Effect of X-irradiation on the size of the mutated sector in diploid and tetraploid rice

Summary An investigation was carried out of the effect of X-irradiation on the size of the mutated sector in three diploids (A.S.D.8, T.N. 1 and G.E.B. 24) and the autotetraploid of G.E.B 24 of Oryza sativa L., by analyzing the M₂ segregation ratios of chlorophyll mutants. A total of 612 segregating M₁ panicle progenies in the diploids and 284 panicle progenies in the tetraploid was studied in the M₂ generation for the ratio of normal to chlorophyll mutant seedlings. Of the 573 segregating M₁ panicles analysed by chi square test in the diploids, 206 showed good fit with a 31 ratio, while the rest of the progenies (367) deviated significantly in both directions from the expected 31 segregation. This was interpreted as the result of M₁ panicles developing from both single cell and multicellular initials following X-irradiation, the former segregating for 31, the latter significantly deviating from 31. An assessment of fertility (as judged from the number of M₂ seedlings per mutated M₁ panicle), related to the distribution of chi square deviations from the 31 ratio in the diploids, indicated that the M₁ panicle progenies deriving from single cell initials had low fertility but those originating from multicellular initials ranged from low to high fertility. A comparison of the distribution pattern of the M₂ segregation ratios of chlorophyll mutant phenotypes in the diploid and autotetraploid G.E.B. 24 indicated that, in the tetraploid, a larger sector and only one, or fewer cell initials than in the diploid species, are involved.     

A CYTOGENETIC ANALYSIS OF CERTAIN POLYPLOIDS IN GRINDELIA (COMPOSITAE)

Two diploid taxa, Grindelia procera and G. camporum, and 3 tetraploid ones, G. camporum, G. hirsutula, and G. stricta, have been studied to ascertain their interrelationships. Meiosis in diploid parental strains was regular, the common chromosome configuration being 5 rod bivalents and 1 ring bivalent. The average chiasmata frequency per chromosome was 0.60. Pollen fertility was about 90% in all strains examined. Diploid interspecific hybrids had normal meiosis with an average chiasmata frequency of 0.56 per chromosome. No heterozygosity for inversions or interchanges was detected, and pollen fertility was above 85%. Meiosis in parental tetraploid strains was characterized by the presence of quadrivalents in addition to a complementary number of bivalents. The average chiasmata frequency per chromosome was 0.59 and pollen fertility was generally about 80%. Tetraploid interspecific hybrids also had quadrivalents, normal meiosis, and high pollen fertility. Close genetic relationships between the diploids and between the tetraploids are indicated, and geographical, ecological, and seasonal barriers to gene exchange exist. Attempts to obtain hybrids between diploids and tetraploids were successful in a few cases. The hybrids were tetraploid and had normal meiosis and fertility similar to parental and F₁ tetraploids. Their origin was by the union of unreduced gametes of the diploid female parent and normal pollen from the tetraploid parent. On the basis of chromosome homology, normal meiosis, plus high fertility exhibited in the diploid, tetraploid, and diploid X tetraploid interspecific hybrids, these species of Grindelia are considered to be a part of an autopolyploid complex. Gene exchange between diploids and diploids, tetraploids and tetraploids, and diploids and tetraploids is possible. Tetraploid G. camporum may have originated by hybridization between G. procera and diploid G. camporum with subsequent doubling of chromosomes and selection for the combined characteristics of the diploids.     

GENETIC CONSEQUENCES OF AUTOPOLYPLOIDY IN TOLMIEA (SAXIFRAGACEAE)

Although there is an extensive literature on the genetic attributes of allopolyploids, very little information is available regarding the genetic consequences of autopolyploidy in natural populations. We therefore addressed the major predicted genetic consequences of autopolyploidy using diploid and tetraploid populations of Tolmiea menziesii. Individual autotetraploid plants frequently maintain three or four alleles at single loci: 39% of the 678 tetraploid plants exhibited three or four alleles for at least one locus. Heterozygosity was also significantly higher in autotetraploid populations than in diploid populations: H_° = 0.070 and 0.237 in diploid and tetraploid Tolmiea, respectively. Most of the genetic diversity in T. menziesii is maintained within populations (ratio of gene diversity within populations to mean total genetic diversity = 0.636). The total genetic diversity due to differentiation between the two cytotypes is only 0.055. Such a low degree of differentiation between cytotypes would be expected between a diploid and its autotetraploid derivative. Most diploid and all tetraploid populations examined are in genetic equilibrium. Diploid and tetraploid Tolmiea share three or four alleles at six of eight polymorphic loci. This suggests that either autotetraploid Tolmiea was formed via crossing of genetically different diploids (perhaps via a triploid intermediate) or autopolyploidy occurred more than once in separate individual plants, followed by later crossing of autotetraploids.     

Other tetraploid species and conspecific diploids as sources of genetic variation for an autotetraploid

? Premise of the study: Most plants are polyploid and have more than two copies of the genome. The evolutionary success of polyploids is often attributed to their potential to harbor increased genetic variation, but it is poorly understood how polyploids can attain such variation. Because of their formation bottleneck, newly formed tetraploids start out with little variation. Tetraploids may attain genetic variation through a combination of new mutations, recurrent formation, and gene exchange with diploid ancestors or related tetraploid species. We explore the role of gene exchange and introgression in autotetraploid Rorippa amphibia, a species that harbors more genetic variation than its diploid ancestors. ? Methods: We crossed autotetraploid R. amphibia to diploid conspecifics and tetraploid R. sylvestris and backcrossed resulting F(1) hybrids. We used flow cytometry to determine the ploidy of all progeny. ? Key results: Tetraploids of R. amphibia and R. sylvestris were interfertile; F(1) hybrids were fertile and could backcross. Crosses between diploids and tetraploids yielded a small number of viable, often tetraploid progeny. This indicates that unreduced gametes can facilitate gene flow from diploids to tetraploids. We detected a frequency of unreduced gametes of around 2.7 per 1000, which was comparable between diploids and tetraploids. ? Conclusions: Introgression from tetraploid R. sylvestris provides a realistic source of variation in autotetraploid R. amphibia. Only in a scenario where other compatible partners are absent, for example immediately after tetraploidization, gene flow through unreduced gametes from diploids could be an important source of genetic variation for tetraploids.     

Microsatellite Analysis of Genetic Variation and Population Genetic Differentiation in Autotetraploid and Diploid Rice

Genetic diversity and population genetic structure of autotetraploid and diploid populations of rice collected from Chengdu Institute of Biology, Chinese Academy of Sciences, were studied based on 36 microsatellite loci. Among 50 varieties, a moderate to high level of genetic diversity was observed at the population level, with the number of alleles per locus (A _e) ranging from 2 to 6 (mean 3.028) and polymorphism information content ranging from 0.04 to 0.76 (mean 0.366). The expected heterozygosity (H _e) varied from 0.04 to 0.76 (mean 0.370) and Shannon’s index (I) from 0.098 to 1.613 (mean 0.649). The autotetraploid populations showed slightly higher levels of A _e, H _e, and I than the diploid populations. Rare alleles were observed at most of the simple sequence repeat loci in one or more of the 50 accessions, and a core fingerprint database of the autotetraploid and diploid rice was constructed. The F-statistics showed genetic variability mainly among autotetraploid populations rather than diploid populations (F _st = 0.066). Cluster analysis of the 50 accessions showed four major groups. Group I contained all of the autotetraploid and diploid indica maintainer lines and an autotetraploid and its original diploid indica male sterile lines. Group II contained only the original IR accessions. Group III was more diverse than either Group II or Group IV, comprising both autotetraploid and diploid indica restoring lines. Group IV included a japonica cluster of the autotetraploid and diploid rices. Furthermore, genetic differences at the single-locus and two-locus levels, as well as components due to allelic and gametic differentiation, were revealed between autotetraploid and diploid varieties. This analysis indicated that the gene pools of diploid and autotetraploid rice were somewhat dissimilar, as variation exists that distinguishes autotetraploid from diploid rices. Using this variation, we can breed new autotetraploid varieties with some important agricultural characters that were not found in the original diploid rice varieties.     

Habitat differentiation, hybridization and gene flow patterns in mixed populations of diploid and autotetraploid Dactylorhiza maculata s.l. (Orchidaceae)

Detailed ecological, morphological and molecular analyses were performed in mixed populations of diploid and autotetraploid Dactylorhiza maculata s.l. in Scandinavia. Comparisons were made with pure populations of either diploid ssp. fuchsii or tetraploid ssp. maculata. It was shown that mixed populations are the result of secondary contact between ssp. fuchsii and ssp. maculata. No patterns of recent and local autopolyploidization were found. Morphology and nuclear DNA markers (internal transcribed spacers of nuclear ribosomal DNA) showed that diploids and tetraploids from mixed populations have similar levels of differentiation to diploids and tetraploids from pure populations. Vegetation analyses, as well as analyses of environmental variables, revealed that diploid and tetraploid individuals in mixed populations are ecologically well differentiated on a microhabitat level. Diploids and tetraploids in pure populations have wider ecological amplitudes than they do in mixed populations. Triploid hybrids grew in intermediate microhabitats between diploids and tetraploids in the mixed populations. Plastid DNA markers indicated that both diploids and tetraploids may act as the maternal parent. Based on morphology and nuclear markers triploids are more similar to tetraploids than to diploids. There were indications of introgressive gene flow between ploidy levels. Plastid markers indicated that gene flow from diploid to tetraploid level is most common, but nuclear markers suggested that gene flow in opposite direction also may occur. Similar patterns of differentiation and gene flow appeared in localities that represented contrasting biogeographic regions. Disturbance and topography may explain why hybridization was slightly more common and the differentiation patterns somewhat less clear in the Scandinavian mountains than in the coastal lowland. An erratum to this article can be found at     

CHLOROPLAST-DNA AND ALLOZYMIC VARIATION IN DIPLOID AND AUTOTETRAPLOID HEUCHERA GROSSULARIIFOLIA (SAXIFRAGACEAE)

Diploid and autotetraploid populations of Heuchera grossulariifolia occur throughout mountainous regions of the Pacific Northwest. Controlled greenhouse crosses indicated that the two cytotypes are largely reproductively isolated. Fourteen diploid and 11 tetraploid populations were analyzed electrophoretically. Individual tetraploid plants expressed up to four alleles per isozyme locus, and tetraploid populations had significantly higher levels of heterozygosity than diploids. Mean observed heterozygosity was 0.159 for tetraploid populations and 0.058 for diploid populations. The patterns of allelic distribution between cytotypes suggested multiple origins of autotetraploids. This hypothesis was supported by restriction-site analysis of chloroplast-DNA (cpDNA) variation which indicated that there had been at least three independent origins of tetraploids. Electrophoretic data, in conjunction with a cpDNA-based phylogeny and geographic distribution of populations, suggest that autopolyploid populations evolved several times as migration of diploids occurred down river systems. This study further supports the contention that autopolyploidy can be a common and successful speciation process in some groups of plants.     

The effect of B chromosomes on homoeologous pairing in species hybrids

The effect of B chromosomes on chromosome pairing at meiosis was investigated in the species hybrid Lolium temulentum x L. perenne at both the diploid and tetraploid level. The presence of B chromosomes drastically reduced association of homoeologous chromosomes in both the diploids and tetraploids. This was evident from the high frequency of univalents recorded in PMC's of diploid hybrids with B's and from the predominantly bivalent association of homologous chromosomes in tetraploids of this type. In the absence of B's homoeologous pairing was extensive giving a high frequency of bivalents in the diploids and multivalents as well as bivalents and univalents in the tetraploids.     

PLANT POLYPLOIDY AND POLLINATION: FLORAL TRAITS AND INSECT VISITS TO DIPLOID AND TETRAPLOID HEUCHERA GROSSULARIIFOLIA

In many polyploid species, polyploids often have different suites of floral traits and different flowering times than their diploid progenitor species. We hypothesized that such differences in floral traits in polyploids may subsequently affect their interactions with pollinating and other insect visitors. We measured floral morphology and flowering phenology in 14 populations of diploid and autotetraploid Heuchera grossulariifolia Rydb. (Saxifragaceae), determined if repeated evolution of independent polyploid lineages resulted in differentiation in floral morphology among those lineages, and ascertained if there was a consistent pattern of differentiation among genetically similar diploid and autotetraploid populations. In addition, we evaluated the differences in suites of floral visitors within a natural community where diploids and autotetraploids occur sympatrically. Overall, flowers of autotetraploid plants were larger and shaped differently than those of diploids, had a different flowering phenology than that of diploids, and attracted different suites of floral visitors. In comparison with flowers of diploids, tetraploid floral morphology varied widely from pronounced differences between cytotypes in some populations to similar flower shapes and sizes between ploidal levels in other populations. Observations of floral visitors to diploids and autotetraploids in a natural sympatric population demonstrated that the cytotypes had different suites of floral visitors and six of the 15 common visitors preferentially visited one ploidy more frequently. Moreover, we also found that floral morphology differed among independent autotetraploid origins, but there was no consistent pattern of differentiation between genetically similar diploid and autotetraploid populations. Hence, the results suggest that the process of polyploidization creates the potential for attraction of different suites of floral visitors. Multiple origins of polyploidy also presents the opportunity for new or different plant-insect interactions among independent polyploid lineages. These differences in turn may affect patterns of gene flow between diploids and polyploids and also among plants of independent polyploid origin. Polyploidy, therefore, may result in a geographic mosaic of interspecific interactions across a species' range, contributing to diversification in both plant and insect groups.     

Origins of polyploids: an example from peonies (Paeonia) and a model for angiosperms

The majority of tetraploid peonies are allopolyploids derived from crosses between phylogenetically distinct diploid lineages. Tetraploid Paeonia obovata was previously considered to be an autopolyploid because it is morphologically indistinguishable from the diploid of the same species. The presence of the Adh2 gene in tetraploid P. obovata but the inability to amplify the Adh2 gene from Chinese diploids of P. obovata, however, suggests that the tetraploid was not an autotetraploid derivative of the geographically adjacent diploid populations in China. The Adh gene phylogenies rather suggest that the tetraploid originated from crosses between two geographical races of diploid P. obovata distributed in China and Japan. The intermediate status of tetraploid P. obovata between auto‐ and allopolyploidy highlights the need for population genetic analyses of polyploid origins along the continuous range of genomic divergence. Here we present a model that describes the probabilities of polyploid formation and establishment as a function of genomic divergence between diploid progenitors. The probability of polyploid formation (P_f) is obtained from the multiplication of the probability of production of unreduced gametes (P_g) and the probability of ‘hybridization’ (P_h). P_f stays relatively stable when the genomic divergence is low, and then decreases progressively rapidly with the increase of genomic divergence between diploid progenitors. The probability of polyploid establishment (P_e), which depends on the rate of appearance of stable beneficial gene combinations and the rate of fertility restoration, is positively correlated with the genomic divergence of diploid parents. Multiplication of P_f and P_e gives an overall probability of polyploid origins (P_o) that varies continuously along the genomic divergence between diploid progenitors. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82 , 561–571.     

Isozymes as molecular markers for diploid and tetraploid individuals ofNajas marina (Najadaceae)

The genetic variability of five natural populations ofNajas marina L., i.e. one diploid of subsp.marina (Europe), two of subsp.intermedia (Europe) and both a diploid (C. Africa) and a tetraploid (Middle East) of subsp.armata, has been estimated by means of electrophoretic studies. These populations differ in their morphology and karyotype. Emphasis is placed on the characteristics and status of a tetraploid cytotype from Merkaz Sappir (Israel). Almost all the variation observed is expressed in seed alcohol dehydrogenase (ADH). The differences are in a unique allele of theAdh-2 locus and in the formation of novel heteromeric isozymes.Adh genes in seeds can be used as a marker for the autotetraploid character. The other enzyme systems tested failed in this respect. The genetic variability based on 23 loci is rather low. Nevertheless, the autotetraploid population has a higher or equal ratio of polymorphic loci than the related diploids. Cluster analysis illustrated not only thatNajas marina subsp.marina has diverged much from subsp.intermedia and subsp.armata, but also showed the difference between the latter two taxa, as well as the intermediate position of the autotetraploid population.     

ADAPTATION OF DIPLOID AND TETRAPLOID CHAMERION ANGUSTIFOLIUM TO ELEVATION BUT NOT LOCAL ENVIRONMENT

Polyploid organisms often have different geographic ranges than their diploid relatives. However, it is unclear whether this divergence is maintained by adaptation or results from historical differences in colonization. Here, we conducted a reciprocal transplant experiment with diploid and autotetraploid Chamerion angustifolium to test for adaptation at the ploidy and population level. In the Rocky Mountains, pure diploid populations occur at high elevations and pure autotetraploid populations occur at low elevations with mixed ploidy populations between. We planted 3134 seedlings in 2004 and 3890 juveniles (bolting) in 2005 among nine plots, three in each of the diploid, mixed ploidy, and tetraploid zones, and monitored survival until 2008. For both seedlings and juvenile plants, elevation significantly influenced survival. The juvenile plants also showed a significant ploidy by elevation interaction, indicating that diploids and tetraploids survived best at their native elevations. In contrast, we found no evidence of local adaptation to plot within elevation. This suggests that the current distribution of diploids and tetraploids across elevations is the result of adaptation and that genome duplication may have facilitated the invasion of lower elevation habitats by limiting the movement of maladapted alleles from diploid populations at higher elevations.     

AUTOPOLYPLOIDY IN HEUCHERA MICRANTHA (SAXIFRAGACEAE)

Heuchera micrantha (Saxifragaceae) is a morphologically variable species comprising five varieties: diversifolia, erubescens, hartwegii, micrantha, and pacifica. Both diploids (2n = 14) and tetraploids (2n = 28) occur within the species. The tetraploid cytotype occupies the central portion of the geographic range of the species, whereas diploids occur primarily in the southern and northern portions of the range. Both diploids and tetraploids have been detected within vars. diversifolia, pacifica, and hartwegii. All counts for vars. erubescens and micrantha are diploid and tetraploid, respectively. Several lines of evidence suggest that tetraploid H. micrantha is of autopolyploid origin. The species is distinct morphologically and is also well separated geographically from other closely related species in subsection Micranthae. The two cytotypes are karyologically identical, possess nearly the same suite of allozymes, and have a very high genetic identity (Ī = 0.971). Significantly, an earlier study documented tetrasomic inheritance in the tetraploid cytotype. Following theoretical expectations, the mean number of alleles per locus, proportion of loci that are polymorphic, and observed heterozygosity are significantly higher for the autotetraploid than for the diploid. The occurrence of both cytotypes in three of the varieties suggests that autopolyploidy may have occurred several times independently in H. micrantha. This was further substantiated by discriminant analysis using morphological characters, which provided evidence for a minimum of two separate origins for the autopolyploid cytotype.     

PHYLOGENETIC IMPLICATIONS FROM CUTICULAR WAX ANALYSES IN EPILOBIUM SECT. ZAUSCHNERIA (ONAGRACEAE)

Analysis of n-alkane constitutents of the leaf cuticle have been utilized to elucidate phylogenetic relationships within Epilobium section Zauschneria. Specimens representing 136 populations were greenhouse-grown under the same conditions. Crosses were made between representatives of all taxa within the section. The resulting F₁ generations were also grown under the same conditions. Cuticle samples were removed in chloroform, purified with column chromatography and analyzed using gas chromatography and mass spectrometry. Resultant data were computer analyed using discriminant function analysis. Based on percent composition of 12 n-alkanes, each population and taxon can be characterized by a distinctive wax profile. Results suggest that the tetraploids E. canum subsp. canum and E. canum subsp. latifolium have been derived from the anastomosis of two distinct diploid genomes, E. canum subsp. canum “angustifolium” and E. septentrionale. Biochemical evidence from extant populations of E. canum are in agreement with geographic positions and ranges of the diploid species involved. Greenhouse hybridizations between the respective diploids, E. canum subsp. canum “canum” and E. septentrionale, result in F₁ progeny possessing phenotypes similar to naturally occurring tetraploid E. canum populations. Wax compositions of the natural tetraploid populations are intermediate between diploids and vary depending on geographical proximity to diploid species. Past taxonomic treatments of the genus have placed the tetraploid Zauschneria arizonica within the species Z. californica (now E. canum). Analysis of wax compositions now suggets that Z. arizonica had an independent origin which involved, at least in part, the diploid species E. canum subsp. garrettii from Utah.     

Alcohol dehydrogenase activity in diploid and autotetraploid Phlox

The effect of polyploidy on the activity of alcohol dehydrogenase was examined in a series of diploid and synthetic autotetraploid Phlox drummondii. In most cases autotetraploids had about twice as much activity as the corresponding diploids, in two cases autotetraploids had about 1.5 times more activity, and in one wild seed pair the activity of the tetraploid was somewhat lower.This study was supported in part by NSF grants (DEB 76-19914, DAL; 4392, AMT, BMS 75-19621, ML).     

Quantitative cytogenetic analyses of autoploid and alloploid taxa in the Helianthus ciliaris group (Compositae)

Diploidy, tetraploidy, and hexaploidy occur in the Helianthus ciliaris complex. Quantitative cytogenetic analysis shows that both auto- and allotetraploids and auto-allohexaploids occur in H. ciliaris. There is evidence for pairing control mutations in and among populations of both cytotypes, and this should be expected with increasing age in any normal diploid or autopolyploid population. The autotetraploid populations contain the model genomes AAAA and the hexaploid AAAABB. The B genome may have been derived from diploid H. laciniatus whose range overlaps the tetraploid cytotype in Texas and New Mexico and may have provided the drought tolerance necessary for the hexaploid H. ciliaris cytotype to become a successful weed in more arid regions of its distribution.     

Genetic and embryological evidences of apomixis at the diploid level in <Emphasis Type="Italic">Paspalum rufum</Emphasis> support recurrent auto-polyploidization in the species

Gametophytic apomixis is an asexual mode of reproduction by seeds. This trait is present in several plant families and is strongly associated with polyploidy. Paspalum rufum is a forage grass with sexual self-incompatible diploids (2n = 2x = 20) and aposporous-apomictic pseudogamous tetraploids (2n = 4x = 40). In previous work embryological observations of the diploid genotype Q3754 showed 8.8–26.8% of the ovaries having one meiotic plus an aposporous-like embryo sac, suggesting some capability for apomictic reproduction. The objective of this work was to characterize progenies derived from Q3754 to determine if aposporous sacs were functional and generated progenies via apomixis at the diploid level. Re-examination of Q3754 ovaries showed that 12.5% of them contained one sexual plus an aposporous sac confirming previous results. Progeny tests were carried out on two experimental families (H₁ and S₁) employing heterozygous RAPD marker loci. Family H₁ was obtained crossing Q3754 with a natural diploid genotype (Q3861) and S₁ derived from the induced self-pollination of Q3754. Genetic analysis of H₁ showed that all individuals derived from sexual reproduction. However, 5 out of 95 plants from S₁ showed the same heterozygous state as the mother plant for 14 RAPD loci suggesting a clonal origin. Further experiments, designed to test the functionality of aposporous sacs by flow cytometric analyses, were carried out on a third family (M₁) obtained by crossing Q3754 with the tetraploid plant Q3785. Histograms of 20 M₁ plants showed 15 diploids (75%), 4 triploids (20%) and 1 tetraploid (5%). Triploids and the tetraploid may have originated from functional aposporous embryo sacs. Likewise, the reconstruction of the developmental route of 40 individual seeds demonstrated that 11 of them (27.5%) derived from fertilized aposporic sacs. The results presented in this work indicate that gametophytic apomixis is effectively expressed at the diploid level in Paspalum rufum and could be the foundation of a recurrent auto-polyploidization process in the species.     

Ploidy level and origin of the European invasive weed Senecio inaequidens (Asteraceae)

Native to South-Africa, species of the Senecio inaequidens complex are presently invasive in Europe, Australia and South-America. Previously, different ploidy levels have been found in these different areas, with only tetraploid individuals reported in Europe, and only diploids in South-Africa and Australia. In the present study chromosome counts and flow cytometry were used to survey DNA ploidy levels in a large sample of 66 native and 21 European invasive populations. One Mexican individual was also added to the study. We found only tetraploid individuals occurring in Europe, whereas both ploidy levels, diploid and tetraploid, were found in South-Africa. Moreover, based on genome size, we suggest that two largely allopatric varieties of diploids exist in South-Africa. The Mexican individual was diploid. We suggest that European tetraploid individuals come from South-Africa and hypothesize that a hybridization event between the two DNA types of diploids occurred in the Lesotho area. The taxonomic difficulties surrounding species of theS. inaequidens complex are briefly discussed.     

CYTOGENETICS OF AGROPYRON FERGANENSE AND ITS HYBRIDS WITH SIX SPECIES OF AGROPYRON,ELYMUS, AND SITANION

Meiosis and mode of reproduction are described in Agropyron ferganense Drob., a perennial forage grass from Central Asia. This species is diploid (2n = 14); it exhibits normal meiosis and reproduces by cross-pollination. Hybrids were produced between A. ferganense and six species with known genome formulas: 1) North American A. spicatum (Pursh) Scribn. & Smith, an SS diploid (2n = 14), 2) Middle Eastern A. libanoticum Hack., an SS diploid (2n = 14), 3) North American A. dasystachyum (Hook.) Scribn., an SSHH tetraploid (2n = 28), 4) Eurasian A. caninum (L.) Beauv., an SSHH tetraploid (2n = 28), 5) North American Sitation hystrix (Nutt.) J. G. Smith, an SSHH tetraploid (2n = 28), and 6) South American Elymus patagonicus Speg., an SSHHHH hexaploid (2n = 42). Almost complete chromosome pairing in the A. ferganense x A. spicatum and A. libanoticum hybrids demonstrated that A. fergenanse is an SS diploid, but it is genetically isolated from the other SS diploids because of high sterility in the F₁ hybrids. S-genome diploids form a network of species that extend from the Middle East through Central Asia to western North America. Frequent occurrence of seven univalents and seven bivalents at metaphase I in the triploid hybrids of A. ferganense x A. dasystachyum, A. caninum and S. hystrix was consistent with the proposed genome formulas of SS for A. ferganense, SSHH for the three tetraploid species, and SSH for the hybrids. Chromosome pairing was highly variable in the A. ferganense x E. patagonicus hybrids; however, some cells had almost complete bivalent pairing, an expected observation in an SSHH hybrid from a cross between an SS diploid (A. ferganense) and an SSHHHH hexaploid (E. patagonicus). Various options were considered concerning the appropriate generic classification of the S-genome diploids, which are now commonly placed in Agropyron. The inclusion of these species in the genus Eiytrigia, as advocated by some Soviet taxonomists, appears to be a reasonable decision.     

Isozymes of Eleusine (Gramineae) and the origin of finger millet

The predominantly African grass genus Eleusine comprises nine species, including diploids and tetraploids based on n = 8, 9, and 10. Among the polyploids are the important crop finger millet, Eleusine coracana subsp. coracana, and its putative wild ancestor, E. coracana subsp. africana. Eleusine coracana is believed to be an allotetraploid derived by hybridization between E. indica and an unknown diploid. To evaluate this hypothesis, 16 isozyme loci coding nine enzymes were compared among seven of the nine Eleusine species (E. intermedia and E. semisterilis were unavailable). Genetic variability differed substantially among diploid species, ranging from P = 0.563, A = 1.6, H = 0.208 in E. indica to P = 0.188, A = 1.2, H = 0.042 in E. jaegeri. The diploids tended to be genetically distinct, with values of Rogers' Similarity ranging from S = 0.294 (E. jaegeri/floccifolia) to S = 0.794 (E. indica/tristachya). Both subspecies of the tetraploid E. coracana exhibited fixed heterozygosity at several loci, verifying their hypothesized allotetraploid status. Both tetraploids also possessed E. indica marker alleles at all loci, corroborating ancestry by this taxon. Genotypes of the non-indica ancestor, inferred separately for each tetraploid, differed substantially from all candidate diploids and also from each other. These data indicate that 1) none of the candidate diploids investigated is likely to have been the non-indica ancestor of E. coracana, and 2) the non-indica ancestor of the wild tetraploid may differ from that of the crop. The latter conclusion is inconsistent with the complete chromosomal homology exhibited between the two tetraploid subspecies, indicating the need for additional evidence bearing on their relationships.