male sterility were generated in some test crosses. This raises the question as to whether the male sterility found in natural dandelion apomicts, is of cytoplasmic or of nuclear genetic nature. As many breeding systems in Taraxacum are involved in shaping population structure, it will be difficult to predict the evolutionary consequences of nuclear-cytoplasmic male-sterility for this species complex.

The occurrence of phenotypically complementary apomixis-recombinants in crosses between sexual and apomictic dandelions (Taraxacum officinale)

Apomictic seed development in dandelion (Taraxacum officinale) involves (1) restitutional meiosis (diplospory), (2) egg cell parthenogenesis, and (3) autonomous endosperm development. The question is whether these elements of apomixis are controlled by one single gene or by several independent genes. Five triploid non-apomictic hybrids, obtained in diploid sexual × triploid apomict crosses were characterized using cyto-embryological and genetic methods. Nomarski-differential interference contrast microscopy and the transmission of microsatellite markers and ploidy levels indicated that the hybrids combined elements of the apomictic and the sexual developmental pathway. Hybrids form two complementary groups with respect to the presence or absence of parthenogenesis and autonomous endosperm development. The occurrence of complementary apomixis-recombinants suggests that parthenogenesis and autonomous endosperm development in Taraxacum are regulated independently by different genes. This study also indicates that early embryo development is independent of endosperm formation, but that endosperm is essential for later embryo growth.

The origin of intermediate species of the genus Sorbus.

The genus Sorbus in Europe contains five diploid species, Sorbus aria, Sorbus aucuparia, Sorbus torminalis, Sorbus chamaemespilus and Sorbus domestica, classified into five different subgenera. The subgenus Aria (or the S. aria aggregate) contains apomictic triploid and tetraploid species. Within the genus there are, in addition, a number of species, morphologically intermediate between plants of the four main subgenera, which are considered to reproduce partly, or exclusively, by apomixis. These are believed to have originated by hybridisation between species in the S. aria aggregate and a species from another subgenus (either S. aucuparia, S. torminalis or S. chamaemespilus). We have used restriction fragment length polymorphism (RFLP) analysis on a total of 178 Sorbus accessions to test this model. The genome relationships of the different groups have been assessed, and the hybrid nature of the major intermediate groups is unequivocally demonstrated. Polyploid species in the S. aria aggregate show genetic variation, indicating the possibility of multiple origins and/or facultative apomictic breeding behaviour. A major finding, confirmed by microsatellite analysis, is that the 'intermediate' species S. intermedia is shown to have genomes from S aria, S aucuparia and S torminalis. Polymorphic mitochondrial DNA markers were used to determine the direction of the crosses that gave rise to new 'hybrid species'; in the majority of cases the pollen was provided by the parent from the S aria aggregate.

Comparative cyto-embryological investigations of sexual and apomictic dandelions ( Taraxacum ) and their apomictic hybrids

In the autonomous apomictic Taraxacum offinale (common dandelion), parthenogenetic egg cells develop into embryos and central cells into endosperm without prior fertilisation. Unreduced (2n) megaspores are formed via meiotic diplospory, a nonreductional type of meiosis. In this paper, we describe the normal developmental pathways of sexual and apomictic reproduction and compare these with the development observed in the apomictic hybrids. In sexual diploids, a standard type of megasporogenesis and embryo sac development is synchronised between florets in individual capitula. In contrast, we observed that megasporogenesis and gametogenesis proceeded asynchronously between florets within a single capitulum of natural triploid apomicts. In addition, autonomous endosperm and embryo development initiated independently within individual florets. Parthenogenetic initiation of embryo development in outdoor apomicts was found to be temperature-dependent. Egg cells produced in natural apomicts were not fertilised after pollination with haploid pollen grains although pollen tubes were observed to grow into their embryo sacs. Both reductional and diplosporous megasporogenesis were observed in individual inflorescences of triploid apomictic hybrids. Embryo and endosperm development initiated independently in natural and hybrid apomicts. [KEYWORDS: apomixis, autonomous endosperm, diplospory embryo, development, parthenogenesis]

Cytological and Reproductive Studies of Japanese Diplazium (Woodsiaceae; Pteridophyta). III. The Cytological Complexity of Species Groups with Simply Pinnate to Bipinnatifid Leaves

with simply pinnate or bipinnatifid leaves. Diplazium wichurae var. wichurae, D. wichurae var. amabile, D. okudairae, and D. pin-faense are sexual diploids (2n=82; n=41II); D.× kidoi and D. × okudairaeoides are sterile diploids (2n= 82; meiosis irregular); D. donianum var. donianum is an apomictic triploid (2n=123; n=123II); D. donianum var. aphanoneuron is a sterile triploid (2n=123; meiosis irregular); D. crassiusculum, D. cavalerianum, D. incomptum, D. longicarpum, and D. pullingeri are sexual tetraploids (2n= 164; n=82II); and D. lobatum is an apomictic tetraploid (2n=164; n=164II). This is the first report of the chromosome numbers of D. lobatum, D. crassiusculum, D. incomptum, D. longicarpum, D. pullingeri, and D. × okudairaeoides, as well as the mitotic chromosome numbers of D. wichurae var. amabile, D. okudairae, D. pinfaense, and D. ×kidoi. The mitotic chromosome number, meiotic behavior, sterility, and allozyme analysis confirm that D. × kidoi and D. × okudairaeoides are hybrids between D. pin-faense and D. wichurae var. wichurae and D. okudairae and D. wichurae var. wichurae, respectively. Diplazium with simply pinnate to bipinnatifid leaves displayed an extraordinary cytological and reproductive complexity: a polyploidal series with diploids to hexaploids, sexual and apomictic reproduction, and natural hybridization.

Cytological, Morphological, Genetic, and Molecular Phylogenetic Studies on Intraspecific Differentiations within Diplazium doederleinii (Woodsiaceae: Pteridophyta)

Two cytotypes, an apomictic triploid (2n=123) and an apomictic tetraploid (2n=164), were found in Japanese Diplazium doederleinii. The cytotypes are clearly distinguishable from each other in phytogeographical, ecological, and morphological characteristics. The tetraploid is smaller in relation to several morphological characteristics and occurs in more northerly regions than the triploid. The distributions of the two cytotypes do not overlap. The leaves of the triploids are evergreen, while those of the tetraploids are summer‐green. Allozyme analysis indicated that one and four multilocus genotypes are contained in the tetraploid and triploid, respectively. The nucleotide sequence of the rbcL gene did not vary within the same cytotype, except for a single triploid with the same rbcL sequence as the tetraploid. There are only two nucleotide differences between the triploid and tetraploid races. These results indicate that the apomictic triploids are of recurrent origin from an undiscovered or extinct ance...

Meiotic recombination in sexual diploid and apomictic triploid dandelions (Taraxacum officinale L.).

Taraxacum officinale L. (dandelion) is a vigorous weed in Europe with diploid sexual populations in the southern regions and partially overlapping populations of diploid sexuals and triploid or tetraploid apomicts in the central and northern regions. Previous studies have demonstrated unexpectedly high levels of genetic variation in the apomictic populations, suggesting the occurrence of genetic segregation in the apomicts and (or) hybridization between sexual and apomictic individuals. In this study we analysed meiosis in both sexual diploid and apomictic triploid plants to find mechanisms that could account for the high levels of genetic variation in the apomicts. Microscopic study of microsporocytes in the triploid apomicts revealed that the levels of chromosome pairing and chiasma formation at meiotic prophase I were lower than in that of the sexual diploids, but still sufficient to assume recombination between the homologues. Nomarski DIC (differential interference contrast) microscopy of optically cleared megasporocytes in the apomicts demonstrated incidental formation of tetrads, which suggests that hybridization can occur in triploid apomicts.

Selection on apomictic lineages of Taraxacum at establishment in a mixed sexual–apomictic population

Abstract A species’ mode of reproduction, sexual or asexual, will affect its ecology and evolution. In many species, asexuality is related to polyploidy. In Taraxacum, apomicts are triploid, and sexuals are diploid. To disentangle the effects of ploidy level and reproductive mode on life-history traits, we compared established apomictic Taraxacum genotypes with newly synthesized apomictic genotypes, obtained from diploid–triploid crosses. Diploid–triploid crossing is probably the way that most apomictic lineages originate. New genotypes had on average a much lower seed set than established genotypes. Established genotypes differed on average from new genotypes, in particular under shaded conditions: the established genotypes had longer leaves and flowered later. The differences between new and established triploids resembled the differences that have been found between sexual diploids and established apomictic triploids. We conclude that ploidy differences alone are not directly responsible for observed differences between sexual diploid and apomictic triploid dandelions.

Crosses between sexual and apomictic dandelions (Taraxacum). II. The breakdown of apomixis.

Some dandelions are diplosporous gametophytic apomicts. In order to study the inheritance and breakdown of apomixis, crosses were made between diploid sexuals and triploid apomicts. To investigate their breeding system, four nonapomictic diploid and 10 nonapomictic triploid hybrids were pollinated with diploids and the progenies were analysed. Seed fertility was significantly reduced in two diploid hybrids. Nine triploid hybrids were fertile and could be classified into three types, with respect to the composition of their progenies. Type A produced n+n hybrids. Type B produced either a mixture of n + n and 2n + n hybrids, or a mixture of pseudogamous 2n + 0 apomicts and 2n + n hybrids. Type C produced exclusively 2n + n hybrids. Inheritance of a microsatellite marker strongly suggested that 2n egg cells in type C plants were produced by a first division restitution mechanism. As in apomicts, microsporogenesis in type C plants was reductional. This suggests that type C plants are diplosporous plants that lack parthenogenesis. Such plants are very rare in other apomictic plant species. It is concluded that 'elements of apomixis', diplospory and parthenogenesis, can be uncoupled. This is inconsistent with the single-locus model for apomixis in Taraxacum as suggested by Mogie (1992). Instead, our results suggest that several loci are involved in the genetic control of apomixis in Taraxacum.

Crosses between sexual and apomictic dandelions (Taraxacum). I. The inheritance of apomixis.

Some dandelions, Taraxacum, are diplosporous gametophytic apomicts. Crosses between closely related diploid sexuals and triploid apomicts were made to study the inheritance of apomixis. Seed-set was less than one-third of that in diploid x diploid crosses, probably because of the inviability of aneuploid pollen or zygotes. Almost 90% of the viable offspring were diploid and the result of selfing, as was shown by a discriminating allozyme marker. Aneuploid outcross pollen had a mentor effect on self-pollen, causing a breakdown of the sporophytic self-incompatibility system. A similar phenomenon has been reported before in wide crosses. Of the 26 allozyme-confirmed hybrids, four were diploids, 15 were triploids and seven were tetraploids. Diploid hybrids were significantly less frequent than triploid hybrids, suggesting either low fitness of haploid pollen or more numerous formation of diploid pollen. Emasculation and bagging of flowers indicated apomictic seed-set in none of the diploid, in one-third of the triploid and in all of the tetraploid hybrids. All apomictic hybrids showed partial seed-set, but additional cross-pollination did not increase seed-set. Cytological analysis of the F2 progeny confirmed that partial apomixis was caused by semisterility and not by residual sexuality (facultative apomixis). The difference in segregation for apomixis between triploid and tetraploid hybrids may be because the triploids originated from partially reduced diploid pollen grains, whereas the tetraploids originated from unreduced triploid pollen grains.

Crosses between sexual and apomictic dandelions (Taraxacum). II. The breakdown of apomixis

Some dandelions are diplosporous gametophytic apomicts. In order to study the inheritance and breakdown of apomixis, crosses were made between diploid sexuals and triploid apomicts. To investigate their breeding system, four nonapomictic diploid and 10 nonapomictic triploid hybrids were pollinated with diploids and the progenies were analysed. Seed fertility was significantly reduced in two diploid hybrids. Nine triploid hybrids were fertile and could be classified into three types, with respect to the composition of their progenies. Type A produced n+n hybrids. Type B produced either a mixture of n + n and 2n + n hybrids, or a mixture of pseudogamous 2n + 0 apomicts and 2n + n hybrids. Type C produced exclusively 2n + n hybrids. Inheritance of a microsatellite marker strongly suggested that 2n egg cells in type C plants were produced by a first division restitution mechanism. As in apomicts, microsporogenesis in type C plants was reductional. This suggests that type C plants are diplosporous plants that lack parthenogenesis. Such plants are very rare in other apomictic plant species. It is concluded that ‘elements of apomixis’, diplospory and parthenogenesis, can be uncoupled. This is inconsistent with the single-locus model for apomixis in Taraxacum as suggested by Mogie (1992). Instead, our results suggest that several loci are involved in the genetic control of apomixis in Taraxacum.

Crosses between sexual and apomictic dandelions (Taraxacum). I. The inheritance of apomixis

Some dandelions, Taraxacum, are diplosporous gametophytic apomicts. Crosses between closely related diploid sexuals and triploid apomicts were made to study the inheritance of apomixis. Seed-set was less than one-third of that in diploid × diploid crosses, probably because of the inviability of aneuploid pollen or zygotes. Almost 90% of the viable offspring were diploid and the result of selfing, as was shown by a discriminating allozyme marker. Aneuploid outcross pollen had a mentor effect on self-pollen, causing a breakdown of the sporophytic self-incompatibility system. A similar phenomenon has been reported before in wide crosses. Of the 26 allozyme-confirmed hybrids, four were diploids, 15 were triploids and seven were tetraploids. Diploid hybrids were significantly less frequent than triploid hybrids, suggesting either low fitness of haploid pollen or more numerous formation of diploid pollen. Emasculation and bagging of flowers indicated apomictic seed-set in none of the diploid, in one-third of the triploid and in all of the tetraploid hybrids. All apomictic hybrids showed partial seed-set, but additional cross-pollination did not increase seed-set. Cytological analysis of the F2 progeny confirmed that partial apomixis was caused by semisterility and not by residual sexuality (facultative apomixis). The difference in segregation for apomixis between triploid and tetraploid hybrids may be because the triploids originated from partially reduced diploid pollen grains, whereas the tetraploids originated from unreduced triploid pollen grains.

Cytological and Reproductive Studies on Japanese Diplazium (Woodsiaceae; Pteridophyta): Apomictic Reproduction in Diplazium with Evergreen Bi- to Tripinnate Leaves

, including polymorphic terrestrial species with evergreen bi- to tripinnate leaves. Diplazium hachijoense, D. virescens var. virescens, var. conterminum, var. okinawaense, and two other unnamed varieties, D. kawakamii var. kawakamii, D. dilatatum var. heterolepia, D. taiwanense, D. × kawabatae (=D. dilatatum × taiwanense), D. × takii (=D. hachijoense × virescens var. virescens), and D.× nakamurae (= D. hachijoense × virescens var. conterminum) are apomictic triploids (2n=n=123). Diplazium amamianum and D. esculentum are sexual diploids (2n=82, n=41) and D. subtripinnatum is a sexual tetraploid (2n= 164, n=82). Diplazium dilatatum var. dilatatum includes both sexual diploid and apomictic triploid populations. Cultivated gametophytes of six triploid taxa produced sporophytes apogamously, confirming their apomictic reproduction. All three putative hybrids, D. × kawabatae, D. × takii, and D. × nakamurae, are triploid, apomictic, and fertile taxa, therefore they are not the result of hybridization between known pairs of Japanese Diplazium plants.

Karyology, chorology and bioecology of the genus Limonium (Plumbaginaceae) in Sardinia

The genus Limonium is very differentiated in Sardinia. Thirty‐eight prevalently endemic (76%) species are known at present. Most of them (21 species) are triploid with a basic number x = 9; some species are widely distributed and, because of their scarce morphological affinities, presumably ancient; others have a more or less restricted distribution area, are weakly differentiated and probably of recent origin. There is also a diploid group with 2n = 18, which occurs in coastal areas, represented generally by obliged allogamous species (14) with an incompatible pollen/stigma heteromorphic system sometimes associated with heterostyly. The diploids have developed coastal belt vicariant species, territorially associated with the system of triploids. Part of the triploids of recent origin is clearly derived from a process of allotriploidy, i.e. hybridisation between a diploid and a triploid living next to each other. In Corsica and Sardinia, due to scarcity of tetraploids, the genus Limonium is characterised by a high diploid/triploid ratio. The same is true, though less clearly, also in other Mediterranean territories. The coexistence of amphimictic diploids of gradual geographic differentiation with, in part, weakly differentiated apomictic triploids poses the taxonomic problem of the contemporaneous adoption of two distinct concepts of species.

Apomixis in diploid × triploid Tripsacum dactyloides hybrids

Apomixis can hinder plant improvement if sexual or apomictic germ plasm of the appropriate ploidy level, required for effective manipulation of apomixis, is not available. To develop needed germ plasm and increase knowledge of apomixis in Tripsacum dactyloides, diploid (2n = 2x = 36) sexual plants were crossed with a highly apomictic triploid (2n = 3x = 54) accession in an attempt to transfer apomixis from a polyploid to the diploid level. The fertility of most hybrids was very low, 69% had 10% or less seed set. A selected subset of 48 hybrids, including most of the more fertile plants, was studied cytologically. The chromosome number of these hybrids ranged from 2n = 36 to 2n = 54. All but 3 of 46 of the 48 hybrids showed indications of apomictic development. Those with near-diploid chromosome numbers were primarily sexual. Highly apomictic hybrids, based on percent diplosporous ovules, had 43 or more chromosomes. Fertility of these hybrids was not related to chromosome number. Fertility of sexual to moderately apomictic plants was reduced as chromosome number increased. Apomixis in T. dactyloides is facultative. Backcrosses are needed to develop germ plasm with a high level of apomixis and near-diploid chromosome numbers.Key words: eastern gama-grass, chromosome number, fertility, mode of reproduction, germ plasm.

Studies in Crataegus (Rosaceae, Maloideae). XIX. Breeding behavior in Ontario Crataegus series Rotundifoliae

Ontario members of Crataegus series Rotundifoliae display breeding behavior that is characteristic of apomictic taxa. Based on a series of analyses, including histological studies of ovule development, chromosome counts, pollen stainability and pollination studies, and phenological analyses, it appears that most of the Ontario Rotundifoliae are tetraploid (2n = 2x = 68), self-fertile, pseudogamous, facultative apomicts. Some triploid (2n = 3x = 51) apomicts are also present in C. dodgei var. flavida (Sarg.) Smith & Phipps. The results presented are congruent with previous studies of Crataegus cytogenetics. Moreover, these results have assisted the taxonomic analysis of the series by permitting the identification of predominantly apomictic microspecies as well as possible hybrid taxa.

Cytogenetics and breeding behavior of an apomictic triploid in bahiagrass

Cytogenetics and breeding behavior of an apomictic triploid in bahiagrass

Eupatorium godfreyanum (Asteraceae), a "New" Species from Eastern United States

Eupatorium godfreyanum is described as a stable triploid apomict linkingE. sessilifolium L. withE. rotundifolium L. The nameE. vaseyi Porter, which has often been used for this plant, does not properly apply, inasmuch as the type collection is a different apomict, linkingE. sessilifolium withE. album L.

Enzyme Variability and Modes of Evolution in Bommeria (Pteridaceae)

As is true of other plants inhabiting xeric environments, desert ferns may be mor- phologically similar because of convergent evolution. Since phylogenetically important reproduc- tive features in these plants may be affected by vegetative modifications, electrophoretically de- tectable variability in 11 enzymes coding for 13 loci has been analyzed to provide an expanded, comparative data base for defining systematic relationships and population structure in the four species of Bommeria. Contrary to the ad hoc assumption that because Bommeria species have relatively high chromosome numbers they should be polyploid, enzyme studies suggest that all species are genetic diploids. These studies also demonstrate that genetic variability is partitioned among the individuals within populations and that the sexual species are outcrossing. Analyses of allelic frequencies indicate that the four species are genetically more distinct from one another than are angiosperm congeners. Bommeria subpaleacea and B. ehrenbergiana emerge as the most similar taxa with B. pedata and B. hispida being more distant. The enzyme data, combined with other characters, support the hypotheses that the mountains of west-central Mexico are the center of origin of Bommeria and that the triploid apomict B. pedata was not derived through hybridization among extant sexual species. Although the corroborative morphological, biogeographical, and enzymatic data present a reasonable phylogenetic hypothesis, the extraordinary genetic distance among these congeners suggests that Bommeria may be polyphyletic, composed of species that are grouped together because of superficial resemblance. Evolution in plants of warm deserts has been characterized by considerable vegetative and physiological convergence (Orians and Solbrig 1977). In assessing characters for phylogenetic analysis of these plants, it is necessary to con- sider carefully the role that the environment may have played in directing morphological modifications. Among angiosperms, xero- morphic, perennial members of the Cactaceae, Euphorbiaceae, and Asclepiadaceae share many vegetative features but may be unambiguously separated into distinct evolutionary lines based on floral characters. Since flowers are relatively ephemeral, their features are less affected by environmental pressures and can be used as reasonably accurate indicators of phylogenetic relationships. Among the ferns, there is a suite of genera commonly associated with xeric habitats. These share many of the features considered to be adaptations for survival in environments hav- ing frequent droughts, low humidity, and high insolation. Plants in genera such as Cheilanthes, Notholaena, Pellaea, and Bommeria are small, have highly dissected leaves, and are often clothed

Biosystematics of <i>Adiantum lunulatum</i> Burm. Complex in India with Special Reference to W. Himalayan Taxa

A diploid apomict, in addition to the already known triploid apomict is described from the W. Himalayas in the widely ranging species Adiantum lunulatum Burm. On the basis of morphological evidence it is suggested that the whole complex be split up into two, viz., A. lunulatum (s.s.) comprising the universally known triploid apomict, the diploid and tetraploid sexual from S. India and the diploid apomict from W. Himalayas and S. India. The other species, A. teestae Verma would comprise of the E. Himalayan diploid sexual and diploid apomict forms together with the tetraploid sexual. Chromosomal associations in the 16-spore mother-celled sporangia have been analysed and on this eviednce the origin of the two polyploid types in Adiantum lunulatum is discussed. It is argued that the diploid apomict A. lunulatum (sensu stricto) probably originated in a previously sexual taxon. Genetically speaking, the diploid apomict is AA', while the triploid is AA'B in constitution but still possessing approximately X pairs+X univalents.