Putative Interspecific Hybrids Research Articles

Cytogenetic evidence of hybridization in Prosopis alba, P. hassleri, and P. fiebrigii in the semiarid-humid Chaco transition

Hybridization is a common and widespread phenomenon in vascular plants, leading to genetic exchange between related species. Interspecific hybridization has been extensively reported in Prosopis, a key genus of the Gran Chaco Americano, when species grow in sympatry or have adjacent distributions. Here, we performed a cytogenetic study in a hybrid zone of Prosopis alba, P. hassleri, and P. fiebrigii in southern South America. We characterized the karyotypes of pure species and the putative interspecific hybrids using chromosome fluorescent banding. Two karyotypes according to the heterochromatin pattern were identified, one corresponding to P. alba and the other to P. hassleri and P. fiebrigii; hybrid individuals presented P. alba or P. hassleri/P. fiebrigii karyotypes. The karyotypic formula and the number of nucleolar organizer regions were the same for all the studied morphotypes. Chromosome similarity among different species of the genus allows a continued hybridization and introgression between species and lends support to the hypothesis of a relatively recent diversification of Prosopis. Studying karyotype variation within a population could be a novel approach to the cytogenetic study of hybrids in the future.

A novel stem vegetable germplasm of interspecific hybrids between Brassica juncea and Brassica oleracea

ABSTRACT Interspecific hybridisation can fuse different genomes into a hybrid that could derive possible a novel germplasm for vegetable improvement. Tuber mustard (Brassica juncea Coss. var. crassicaulis Chen et Yang, AABB) and kohlrabi (Brassica oleracea L. var. caulorapa, CC) belong to different species of stem vegetables in Brassica. In present study, interspecific hybridisation between tuber mustard and purple kohlrabi was conducted successfully. Through ovary culture, four putative interspecific hybrids were obtained and three of them were identified as true hybrids by morphology, cytology and molecular analysis with the hybrid acquisition frequency of 0.68%. After chromosome doubling, one hybrid doubled completely as allohexaploidy (AABBCC) with 54 chromosomes, seven hybrids (ABC) with 27 ones non-doubled, and four hybrids doubled partially as the chimeras with above two types of chromosome. The allotriploids exhibited low fertility with vigour pollen rate of 7.54% and low seed-setting rates of 0.06, however, their allohexaploids exhibited high fertility with vigour pollen rate of 51.74% and high seed-setting rate of 5.88. This allohexaploid hybrid showed unique phenotypes with the normal bigger anther, better developed pods, larger expanding stem than the allotriploid ones. The above two types of hybrids will act as new germplasm of Brassica stem vegetables in breeding programme.

Alternative facts: a reconsideration of putatively natural interspecifichybrid specimens in the genus Heliconius (Lepidoptera: Nymphalidae).

Mallet et al. (2007 BMC Evolutionary Biology, 7, 28) employed a database of putative interspecific hybrid specimens of the genus Heliconius to advance a hypothesis of "the species boundary as a continuum." Here, each of those specimens, as well as subsequently documented specimens, is individually reassessed regarding its phenotype, potential parentage and chain of custody in collections. Using a quantified scale of reliability, most of the specimens are interpreted differently than Mallet et al.'s identifications, and the actual number of interspecific hybrids is estimated to be much smaller than they proposed. To be specific, of 163 putative hybrid specimens examined, 11% suffered from ambiguous identity, 5% from confounding issues with their data labels, 50% were arguably intraspecific (depending upon alternative species concepts), and 22% were almost certainly reared, commercial specimens. Only eleven of the specimens meet the criteria established here to be legitimate and reliable interspecific hybrids, and all of those are between closely-related species. This result has potentially important implications for current hypotheses of frequent genomic introgression of wing pattern alleles among Heliconius clades.

Control of Phytophthora species in plant stock for habitat restoration through best management practices

Emergent plant pathogens represent one of the most significant threats to biodiversity, and exotic Phytophthora species have recently emerged as a serious problem in restored habitats in California and in nurseries producing the plant stock. It is hypothesized that ‘best management practices’ prescribed through a Phytophthora Prevention Programme (PPP) could be useful in minimizing phytophthora disease incidence. To understand the magnitude of the problem and the efficacy of the PPP, plants in restoration nurseries were evaluated for (i) the Phytophthora species assemblage present in the absence of the PPP, and (ii) the effectiveness of the PPP to reduce them. Sampling included 203 plants grown in the absence of the PPP, and 294 grown implementing the PPP. Only samples collected in the absence of the PPP were Phytophthora‐positive, and cumulatively yielded 55 isolates from 13 different taxa, including 1 putative interspecific hybrid genotype. There were 21 novel Phytophthora–plant species combinations. The most common Phytophthora species was P. cactorum. Four plant species had the highest disease incidence, namely: Diplacus aurantiacus (50 ± 11.2%), Heteromeles arbutifolia (33 ± 9.6%), Ceanothus thyrsiflorus (30 ± 8.4%), and Frangula californica (30 ± 8.4%). Disease incidence in nurseries after the implementation of the PPP dropped to zero (P < 0.001), and was unaffected to any significant degree by nursery differences, or plant species tested. This study identifies a large number of novel ‘plant species × Phytophthora species’ combinations, and provides for the first time strong evidence that the PPP significantly reduced Phytophthora in plant stock for habitat restoration.

Integrating a comprehensive DNA barcode reference library with a global map of yews (Taxus L.) for forensic identification.

Rapid and accurate identification of endangered species is a critical component of biosurveillance and conservation management, and potentially policing illegal trades. However, this is often not possible using traditional taxonomy, especially where only small or preprocessed parts of plants are available. Reliable identification can be achieved via a comprehensive DNA barcode reference library, accompanied by precise distribution data. However, these require extensive sampling at spatial and taxonomic scales, which has rarely been achieved for cosmopolitan taxa. Here, we construct a comprehensive DNA barcode reference library and generate distribution maps using species distribution modelling (SDM), for all 15 Taxus species worldwide. We find that trnL-trnF is the ideal barcode for Taxus: It can distinguish all Taxus species and in combination with ITS identify hybrids. Among five analysis methods tested, NJ was the most effective. Among 4,151 individuals screened for trnL-trnF, 73 haplotypes were detected, all species-specific and some population private. Taxonomical, geographical and genetic dimensions of sampling strategy were all found to affect the comprehensiveness of the resulting DNA barcode library. Maps from SDM showed that most species had allopatric distributions, except T. mairei in the Sino-Himalayan region. Using the barcode library and distribution map data, two unknown forensic samples were identified to species (and in one case, population) level and another was determined as a putative interspecific hybrid. This integrated species identification system for Taxus can be used for biosurveillance, conservation management and to monitor and prosecute illegal trade. Similar identification systems are recommended for other IUCN- and CITES-listed taxa.

English

Wild relatives of eggplants represent a good source of variation for breeding programmes, in particular for traits related to biotic and abiotic stresses and also fruit quality traits. However, wild species remain largely unexploited for eggplant breeding compared to other crops like tomato. Seven cultivated eggplant accessions (SM001-02, SM001-04, SM001-06, SM001-07, SA002-02, SA002-03 and SMA003-03) were crossed with three wild accessions (ST004-03 (Solanum tovum), San005-01 (Solanum anguivi) and SA002-08 (Solanum aethiopicum)) in an open field using completely randomized design. The success of fruit and seed set as well as seed germination depended on the cross combination and the direction of the cross. In this regard, no fruit set was recorded when the wild accessions were used as female parents. The highest fruit set and mean number of seeds/fruit was obtained from the crosses Sm001-07 × ST004-03 (6%; 264 seeds) and Sm001-07 × San005-01 (5.7%, 114 seeds), respectively. The germination of hybrid seeds was recorded in only three crosses, SM001-07 × ST004-03, SM001-07 × San005-01 and SA002-02 × San005-01 with germination range from 3.3 to 16.6%. However, plantlets from these seeds did not survive after two weeks of germination. The hybridity of the putative interspecific F1 hybrids (through tissue culture) was confirmed with a morphological marker. These hybrids obtained will contribute to broadening the genetic background of cultivated eggplant species used in this study and to the genetic enhancement of this crop. Key words: Crossability, eggplant, fruit set, germination, introgression, seed set, wild relatives.

Generation of interspecific hybrids between Trifolium vesiculosum and T. alexandrinum using embryo rescue

Interspecific hybrids were developed between Trifolium alexandrinum cultivar Wardan × Trifolium vesiculosum and T. alexandrinum cultivar BL1 × T. vesiculosum through embryo rescue, as the crosses failed to set seed under natural conditions. Trifolium vesiculosum was used as a donor/male parent in this study as it is reported to possess tolerance to stem rot and high forage yield. Fertilization in crossed florets of the crosses was manifested from the recovery of swollen ovaries ( 93.00%) of the swollen ovaries. The hybrid embryos at various developmental stages (heart, torpedo and cotyledonary) were rescued at a frequency of 2.56% from Wardan × T. vesiculosum and 6.12% from BL1 × T. vesiculosum. Differentiation occurred only in the cotyledonary stage embryos, resulting in 17 putative interspecific hybrid plantlets. The assessment of plantlet hybridity through SSR markers (for the alleles inherited from the donor parent), micromorphological leaf traits (leaf texture and stomata) and morphological characters (plant height, leaflet length and width) confirmed production of two interspecific hybrids designated as AV1 and BV3 representing both the crosses. AV1 displayed moderate resistance and BV3 was resistant to stem rot.

The genetic relationship and structure of some natural interspecific hybrids in Prunus subgenus Prunophora, based on nuclear and chloroplast simple sequence repeats

In the subgenus Prunophora of the genus Prunus, many transitional traits presented in interspecific hybrids, the so-called ‘new species’, were frequently named due to the complicated botanical classification system. In this study, we used 16 nuclear simple sequence repeats (nSSRs) and 10 chloroplast simple sequence repeats (cpSSRs) to evaluate genetic relationships among 42 accessions, which included 15 putative interspecific hybrids, and then to reveal the speciation and differentiation in the subgenus Prunophora. In total, 231 and 27 alleles were observed in nSSRs and cpSSRs respectively; and with cpSSRs 20 haplotypes were revealed among the accessions. Furthermore, the haplotype and genetic structure analysis implied that (1) Prunus simonii Carr. might be a subspecies or a forma of Prunus salicina L., rather than an interspecific hybrid between P. salicina and Prunus armeniaca L., (2) Prunus limeixing Zhang et Wang was derived from a natural hybrid with P. salicina as its maternal progenitor and P. armeniaca as the female parent, and (3) Prunus cathayana Fu et al. (or kernel-using apricot) was an interspecific hybridization species of P. armeniaca (maternal parent) and Prunus sibirica L. (female parent). These results will be useful for clarifying the problems in the botanical classification, and facilitate the conservation and management of plum and apricot genetic resources in the Chinese National Germplasm Repository for Plums and Apricots.

Genetic relationships of boxwood (Buxus L.) accessions based on genic simple sequence repeat markers

Boxwoods (Buxus L., Buxaceae) are popular woody landscape shrubs grown for their diverse forms and broad-leaved evergreen foliage. We used genic simple sequence repeat (genic-SSR) markers to assess genetic diversity and relatedness of 275 accessions from the National Boxwood Collection at the U.S. National Arboretum. Flow cytometry was conducted to determine the relative ploidy of each accession. Genic-SSR loci were highly variable among the accessions, detecting an average of 6.7 alleles per locus based on 17 primer pairs. Data were analyzed with a distance matrix based on Jaccard’s similarity index, followed by Unweighted Pair Group Method with Arithmetic Mean clustering. Two major clusters were identified, with four subclusters consisting of individual accessions from B. balearica Lam., B. bodinieri Levl., B. harlandii Hance, B. microphylla Siebold et Zuccarini, B. sempervirens L., B. sinica (Rehd. et Wils.) M. Cheng, and their putative interspecific hybrids. The accessions generally clustered by cultivar, provenance, or species. Clustering within each group typically reflected breeding pedigrees, when known, and the clusters were supported by bootstrap results. This information will be used for breeding programs and collection management, and for identifying possible sources of disease tolerance for boxwood blight and other diseases and pests.

Interspecific Hybridization between Eggplant and Wild Relatives from Different Genepools

Wild relatives represent a source of variation for many traits of interest for eggplant (Solanum melongena) breeding, as well as for broadening the genetic base of this crop. However, interspecific hybridization with wild relatives has been barely used in eggplant breeding programs. As initiation of an introgression breeding program we performed 1424 interspecific hybridizations between six accessions of eggplant from the Occidental and Oriental groups and 19 accessions of 12 wild species from the primary (Solanum incanum and Solanum insanum), secondary (Solanum anguivi, Solanum dasyphyllum, Solanum lichtensteinii, Solanum linnaeanum, Solanum pyracanthos, Solanum tomentosum, and Solanum violaceum), and tertiary (Solanum elaeagnifolium, Solanum sisymbriifolium, and Solanum torvum) genepools. Fruit set, hybrid seed, and seed germination were obtained between Solanum melongena and all wild species of the primary and secondary genepools. The highest fruit set percentage and quantity of seeds per fruit were obtained with the two primary genepool species S. incanum and S. insanum as well as with some secondary genepool species, like S. anguivi, S. dasyphyllum, or S. lichtensteinii, although some differences among species were observed depending on the direction of the hybridization. For small-fruited wild species, the number of seeds per fruit was lower when using them as maternal parent. Regarding tertiary genepool species, fruit set was obtained only in interspecific hybridizations of eggplant with S. sisymbriifolium and S. torvum, although the fruit of the former were parthenocarpic. However, it was possible to rescue viable interspecific hybrids with S. torvum. In total we obtained 58 interspecific hybrid combinations (excluding reciprocals) between eggplant and wild relatives. Some differences were observed among S. melongena accessions in the degree of success of interspecific hybridization, so that the number of hybrid combinations obtained for each accession ranged between 7 (MEL2) and 16 (MEL1). Hybridity of putative interspecific hybrid plantlets was confirmed with a morphological trait (leaf prickliness) and 12 single nucleotide polymorphism markers. The results show that eggplant is amenable to interspecific hybridization with a large number of wild species, including tertiary genepool materials. These hybrid materials are the starting point for introgression breeding in eggplant and in some cases might also be useful as rootstocks for eggplant grafting.

Notes on Malesherbia (Passifloraceae) in Peru: a new species from southern Peru, a new record and a first report on interspecific hybridization in Malesherbia

Malesherbia fatimae is described as a new species from the Province Condesuyos, Department of Arequipa, Peru. The new species is clearly differentiated from all know species of the genus by forming a strictly erect, laxly branched shrub with densely tomentose indument and from the perennial Peruvian members by an absence of the glandular trichomes. No clear affinities to know species present themselves, but the new species evidently belongs to Malesherbia sect. Malesherbia. At the type locality a putative hybrid with the morphologically highly divergent Malesherbia tenuifolia was observed, showing morphologically intermediate characters between M. fatimae and M. tenuifolia. Malesherbia tenuifolia is here for the first time reported from Peru, bringing the species total for the county to 12. A second putative interspecific hybrid, Malesherbia ardens × M. arequipensis, is reported from Department Moquegua. This indicates that species of Malesherbia may hybridize when eco-geographical separation breaks down due to habitat modification. The new species and the two novel hybrids are described and illustrated.

Correction: in the wild hybridization of annual datura species as unveiled by morphological and molecular comparisons.

The present work aimed to verify whether intermediate variants were natural crosses between Datura species (D. stramonium forms and D. ferox). Their existence has been long ago insinuated but has not been studied using morphological features and molecular tools. The variants differed in stem coloring, upper bearing forks, and fruit characters. Principal Components Analysis of 11 morphological characteristics showed that D. ferox and D. stramonium (forms stramonium and tatula) were quite different and the putative hybrids were intermittent. The D. ferox × D. stramonium f. tatula was closer to the latter of its parents. Sequencing analysis revealed identical amplified trnL intron in all variants and a 100% homology with D. stramonium accession number EU580984.1 suggested that this plastid cannot discern Datura variants. However, genomic analysis with URP markers indicated that the hybrids had >60% genetic makeup similarity with both parents suggesting that the intermediate variants were putative inter-specific hybrids. Moreover, the dendrogram stemmed from cluster analysis of the fingerprint profile of variants placed D. stramonium and D. ferox in different branches indicating their genetic differentiation from each other as well as from their hybrids. The findings suggest that the natural hybridization of annual Datura species occurs. Extrapolating, this hybridization could be the first step for speciation. More possibly, it can alter population composition, its weediness and adaptability to local conditions.

Morphological, cytological and molecular analyses of a synthetic hexaploid derived from an interspecific hybrid between Gossypium hirsutum and Gossypium anomalum

Gossypium anomalum represents an inestimable source of genes that could potentially be transferred into the gene pool of cultivated cotton. To resolve interspecific hybrid sterility problems, we previously treated triploid hybrids derived from a cross between Gossypium hirsutum and G. anomalum with 0.15% colchicine and obtained a putative fertile hexaploid. In this study, we performed morphological, molecular and cytological analyses to assess the hybridity and doubled status of putative interspecific hybrid plants. Most of the morphological characteristics of the putative hexaploid plants were intermediate between G. hirsutum and G. anomalum. Analysis of mitotic metaphase plates revealed 78 chromosomes, confirming the doubled hybrid status of the hexaploid. Genome-wide molecular analysis with different genome-derived SSR markers revealed a high level of polymorphism (96.6%) between G. hirsutum and G. anomalum. The marker transferability rate from other species to G. anomalum was as high as 98.0%. The high percentage of polymorphic markers with additive banding profiles in the hexaploid indicates the hybridity of the hexaploid on a genome-wide level. A-genome-derived markers were more powerful for distinguishing the genomic differences between G. hirsutum and G. anomalum than D-genome-derived markers. This study demonstrates the hybridity and chromosomally doubled status of the (G. anomalum×G. hirsutum)2 hexaploid using morphological, cytological and molecular marker methods. The informative SSR markers screened in the study will be useful marker resources for tracking the flow of G. anomalum genetic material among progenies that may be produced by future backcrosses to G. hirsutum.

In the wild hybridization of annual Datura species as unveiled by morphological and molecular comparisons.

BackgroundThe present work aimed to verify whether intermediate variants were natural crosses between Datura species (D. stramonium forms and D. ferox). Their existence has been long ago insinuated but has not been studied using morphological features and molecular tools. The variants differed in stem coloring, upper bearing forks, and fruit characters.ResultsPrincipal Components Analysis of 11 morphological characteristics showed that D. ferox and D. stramonium (forms stramonium and tatula) were quite different and the putative hybrids were intermittent. The D. ferox × D. stramonium f. tatula was closer to the latter of its parents. Sequencing analysis revealed identical amplified trnL intron in all variants and a 100% homology with D. stramonium accession number EU580984.1 suggested that this plastid cannot discern Datura variants. However, genomic analysis with URP markers indicated that the hybrids had >60% genetic makeup similarity with both parents suggesting that the intermediate variants were putative inter-specific hybrids. Moreover, the dendrogram stemmed from cluster analysis of the fingerprint profile of variants placed D. stramonium and D. ferox in different branches indicating their genetic differentiation from each other as well as from their hybrids.ConclusionsThe findings suggest that the natural hybridization of annual Datura species occurs. Extrapolating, this hybridization could be the first step for speciation. More possibly, it can alter population composition, its weediness and adaptability to local conditions.

Genetic Relationships in Zoysia Species and the Identification of Putative Interspecific Hybrids Using Simple Sequence Repeat Markers and Inflorescence Traits

Zoysia Willd. are warm‐season turfgrasses used throughout the southern United States and upward into the transition zone for their superior heat and drought tolerances and their relatively low input requirements. Understanding the population structure present within Zoysia germplasm can assist plant breeders in exploiting available variation. The objectives of this study were to assess simple sequence repeat (SSR) allelic diversity within and among Zoysia spp., evaluate the genetic constitution of putative interspecific hybrids, and determine if Zoysia spp. and hybrids can be differentiated by inflorescence traits. Sixty‐two Zoysia accessions selected as a subset of genotypically and phenotypically diverse genotypes were evaluated with 50 SSR markers and six inflorescence traits. Both an unweighted pair group method with arithmetic averaging (UPGMA) phylogram and principal coordinate analysis (PCO) analysis revealed a continuum of genetic variation present within and among the species. The model‐based program STRUCTURE revealed two distinct subpopulations within Z. japonica Steud. as well as a distinct Z. matrella (L.) Merr. subpopulation. Based on SSR analysis, 23 Zoysia accessions evaluated had evidence of admixture. Six inflorescence phenotypes also revealed a continuous range of variation. This study was able to verify the presence of hybrids between Z. japonica and Z. matrella and provided further evidence for the hypothesis that Zoysia spp. are subpopulations or ecotypes within one species and not separate species.

Genetic diversity, population structure and phylogenetic inference among Italian Orchids of the Serapias genus assessed by AFLP molecular markers

The means by which new species arise remain a fundamental question in the Orchid family evolution. Even if the Orchid-pollinator interactions are highly specific, events of cross-hybridization in Serapias are possible and represent a central issue of the plant's evolutionary biology. Italian Orchids have a wide variety of pollination systems and highly diverse floral traits, considered as some of the main tools for activating the speciation processes. The assess of diversity in Serapias taxa and eventual phylogenetic relationships among Serapias lingua, S. parviflora, S. vomeracea, S. apulica, S. nurrica, S. neglecta, S. cordigera, S. politisii, and some putative inter-specific hybrids were investigated at the DNA level by AFLP markers. The present molecular study on Serapias populations (1) confirms some previous finding, (2) opens new hypotheses on the possible evolutionary and taxonomic relationships existing among the Serapias species, and (3) gives indications about the real consistence of hybrid individuals in a genus characterized by a relevant gene flow.

Natural hybridization of two mussel species Dreissena polymorpha (Pallas, 1771) and Dreissena bugensis (Andrusov, 1897)

Dreissenids display a high diversity of shell morphology, and it is frequently difficult to ascribe some individuals from mixed populations to one of the two species, Dreissena polymorpha (Pallas, 1771) or D. bugensis (Andrusov, 1897). Presumably, such individuals may be interspecific hybrids. We have analyzed species-specific allozyme loci of the typical representatives of these two mussel species and putative interspecific hybrids. A natural interspecific hybrid between D. polymorpha and D. bugensis was discovered for the first time by genetic methods. It has been demonstrated that D. bugensis was a maternal parent.

Evaluating Theobroma grandiflorum for comparative genomic studies with Theobroma cacao

The seeds of Theobroma cacao (cacao) are the source of cocoa, the raw material for the multi-billion dollar chocolate industry. Cacao’s two most important traits are its unique seed storage triglyceride (cocoa butter) and the flavor of its fermented beans (chocolate). The genome of T. cacao is being sequenced, and to expand the utility of the genome sequence to the improvement of cacao, we are evaluating Theobroma grandiflorum, the closest economically important species of Theobroma for its potential use in a comparative genomic study. T. grandiflorum differs from cacao in important agronomic traits such as flavor of the fermented beans, disease resistance to witches’ broom and abscission of mature fruits. By comparing genomic sequences and analyzing viable inter-specific hybrids, we hope to identify the key genes that regulate cacao’s most important traits. We have investigated the utility in T. grandiflorum of three types of markers (microsatellite markers, single-strand conformational polymorphism markers and single nucleotide polymorphism (SNP) markers) developed in cacao. Through sequencing of amplicons of 12 diverse individuals of both cacao and T. grandiflorum, we have identified new intra- and inter-specific SNPs. Two markers which had no overlap of alleles between the species were used to genotype putative inter-specific hybrid seedlings. Sequence conservation was significant and species-specific differences numerous enough to suggest that comparative genomics of T. grandiflorum and T. cacao will be useful in elucidating the genetic differences that lead to a variety of important agronomic trait differences.

Differentiation of three closely related Japanese oak species and detection of interspecific hybrids using AFLP markers

Three white-oak species ( Quercus crispula Blume, Quercus dentata Thunb., and Quercus serrata Thunb.) are native, widely distributed, and prominent species in the temperate deciduous forests of Japan. They are closely related to each other and overlapping morphological variation in some traits is observed, although they differ from each other in appearance. To distinguish these species genetically, we carried out clustering analysis based on Bayesian approach by AFLP markers using morphologically typical trees. Although no completely species-specific markers were obtained, these species could be distinguished and their genetic relationships were evaluated based on differences in frequencies of 66 polymorphic markers, including four that were almost completely species-specific. We also attempted to characterize putative interspecific hybrids between Q. crispula and Q. dentata sampled in a mixed stand. Two programs, HINDEX and STRUCTURE, were successfully used to detect several hybrid individuals without any prior information about their morphological traits. However, STRUCTURE and HINDEX gave conflicting indications regarding the admixture levels in some individuals.

Cytotaxonomy and cytogeography of the goldenaster genus Heterotheca (Asteraceae: Astereae)This paper is one of a selection of papers published in the Special Issue on Systematics Research.

Between 1948 and 2008, 668 chromosome number determinations have been reported for individuals of the goldenaster genus Heterotheca Cass. (Asteraceae: Astereae) from Canada, Mexico, and the United States, including 285 newly reported here. This common group of mostly western North American herbaceous annuals and perennials is subdivided into three sections, the most complex of which is section Phyllotheca . Differences of opinion about the taxonomy of the goldenasters, numerous synonyms, and the lack of a comprehensive treatment of the polymorphic villosa complex and other species complexes in section Phyllotheca made it difficult to organize the data into a meaningful discussion of the cytotaxonomy and cytogeography of this genus. Here I present the geographic distribution of all published counts and cytogeography maps of all species of Heterotheca, assigning taxa following recent taxonomic revisions. The sample sizes for species sampled ranged from 1 to 228 reports. All species have a base number of x = 9. Only diploids (2n = 9II or 2n = 18) were found in Heterotheca oregona (Nutt.) Shinners (the single species of sect. Ammodia ) and in the three species of section Heterotheca. Both diploids and (or) tetraploids were found across 18 sampled species of section Phyllotheca, with the frequency varying among them; nine species are known only at one ploidy level (either diploid or tetraploid; three species each). Only four triploids (2n = 9III or 2n = 27) were encountered in section Phyllotheca; two were putative interspecific hybrids. One new nomenclatural combination is proposed: Heterotheca inuloides var. leptoglossa .