Apomixis is not developmentally conserved in related, genetically characterized Hieracium plants of varying ploidy

Apomixis is facultative in characterized members of the genus Hieracium. The three components that comprise the apomictic mechanism include apospory followed by autonomous embryo and endosperm formation. The time of aposporous embryo sac initiation and mode of embryo sac formation are different in Hieracium piloselloides (D3) and Hieracium aurantiacum (A3.4). Genetic studies have shown that a single dominant locus encodes all three components of apomixis in both species (Bicknell et al. 2000). We histologically examined a range of related, genetically characterized apomictic Hieracium plants derived from D3 and A3.4 to assess conservation of the apomictic mechanism in different genetic backgrounds. The plants varied in ploidy, and also in the amount of DNA introduced from sexual Hieracium pilosella (P4). An apomictic hybrid from a cross between the two apomicts was also examined. The developmental processes observed in the parental apomicts were not conserved in the examined plants and alterations occurred in the components of apomixis. One plant also exhibited adventitious embryony. The results show that other genetic factors can modify apomixis with respect to time of initiation, spatial location, and mode of developmental progression. Both the apomictic locus and the modifiers are essential for efficient penetrance of the trait in Hieracium. Some of the findings in Hieracium correspond with observations in Ranunculus and this is discussed in terms of models for apomictic development and the control of apomixis in crops. Received: 21 June 1999 / Revision accepted: 17 November 1999     

Methods for induction and utilization of variability in the improvement of an apomictic grass,Dichanthium annulatum complex

Summary Many problems and difficulties are encountered in making genetic improvements in plants where both apomixis and polyploidy occur together. From biosystematic studies on an agamic species complex, Dichanthium annulatum, information is presented on: (A) Mechanisms which create variability in apomicts — (i) genome building and reduction, (ii) hybridization between ecotypes of facultative apomicts, (iii) fertilization of unreduced gametes, (iv) introgressive hybridization, (v) preferential pairing and genotypic control of bivalent formation and (vi) induced mutation; (B) Embryo-sac variations, vis-a-vis sexual/apomictic sacs — (i) production of sexual embryo-sac in apomicts, (ii) balance between apomixis and sexual process, (iii) effect of environment and experimental manipulation of the type of embryo-sac; and (C) Heterosis and fixation of apomixis.The utilization and exploitation of these mechanisms and phenomena for accelerating the genetic improvement of apomictic plants is discussed.Mating systems impose certain restrictions on the breeding methodology to be used in the genetic improvement of crop plants. Allogamous species have built-in mechanisms for self-improvement and, for them, the breeding techniques are well worked out. Little information is, however, available on the procedures to be followed for the genetic improvement of apomicts. Recently gathered information on the causal mechanisms of apomixis and its mode of inheritance, the genetic systems which regulate the balance between apomixis and sexuality, the physical and chemical agents for artificial induction of sexuality in apomicts, and the processes which promote variability and adaptive polymorphism in apomicts show a way for the creation, exploitation and fixation of superior genotypes. Such information, based on biosystematic studies on an agamic species complex, Dichanthium annulatum, at the Oklahoma State University, Stillwater, Oklahoma, U.S.A., is presented here.Breeding procedures commonly followed for the genetic improvement of apomicts are outlined below:1. Collection of varieties, strains or ecotypes from diverse sources; 2. Evaluation of the germ plasm for the presence of desirable characters; 3. Building up of selection indices and estimation of genetic parameters; 4. Determination of mode of reproduction and isolation of sexual types or clones; 5. Hybridization using the sexual types; 6. Progeny testing, comparisons, multiplication and release of superior types.Thus, the success of the breeding programme would depend on the range of variability already present in the germ plasm collections, the relative proportion of sexual/apomictic seed produced and the exploitation of variability from the crossbred progenies. Since large collections of plants with different genotypes are not often available, one would like to look for the mechanisms which can create variability in the apomicts. Such mechanisms are as follows.     

DOES HYBRIDIZATION DRIVE THE TRANSITION TO ASEXUALITY IN DIPLOID BOECHERA?

Gametophytic apomixis is a common form of asexual reproduction in plants. Virtually all gametophytic apomicts are polyploids, and some view polyploidy as a prerequisite for the transition to apomixis. However, any causal link between apomixis and polyploidy is complicated by the fact that most apomictic polyploids are allopolyploids, leading some to speculate that hybridization, rather than polyploidy, enables apomixis. Diploid apomixis presents a rare opportunity to isolate the role of hybridization, and a number of diploid apomicts have been documented in the genus Boechera (Brassicaceae). Here, we present the results of a microsatellite study of 1393 morphologically and geographically diverse diploid individuals, evaluating the hypothesis that diploid Boechera apomicts are hybrids. This genus‐wide dataset was made possible by the applicability of a core set of microsatellite loci in 69 of the 70 diploid Boechera species and by our ability to successfully genotype herbarium specimens of widely varying ages. With few exceptions, diploid apomicts exhibited markedly high levels of heterozygosity resulting from the combination of disparate genomes. This strongly suggests that most apomictic diploid Boechera lineages are of hybrid origin, and that the genomic consequences of hybridization allow for the transition to gametophytic apomixis in this genus.     

Stomatal size,ploidy and polyembryony in Eriotheca Stellate Trichome Species Complex (Bombacoideae – Malvaceae) in the Cerrados of Brazil

• Geographic parthenogenesis, range expansion of apomictic plants after climate changes, has been described for Northern Hemisphere gametophytic apomicts. But similar trends have been observed for sporophytic apomicts of Cerrado, the savannas in Brazil. Eriotheca pubescens is a common Cerrado tree, an agamic complex of either hexaploid/polyembryonic apomicts or tetraploid/monoembryonic sexual individuals. Some populations have been described as a new species, Eriotheca estevesiae, all included in the Eriotheca Stellate Trichome Species Complex (ESTSC). Since breeding systems and ploidy are clearly associated with polyembryony and stomatal size, we used these ancillary features to map the reproductive and ploidy level traits of E. pubescens and E. estevesiae.
• Leaves and seeds were collected from individuals of 19 populations. Seeds were evaluated for the presence of polyembryony and leaves for stomatal measurements.
• Eight populations were monoembryonic while another eight were polyembryonic and for other three, the embryonic pattern was not readily verified. E. pubescens polyembryonic and hexaploid populations formed a homogeneous group, but monoembryonic plants were more variable. E. estevesiae populations were monoembryonic with smaller stomata. In contrast, some E. pubescens monoembryonic populations further south presented larger stomata. Despite these outliers, possibly mixed populations, stomatal size and embryonic pattern differed from northern to southern populations.
• Embryonic pattern and stomatal size indicated that northernmost populations of Eriotheca STSC (E. estevesiae) are diploid and sexual. Southernmost populations, mostly polyembryonic and with large stomata, are hexaploid and apomictic. This is in agreement with geographic parthenogenesis and range expansion of apomictic lineages to southern habitats available after the last glacial maximum.

     

INDUCTION OF APOMIXIS BY OUTCROSSING BETWEEN GENETICALLY DIVERGENT ENTITIES OF CALOGLOSSA LEPRIEURII (CERAMIALES,RHODOPHYTA) AND EVIDENCE OF HYBRID APOMICTS IN NATURE1

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.     

Sexual and apomictic development in Hieracium

 Most members of the genus Hieracium are apomictic and set seed without fertilization, but sexual forms also exist. A cytological study was conducted on an apomictic accession of H. aurantiacum (A3.4) and also H. piloselloides (D3) to precisely define the cellular basis for apomixis. The apomictic events were compared with the sexual events in a self-incompatible isolate of H. pilosella (P4). All plants were maintained as vegetatively propagated lines each derived from a single plant. Sexual P4 exhibited characteristic events of polygonum-type embryo sac formation, showed no latent apomitic tendencies, and depended upon fertilization to set seed. In contrast, D3 and A3.4 were autonomous aposporous apomicts, forming both embryo and endosperm spontaneously inside an unreduced embryo sac. The two apomicts exhibited distinct mechanisms, but variation was also observed within each apomictic line. Seeds from apomicts often contained more than one embryo. A degree of developmental instability was also observed amongst germinated seedlings and included variation in meristem and cotyledon number, altered phyllotaxis, callus formation, and seedling fusion. In most cases abnormal seedlings developed into normal plants. Such phenomena were not observed following germination of hybrid seeds derived from crosses between sexual P4 and the apomictic plants. The three plants can now be used in inheritance studies and also to investigate the molecular mechanisms controlling apomixis. Received: 11 February 1998 / Revision accepted: 23 July 1998     

EVIDENCE OF PARTIAL APOMIXIS IN ANTENNARIA MEDIA (ASTERACEAE: INULEAE) DETECTED BY THE SEGREGATION OF GENETIC MARKERS

The interplant variation in sexual and asexual reproduction in an Oregon population of the alpine perennial Antennaria media was investigated. Four polymorphic loci were assayed by enzyme electrophoresis of the progeny of 72 families from two subpopulations of A. media. The population was divided into two spatially distinct subpopulations. A multilocus model, incorporating a mixture of apomixis and random outcrossing, was used to estimate the mating system of pistillate plants both on the population and individual levels with statistical significance of the estimates based on bootstrap methods. The population contained a mixture of sexual individuals, partial apomicts, and obligate apomicts. The first subpopulation contained individuals that were partially apomictic and presumably produced both reduced and unreduced embryo sacs. There was a conspicuous difference in the breeding system composition between the two subpopulations. The first subpopulation had a “female” biased gender ratio and contained mostly obligate apomicts, some partial apomicts, and some outcrossing amphimicts. The second subpopulation, which had a nearly balanced gender ratio, contained mostly amphimicts, some obligate apomicts, but no facultative apomicts. This is the first study to document partial apomixis in individual plants by the use of genetic markers.     

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. Received: 15 July 1998 / Accepted: 13 August 1998     

Sexual Hieracium pilosella plants are better inter-specific,while apomictic plants are better intra-specific competitors

Apomixis, asexual reproduction through seeds, occurs in over 40 plant families. This widespread phenomenon can lead to the fixation of successful genotypes, resulting in a fitness advantage. On the other hand, apomicts are expected to lose their fitness advantage if the environment changes because of their limited evolutionary potential, which is due to low genetic variability and the potential accumulation of deleterious somatic mutations. Nonetheless, some apomicts have been extremely successful, for example certain apomictic accessions of Hieracium pilosella L. from New Zealand, where the plant is invasive. Here, we investigate whether the success of these apomictic accessions could be due to a fitness advantage by comparing the vegetative competitiveness of apomictic H. pilosella from New Zealand with sexual accessions of H. pilosella from Europe. Sexual and apomictic plants were grown either (A) alone (no competition), (B) in competition with the other type (intra-specific competition), (C) in competition with the grass Bromus erectus (inter-specific competition), and (D) in competition with the other type and the grass B. erectus (intra- and inter-specific competition). To distinguish effects of apomixis and the region of origin, different H. pilosella lineages were compared. Furthermore, experiments were carried out to investigate effects of the ploidy level. We show that sexual plants are better inter-specific competitors than apomicts in terms of vegetative reproduction (number of stolons) and vegetative spread (stolon length), while apomicts do better than sexuals in intra-specific competition. The magnitude of the effect was in some cases dependent on the ploidy levels of the plants. Furthermore, apomicts always produced more stolons than sexuals, suggesting potential displacement of sexuals by apomicts where they co-occur.     

Sexual tetraploid and apomictic pentaploid populations ofHieracium pilosella (Compositae)

Five species are recognized inHieracium subgen.Pilosella sect.Pilosellina Fries. Four are diploid (2x, 2n = 18), one (H. pilosella L.) is highly variable morphologically and cytologically (from 2x to 10x), in its mode of reproduction (self-incompatibility, agamospermy, amphimixis, apo-amphimixis) and in its hybridization pattern. A part of this huge agamic complex was analysed by comparing sexual 4x and apomictic 5x plants (crossing and germination experiments, measurements of vegetative reproduction by stolons etc.). In the experimental garden apomictic 5x produced more stolons than the sexual 4x plants and the total length of the stolons per rosette was greater. However, in nature, the competitive potential of the sexual plants seems to be higher, presumably as a result of the higher mortality of ramets in 5x. Sexual 4x plants often grow in dense and grazed grass vegetation, whereas 5x apomicts often occur in dunes with patchy vegetation. Apomicts produce more capitula per rosette, and sexual rosettes form only about 60% of the number of viable achenes as compared to apomictic ones. Therefore, apomicts appear to be characterized by a greater colonizing ability than sexual plants. Apomictic plants produce equal numbers of viable achenes under conditions of both open pollination and isolation. Sexual plants do not form any viable achenes after isolation and produce a somewhat lower percentage of achenes after open pollination than do apomictics. 5xreproduce exclusively apomictically. Apo-amphimixis was never observed in pentaploids and only very rarely in tetraploids. Addition hybrids are very rare. The cross sexual 4x × apomictic 5x failed in 70% of the attempts, but the recombination of genomes carrying genes for apomixis is possible and results in apomictic 4x and sexual 5x, both with a reduced number of viable achenes. In nature sexual and apomictic plants may occur in close proximity. In such cases the germination rate of the achenes of 4x and 5x is lower; this may indicate that apomictic plants fertilize sexual plants in nature (unidirectional gene-flow). 5x plants form euploid gametes carrying two or three genomes. The results of the crossing experiments can be explained in terms ofNogler's theory of monogenic inheritance of apospory.Variation and evolution inHieracium subg.Pilosella sect.Pilosellina I.     

Cytogenetic peculiarities of the genesis of apical meristem cells in case of gametophytic apomixis (with reference to autonomous Asteraceae apomicts)

The cytogenetic peculiarities of the genesis of apical meristem cells in apomicts were analyzed using some Asteraceae species as an example. It has been revealed that the frequency of aneu- and mixoploids is so high among the plants of these species (up to 30–60% of all plants studied or their progeny) that there is every reason to say that their occurrence in apomicts is regular rather than spontaneous. It has been demonstrated that microgametophyte in aposporous facultative apomict Pilosella officinarum is a relatively stable element of the seed reproduction system in terms of the ploidy level.     

Coevolution of apomixis and genome size within the genus<Emphasis Type="Italic"> Hypericum</Emphasis>

Trends concerning coevolution of mode of reproduction and genome size were elucidated by screening both components in 71 species/subspecies of the genus Hypericum. Two independent agamic complexes were identified (sections Ascyreia with ten, and Hypericum with five apomictic species). In the phylogenetically younger section Hypericum, the relative DNA content of apomicts is increased solely by polyploidy. The apomicts of the evolutionarily older section Ascyreia have significantly larger genomes than all other species due to polyploidization and higher DNA content per chromosome. An accumulation of retroelements might be one reason for the larger genomes. The male fertility of the apomicts was reduced compared to sexuals, although all apomicts were facultative pseudogamous, forming reduced male gametes. Another form of apomixis (obligate pseudogamous with unreduced male gametes), probably indicating an escape from interspecific sterility, was found in H. scabrum, the only case of asexual seed formation outside of sections Ascyreia and Hypericum. The described scenario for evolution of apomixis in relation to genome size deserves consideration in harnessing of apomixis.     

Apomixis via recombination of genome regions for apomeiosis (diplospory) and parthenogenesis in Erigeron (daisy fleabane, Asteraceae)

Apomixis in daisy fleabanes (Erigeron annuus and E. strigosus) is controlled by two genetically unlinked loci that regulate, independently, the formation of unreduced female gametophytes (apomeiosis, diplospory) and autonomous seed formation (parthenogenesis). In this work, fully apomictic F2s were regenerated by crossing F1s bearing, separately, these two functional regions. Two triploid (3x = 2n = 27) highly diplosporous F1s served as seed parents to an aneuploid (2x + 1 = 2n = 19) meiotic pollen donor bearing four AFLP markers linked to parthenogenetic seed formation but producing only abortive embryos and endosperm. Of 408 hybrids, 21 (5.1%) produced seed. Nine of these putative apomicts were tetraploids (4x), likely combining an unreduced egg from the diplosporous seed parent and a haploid gamete from the pollen parent (3x + x). The other 12 hybrid apomicts were pentaploid, interpreted as arising from the fusion of an unreduced diplosporous egg with an unreduced sperm cell (3x + 2x). Analysis indicated that all but three of the 21 synthetic apomicts recombined markers linked to diplospory and parthenogenesis. In addition, three additional hybrids combined markers linked to the two functional regions but produced only aborted embryos. The apomicts varied in percentage of diplosporous ovules (4.7–95.3% of all ovules produced) and in percentage of ovules that developed into seed (3.8–58.0%). These results support the hypothesis that apomeiosis and autonomous seed formation are genetically distinct, and that the traits can be separated and recombined to create hybrids exhibiting apomixis at near wildtype levels.     

Synergids and filiform apparatus in the sexual and apomictic dandelions from section Palustria (Taraxacum, Asteraceae)

An evolutionary trend to reduce “unnecessary costs” associated with the sexual reproduction of their amphimictic ancestors, which may result in greater reproductive success, has been observed among the obligatory apomicts. However, in the case of the female gametophyte, knowledge about this trend in apomicts is not sufficient because most of the ultrastructural studies of the female gametophyte have dealt with amphimictic angiosperms. In this paper, we tested the hypothesis that, in contrast to amphimictic plants, synergids in apomictic embryo sacs do not form a filiform apparatus. We compared the synergid structure in two dandelions from sect. Palustria: the amphimictic diploid Taraxacum tenuifolium and the apomictic tetraploid, male-sterile Taraxacum brandenburgicum. Synergids in both species possessed a filiform apparatus. In T. brandenburgicum, both synergids persisted for a long time without any degeneration, in spite of the presence of an embryo and endosperm. We propose that the persistent synergids in apomicts may play a role in the transport of nutrients to the embryo.     

Dispersal and establishment traits provide a colonization advantage for a polyploid apomictic plant

Premise

Apomictic plants (reproducing asexually through seed) often have larger ranges and occur at higher latitudes than closely related sexuals, a pattern known as geographical parthenogenesis (GP). Explanations for GP include differences in colonizing ability due to reproductive assurance and direct/indirect effects of polyploidy (most apomicts are polyploid) on ecological tolerances. While life history traits associated with dispersal and establishment also contribute to the potential for range expansion, few studies compare these traits in related apomicts and sexuals.

Methods

We investigated differences in early life history traits between diploid-sexual and polyploid-apomictic Townsendia hookeri (Asteraceae), which displays a classic pattern of GP. Using lab and greenhouse experiments, we measured seed dispersal traits, germination success, and seedling size and survival in sexual and apomictic populations from across the range of the species.

Results

While theory predicts that trade-offs between dispersal and establishment traits should be common, this was largely not the case in T. hookeri. Apomictic seeds had both lower terminal velocity (staying aloft longer when dropped) and higher germination success than sexual seeds. While there were no differences in seedling size between reproductive types, apomicts did, however, have slightly lower seedling survival than sexuals.

Conclusions

These differences in early life history traits, combined with reproductive assurance conferred by apomixis, suggest that apomicts achieve a greater range through advantages in their ability to both spread and establish.

     

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

The 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).     

Agrobacterium-mediated transformation and regeneration of guayule

In vitro grown shoot tissue of facultative apomictic lines of guayule (Parthenium argentatum Gray), a rubber producing desert shrub, were transformed by Agrobacterium-mediated DNA transfer and regenerated into complete plants. Guayule shoots of lines 11591, UC101 and UC104 were inoculated with A. tumefaciens strains LBA4404 or PC2760 harboring the binary vector pCGN1557. Axillary shoots were regenerated from transformed cells and rooted in vitro in the presence of kanamycin. Genetic transformation in all cases was verified by Southern blot analysis. Transgenic plants were grown to maturity in the greenhouse and, as predicted for apomictic species, all seed produced possessed kanamycin resistance. Because apomicts have limitations for gene transfer by normal sexual crosses, this method offers a new means of transferring genes into this species.Abbreviations BA benzyladenine - EDTA ethylene diamine tetraacetate - kan_R kanamycin resistance - MS salts salts of Murashige and Skoog medium (1962) - NAA naphthalene acetic acid - NPT-II neomycin phosphotransferase - SDS sodium dodecyl sulfate     

Genetic fingerprinting for determining the mode of reproduction in Paspalum notatum, a subtropical apomictic forage grass

 Paspalum is an important genus of the family Gramineae that includes several valuable forage grasses. Many of the species are polyploid and either obligate or facultative apomicts. Cyto-embryological observations of several tetraploid genotypes of P. notatum were performed to determine their mode of reproduction. Afterwards, selfed progenies of the genotypes F131, Q3664 and Q4117 were analysed using RFLP and RAPD genetic fingerprints to identify maternal and non-maternal (aberrant) plants, and to establish the degree of apomictic reproduction. Five maize clones and six primers were used for detecting genetic deviations from the maternal profile. Maize clones umc379, umc384 and umc318 and primers OPG10 and OPI4 were the most informative for discriminating between maternal and aberrant individuals within the progenies of F131 and Q3664. The combined results of three RFLP clones or 4–6 RAPD primers were necessary to ascertain the mode of reproduction in plants F131 and Q3664. The results obtained with the RFLP and RAPD markers were in agreement with the cyto-embryological studies in ascertaining the mode and degree of apomictic reproduction. Plant F131 showed a completely sexual reproductive behaviour, Q3664 an elevated expression of sexuality, while Q4117 was highly apomictic. A fingerprint analysis of an outcrossing population, aimed at the identification of hybrid plants, was also performed. Maize clones um318 and umc379 and primers OPC2 and OPC9 were used. The presence of specific bands belonging to the male parent permitted a rapid and easy detection of hybrids. The methodology described here can be applied both for the characterisation of P. notatum populations and to identify hybrid progenies in Paspalum breeding programs. Received: 5 March 1997 / Accepted: 13 May 1997     

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.