Microsatellite evidence for low genetic diversity and reproductive isolation in tetraploid Centaurea seridis (Asteraceae) coexisting with diploid Centaurea aspera and triploid hybrids in contact zones

Survival of polyploids in nature depends on several factors, including competition from diploid relatives and increased genetic diversity. Unlike other reported Centaurea polyploid complexes, diploid Centaurea aspera and tetraploid Centaurea seridis coexist in hybrid zones with frequent triploid individuals. The polyploid origin of C. seridis, the genetic diversity and population structure of the three cytotypes, and the degree of genetic differentiation among them were analyzed in seven mixed‐ploidy zones, involving different subspecies and ecological conditions. Ploidy was determined by flow cytometry. Microsatellite data suggested an allopolyploid origin of C. seridis. In the contact zones, diploids and tetraploids were genetically differentiated. When compared with the related C. aspera, a low genetic diversity was observed in C. seridis, which is uncommon in tetraploids. Furthermore, although diploid individuals were grouped in a single widespread genetic cluster, tetraploids were grouped in two highly differentiated clusters and showed significant isolation by distance. This genetic pattern in C. seridis may be related to a minimal gene flow with diploid relatives and/or other genetic factors, such as rare polyploidization events, founder effects or an increased selfing rate. Neither taxonomic assignment at subspecies level, nor ecological conditions could explain the genetic differentiation between tetraploid clusters. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 176 , 82–98.     

Gamete production patterns and mating systems in water frogs of the hybridogenetic Pelophylax esculentus complex in north‐eastern Ukraine

Hybridization and polyploidy play an important role in animal speciation. European water frogs of the Pelophylax esculentus complex demonstrate unusual genetic phenomena associated with hybridization, clonality and polyploidy which presumably indicate an initial stage of reticulate speciation. The Seversky Donets River drainage in north‐eastern Ukraine is inhabited by both sexes of the diploid and triploid hybrid P. esculentus and only one parental species Pelophylax ridibundus. Based on the presence of various types of hybrids, all populations studied can be divided into three geographical groups: I) P. ridibundus–P. esculentus without triploids; II) P. ridibundus–P. esculentus without diploid hybrids; and III) P. ridibundus–P. esculentus with a mixture of diploids and triploids. A study of gametogenesis revealed that diploid P. esculentus in populations of the first type usually produced haploid gametes of P. ridibundus and a mixture of haploid gametes that carried one or another parental genome (hybrid amphispermy). In populations of the second type, hybrids are derived from crosses of P. ridibundus males with triploid hybrid females producing haploid eggs with a genome of P. lessonae. Therefore, we suggest that clonal genome duplication in these eggs might be the result of suppression of second polar body formation or extra precleavage endoreduplication. In populations of the third type, some diploid females can produce diploid gametes. Fertilization of these eggs with haploid sperm can result in triploid hybrids. Other hybrids here produce haploid gametes with one or another parental genome or their mixture giving rise to new diploid hybrids.     

Evidence for non-disomic inheritance in a Citrus interspecific tetraploid somatic hybrid between C. reticulata and C. limon using SSR markers and cytogenetic analysis

Artificial tetraploid somatic hybrids have been developed for sterile triploid citrus breeding by sexual hybridization between diploid and tetraploid somatic hybrids. The genetic structure of diploid gametes produced by tetraploid genotypes depends on the mode of chromosome association at meiosis. In order to evaluate tetraploid inheritance in a tetraploid interspecific somatic hybrid between mandarin and lemon, we performed segregation studies using cytogenetic and single sequence repeat molecular markers. Cytogenetic analysis of meiosis in the somatic hybrid revealed 11% tetravalents and 76% bivalents. Inheritance of the tetraploid hybrid was analyzed by genotyping the triploid progeny derived from a cross between a diploid pummelo and the tetraploid somatic hybrid, in order to derive genotypes of the meiospores produced by the tetraploid. A likelihood-based approach was used to distinguish between disomic, tetrasomic, and intermediate inheritance models and to estimate the double reduction rate. In agreement with expectations based the cytogenetic data, marker segregation was largely compatible with tetrasomic and inheritance intermediate between disomic and tetrasomic, with some evidence for preferential pairing of homoeologous chromosomes. This has important implications for the design of breeding programs that involve tetraploid hybrids, and underscores the need to consider inheritance models that are intermediate between disomic and tetrasomic.     

Production of tetraploid plants of non apomictic citrus genotypes

Ploidy manipulation in Citrus is a major issue of current breeding programs aiming to develop triploid seedless mandarins to address consumer demands for seedless fruits. The most effective method to obtain triploid hybrids is to pollinate tetraploid non apomictic cultivars with pollen of diploid varieties. Such non apomictic tetraploid lines are not found in the citrus germplasm and need to be created. In this work we describe a new methodology based on in vitro shoot-tip grafting combined with treatment of the micro-grafted shoot-tip with colchicine and oryzalin to achieve chromosome doubling and a dechimerization procedure assisted by flow cytometry. Stable tetraploid plants of Clemenules, Fina and Marisol clementines and Moncada mandarin have been obtained directly from shoot tip grafting combined with colchicine and oryzalin treatments or after dechimerization of mixoploids plants (2x–4x). These stable tetraploid plants have been used in 4x × 2x hybridizations, to recover over 3,250 triploid hybrids in 3 years.     

Mode of hybridogenesis and habitat preferences influence population composition of water frogs (Pelophylax esculentus complex,Anura: Ranidae) in a region of sympatric occurrence (western Slovakia)

Coexistence of sperm‐dependent asexual hybrids with their sexual progenitors depends on genetic and ecological interactions between sexual and asexual forms. In this study, we investigate genotypic composition, modes of hybridogenetic gametogenesis and habitat preferences of European water frogs (Pelophylax esculentus complex) in a region of sympatric occurrence. Pelophylax esculentus complex comprises parental species P. ridibundus and P. lessonae, whose primary hybridization leads to hybridogenetic lineages of P. esculentus. Hybrids clonally transmit one parental genome and mate with the other parental species, forming a new generation of hybrids. In the region of western Slovakia, we found syntopic occurrence of diploid and triploid hybrids with P. lessonae, syntopic occurrence of all three taxa as well as the existence of pure P. ridibundus populations. All triploid hybrids were exclusively male possessing one ridibundus and two different lessonae genomes (RLL). Sex ratio in diploid hybrids was substantially female‐biased. Irrespective of the population composition, diploid hybrids excluded the lessonae genome from their germ line and produced ridibundus gametes. Contrarily, RLL males unequivocally eliminated the ridibundus genome and produced diploid lessonae sperms. Perpetuation of RLL males in studied populations is most likely achieved by their mating with diploid hybrid females. The composition of water frog populations is also shaped by taxon‐specific habitat preferences. While P. ridibundus preferred larger water bodies (gravelpits, fishery ponds, dead river arms), P. lessonae was most frequently found in marshes and smaller sandpits. Pelophylax esculentus occupied predominately similar habitats as its sexual host P. lessonae.     

Speciation through androgenesis in the stick insect genus Clonopsis (Insecta Phasmatodea)

In Morocco, Clonopsis stick insects showed tangled reproductive interactions actually resulting into a network of phylogenetic relationships known as ‘reticulate evolution’. Peculiar to parthenogenetic C. gallica and C. soumiae (54 and 72 chromosomes, respectively) – closely related to the bisexual C. felicitatis (2n = 36) – is the finding of numerically polyploid karyotypes with a diploid structure. Androgenesis appeared to be the most parsimonious explanation accounting for both the low mitochondrial differentiation and the quick onset of those polyploids with structurally diploid karyotypes, paired with neat nuclear differentiations. According to a proposed model, hybrid triploid females would segregate balanced haploid and diploid 2nd oocytes immediately producing all kinds of parthenogens and androgens. Owing to these peculiar reproductive issues, we felt useful searching for stronger evidence by deeply analysing the mitochondrial genome. This new analysis showed a neat separation of sexual Tetouan haplotypes from the parthenogenetic and androgenetic ones, which are grouped in two slightly overlapping groups by network analysis: Moroccan parthenogens and androgens vs European C. gallica. It could be also envisaged that C. gallica has multiple origins, being a complex of parthenogenetic strains originated through independent hybridizations. The straightforward mechanism originating both triploid and tetraploid parthenogens well fits with both their widely ascertained low mitochondrial differentiation and the geographical closeness of the most similar samples, independently from their specific karyotype. Combining the outcomes of the hybridization events and androgenesis, which completely substitutes hybrid genomes with those of a related paternal species, would conceivably realize the observed picture of species structure and distribution. Owing to the reinforced data set, it now appears much more sensible to support androgenesis as a quick pathway to originate polyploids with numerically and genetically sharply differing chromosome sets, while maintaining, at the same time, high mitochondrial similarity.     

Meiotic analyses ofCucumis hybrids and an evolutionary evaluation of the genusCucumis (Cucurbitaceae)

Meiosis in seven interspecificCucumis hybrids has been analysed i.a. inC. metuliferus ×C. zeyheri, where the parents belong to different sections. In the triploid hybrids a remarkably high number of trivalents has been found. Additional data from literature on geographical distribution, cucurbitacins, flavonoid patterns, isozymes, C-banding, genome size, DNA amount and chloroplast DNA are used to discuss species relationships and evolution. The African cross-compatible group is divided into theMyriocarpus subgroup with the diploid speciesC. africanus, C. myriocarpus subsp.leptodermis and subsp.myriocarpus, and theAnguria subgroup withC. anguria, C. dipsaceus, C. ficifolius, C. prophetarum, C. zeyheri and all polyploids (exceptC. heptadactylus). It is argued that the Asian subg.Melo with x = 7 is derived from the African subg.Cucumis with x = 12; the latter contains all the polyploid species and has the most common basic chromosome number of theCucurbitaceae. This phylogenetic advance is interpreted with concepts of the quantum model of evolution.     

Contrasting reproductive strategies of triploid hybrid males in vertebrate mating systems

The scarcity of parthenogenetic vertebrates is often attributed to their ‘inferior’ mode of clonal reproduction, which restricts them to self‐reproduce their own genotype lineage and leaves little evolutionary potential with regard to speciation and evolution of sexual reproduction. Here, we show that for some taxa, such uniformity does not hold. Using hybridogenetic water frogs (Pelophylax esculentus) as a model system, we demonstrate that triploid hybrid males from two geographic regions exhibit very different reproductive modes. With an integrative data set combining field studies, crossing experiments, flow cytometry and microsatellite analyses, we found that triploid hybrids from Central Europe are rare, occur in male sex only and form diploid gametes of a single clonal lineage. In contrast, triploid hybrids from north‐western Europe are widespread, occur in both sexes and produce recombined haploid gametes. These differences translate into contrasting reproductive roles between regions. In Central Europe, triploid hybrid males sexually parasitize diploid hybrids and just perpetuate their own genotype – which is the usual pattern in parthenogens. In north‐western Europe, on the other hand, the triploid males are gamete donors for diploid hybrids, thereby stabilizing the mixed 2n‐3n hybrid populations. By demonstrating these contrasting roles in male reproduction, we draw attention to a new significant evolutionary potential for animals with nonsexual reproduction, namely reproductive plasticity.     

CHROMOSOME PAIRING IN NATURAL FIRST AND ADVANCED GENERATION LIATRIS HYBRIDS

Chromosome pairing was analyzed in natural F₁ and advanced generation hybrids of crosses between Liatris aspera and L. spicata, and L. aspera and L. cylindracea. The status of the hybrids was determined on the basis of morphological and chromatographic criteria. The F₁ hybrids of L. aspera × L. spicata contained pairing irregularities in 30% of their PMC's as compared to 10% in the hybrid segregates. Notably, translocation figures appeared in 7% of the F₁ PMC's as compared to 0.6% in the segregate PMC's. The F₁ hybrids of L. aspera × L. cylindracea contained irregularities in 19% of their PMC's as compared to 7% in the segregates. Translocation figures occurred in 6% of the F₁ PMC's, but only in 0.7% of the segregate PMC's. The incidence of pairing and level of structural heterozygosity was far lower in the hybrid segregates than was anticipated on the basis of F₁ pairing relationships. Thus it would appear that there is selection for structural homozygosity in the hybrid segregates.     

Analysis of meiosis in triticale (XTriticosecale Wittmack) X rye (Secale cereale L.) F1 hybrids at three ploidy levels

Summary Triticales (XTriticosecale Wittmack) at three ploidy levels (8x, 6x, 4x, x=7) were crossed with diploid rye (Secale cereale L.) to produce a solitary hypopentaploid hybrid (2n=32), and a number of tetraploid (2n=4x=28) and triploid (2n=3x=21) hybrids. The hybrids exhibited a morphology which was intermediate between the parents. The number of bivalents ranged from 1–7 (4.65 per cell) in hypopentaploid, from 2–12 (7.13 per cell) in tetraploid and from 4–9 (6.84 per cell) in triploid hybrids. In 4x and 3x hybrids, trivalents and quadrivalents were also observed at low frequencies (range 0–1; mean 0.01–0.03 per cell). Chiasmata frequency was highest in triploid hybrids (12.44 per cell), lowest in hypopentaploid (5.37 per cell) and intermediate in tetraploids (10.54 per cell). More than 7¹¹ were found in 39.7% pollen mother cells (PMC's) in the 4x hybrids and in 5.0% PMCs in 3x hybrids. It is concluded that an increase in the relative proportion of wheat chromosomes in the hybrids had a slight suppression effect on homologous as well as homoeologous pairing of rye chromosomes. Contrary to this, the relative increase in rye complement promoted homoeologous pairing between wheat chromosomes. In triploid hybrids, the chiasmata frequency as well as the c value were the highest, suggesting that in tetraploid hybrids rye chromosomes had a reduced pairing (low frequency of ring bivalents).     

Hybridization between diploid Centaurea pseudophrygia and tetraploid C. jacea (Asteraceae): the role of mixed pollination,unreduced gametes,and mentor effects

We studied hybridization between the diploid Centaurea pseudophrygia and the tetraploid C. jacea by performing crossing experiments and screening natural populations using flow cytometry. The experiments confirm that the studied species exhibit strong reproductive isolation. Interspecific hybrids were formed at a low frequency, including triploids (originating from reduced gametes) and tetraploids (involving unreduced gametes of the diploids). In contrast, hybrids were almost absent among seeds and adult plants of natural mixed populations and among the offspring from experimental pollinations with a mixture of pollen of both ploidy levels. We found that mixed pollination is an important mechanism for preventing hybridization between plants of different ploidy levels and sustaining the reproduction of the tetraploids. A mentor effect (induced selfing in the presence of pollen of different ploidy levels) was observed in both diploids and tetraploids, reinforcing the reproductive isolation between cytotypes. Higher ploidy levels (pentaploid, hexaploid) involving unreduced gametes of the tetraploid species were identified. Notably, pentaploids were discovered for the first time in Centaurea sect. Jacea. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 93–106.     

Hybridization of crucian carp <Emphasis Type="Italic">Carassius carassius</Emphasis> (Linnaeus, 1758) in Ukrainian reservoirs and the genetic structure of hybrids

The hybridization of crucian carp Carassius carassius inhabiting polyspecific carp populations of Ukraine’s water bodies and the genetic structure of its hybrids were studied using biochemical gene marking and cytometry procedures. The widespread hybridization between C. auratus and C. carassius was justified by a large number of hybrids, which could form populations only represented by hybrid specimens. The hybrids of C. auratus × C. carassius were diploid, triploid, and, in exceptional cases, tetraploid. Females and males were presented in equal numbers and were most likely breeding by hybridogenesis. Alongside with that, some clonal hybrids of C. carassius × C. gibelio-1 were represented by tetraploid females, and one triploid female of C. carassius × Tinca tinca was found. Arguments are presented in favour of hybridization of alien C. auratus and endemic C. carassius and that this hybridization may appear as one of the mechanisms of replacement and suppression of the C. carassius species.     

HYBRIDIZATION AND CYTOTAXONOMY OF DICENTRA

Biosystematic relationships among species assigned to three subgenera of the genus Dicentra were investigated with respect to hybridization and chromosomal constitution and fertility of the hybrids. Four species of subgenus Dicentra, D. formosa, D. eximia, D. nevadensis, and D. peregrina, were intercrossed in various ways to form diploid, triploid, and tetraploid hybrids. Hybrids at the tetraploid level in this subgenus invariably were highly fertile. Triploid hybrids, as expected, were mostly very sterile. Diploid hybrids varied in this respect, but none was highly fertile. Crosses with two of the remaining four species of subgenus Dicentra produced no hybrids, but abundant seed was obtained in one instance. The two species of the subgenus Chrysocapnos, D. chrysaniha and D. ochroleuca, cross to produce a partially fertile tetraploid hybrid, but cross-pollinations involving these species with those of other subgenera failed. The single species of subgenus Hedycapnos, D. spectablis (diploid) produced no hybrids when cross-pollinated with members of the other two sections. These results are fully concordant with presumed affinities based on morphological similarity In addition, preliminary results of hybridization between the monocotyledonous D. peregrina and a number of dicotyledonous species of Dicentra are reported.     

THE EXPERIMENTAL FORMATION OF POLYPLOIDY AND ITS EFFECT IN THE GENUS MUSA

Polyploidy was induced in two Musa species and their hybrid by means of immersing newly germinated diploid seedlings in 0.5% aq. colchicine solution. Tetraploid M. balbisiana plants were taller and more robust but had slower growth rate, droopier leaves, fewer suckers, and scantier root systems than the diploids. Tetraploidy affected fruit size and shape in M. balbisiana and in M. acuminata. Tetraploidy did not affect bunch size (the number of fruit-bearing nodes) in M. acuminata banksii but caused great reduction in bunch size of M. acuminata microcarpa ‘Zebrina.’ Doubling of the chromosome number resulted in doubling of the anthocyanin concentration in the leaves of the pigmented banana plants. Colchicine-induced female sterility was detected in a treated diploid plant of M. acuminata subsp. microcarpa. A number of either female or male, or both female and male sterile plants were encountered in colchicine-induced tetraploid M. acuminata species. Only one secondary M. balbisiana tetraploid plant was found to be partially female sterile. Colchicine treatment of the seedlings resulted in chromosomal irregularities at mitosis. Chromosomes appeared as open chains, rings and multiple configurations at metaphase. In the majority of cases doubled chromsomal complements reverted to their original number. However, there were cases in which diploids were induced to tetraploidy and later reverted to triploidy instead of diploidy. Also, there were cases in which triploids were doubled to hexaploidy and later reverted to tetraploidy instead of triploidy. From the close morphological similarity between a number of triploid and tetraploid M. balbisiana plants and some edible triploid cultigens belonging to the “ABB Group,” it is suggested that the karyotype of some of these cultigens could be “BBB” rather than “ABB” which contains one “A” M. acuminata genome. New pathways and additional evolutionary possibilities are incorporated into Simmonds’ scheme on the evolution of edible bananas.     

Cytogenetic evidence of mixed disomic and polysomic inheritance in an allotetraploid (AABB) Musa genotype

Background and Aims

Edible bananas originated mainly from two wild species, Musa acuminata Colla (AA) and Musa balbisiana Colla (BB), and triploid cultivars with an AAA, AAB or ABB genome are the most widely used. In the present study, chromosome pairing affinities are investigated in a sterile AB Indian variety and in its fertile colchicine-induced allotetraploid (AABB) derivative to determine the inheritance pattern of the tetraploid genotype. The potential implications of interspecific recombination and chromosomal composition of diploid gametes for Musa improvement are presented.

Methods

The pairing of different chromosome sets at diploid and tetraploid levels was investigated through a combination of conventional cytogenetic and genomic in-situ hybridization (GISH) analyses of meiotic chromosomes, leading to a likelihood model of the pairing behaviour. GISH analysis of mitotic chromosomes was also conducted to reveal the chromosome constitution of hybrids derived from crosses involving the allotetraploid genotype.

Key Results

Analysis of chromosome associations at both ploidy levels suggested that the newly formed allotetraploid behaves as a ‘segmental allotetraploid’ with three chromosome sets in a tetrasomic pattern, three sets in a likely disomic pattern and the five remaining sets in an intermediate pattern. Balanced and unbalanced diploid gametes were detected in progenies, with the chromosome constitution appearing to be more homogenous in pollen than in ovules.

Conclusions

Colchicine-induced allotetraploids in Musa provide access to the genetic background of natural AB varieties. The segmental inheritance pattern exhibited by the AABB allotetraploid genotype implies chromosome exchanges between M. acuminata and M. balbisiana species and opens new horizons for reciprocal transfer of valuable alleles.     

Genetic variation in Scandinavian Anthericum liliago (Anthericaceae): allopolyploidy, hybridization and immigration history

 We investigated patterns of isozyme variation and the hierarchic structure of genetic diversity in 25 Scandinavian populations of the lilioid herb, Anthericum liliago. Isozyme data suggest that tetraploid A. liliago has an allopolyploid origin and that A. ramosum may be one of its diploid progenitors. Two populations contained known or suspected hybrids between A. liliago and A. ramosum. Isozymes show that one population from S Sweden contains both triploid (hybrid) and tetraploid individuals whereas a putatively hybrid Danish population represents diploid A. liliago. There is an overall northward and eastward decline in allelic richness in the tetraploid populations, with the highest numbers of alleles in Denmark and SW Sweden. This pattern is consistent with a progressive loss of allelic variation during the species' postglacial colonization of Scandinavia. The between-population component of genetic diversity is 4% (compared with 12% in diploid A. ramosum), the between-region diversity component is 7% and 89% of the total diversity is stored within populations. Received March 13, 2002; accepted September 24, 2002 Published online: December 11, 2002     

Chromosome doubling in vine cacti hybrids

We performed reciprocal crosses between the tetraploid Selenicereus megalanthus and the diploid Hylocereus species, H. undatus and H. polyrhizus. S. megalanthus x H. undatus gave rise to viable hexaploids and 6x-aneuploid hybrids rather than to the expected triploids. No genuine hybrids were obtained in the reciprocal cross. The pollen diameter of the tetraploid S. megalanthus varied widely, indicating the occurrence of unreduced gametes, while that of H. undatus pollen was very uniform, indicating an extremely low frequency of unreduced gametes. This finding suggests that the hexaploids were formed by chromosome doubling after the formation of the hybrid triploid zygote rather than by fusion of unreduced gametes of the two species.     

Cytogenetics of the hybrid Gilia millefoliata × achilleaefolia

Summary Gilia millefoliata andG. achilleaefolia, two annual diploid (n=9) species ofPolemoniaceae, crossed readily in certain combinations but not in others. The F₁ hybrids were vigorous but sterile. They gave rise, apparently by the union of unreduced gametes, to an F₂ generation of tetraploids, which were mostly fertile.Chromosome pairing in the hybrids varied markedly according to the state of nutrition of the plants. The F₁ hybrids formed fewer clear diakinesis figures, fewer bivalents, fewer chiasmata per bivalent, and more attenuated or stretched bivalents when grown in 2 pots of sand than when grown in rich soil (Table 3). A pot-bound allotetraploid individual derived from this hybrid showed the same meiotic irregularities as the starved F₁s until irrigated with a solution of mineral nutrients, after which its chromosomes paired regularly in bivalents (Table 2, Fig. 38).The capacity of the F₁ hybrids to produce polyploids also differed strikingly in the two cultures. The rate of polyploidy of the stunted sand-grown hybrids was 2381 viable tetraploid zygotes per million flowers, while the corresponding figure for the luxuriant field hybrids was only 2.7 per million flowers.For the production of polyploid progeny by diploid parents — a process which should be clearly distinguished from normal fertility — the termpolyploidy rate is proposed. It is suggested that starvation of a structural hybrid may sometimes increase its polyploidy rate by reducing chromosome pairing to the point where restitution nuclei and hence unreduced gametes can be formed.     

Single cross alfalfa (Medicago sativa L.) hybrids produced via 2n gametes and somatic chromosome doubling: experimental and theoretical comparisons

Summary The potential breeding value of 2n gametes from diploid alfalfa (2n = 2x = 16) was tested by comparing single cross alfalfa hybrids produced via 2n = 2x gametes from diploids versus n = 2x gametes from somatic-chromosome-doubled, tetraploid counterparts. Three diploid clones, designated 2x-(rprp), homozygous for the gene rp (conditions 2n gamete formation by a first division restitution mechanism) were colchicine-doubled to produce their tetraploid counterparts, designated 4x-(SCD). These six clones were crossed as males to the same cytoplasmic male sterile clone. Yield comparisons of progeny from the six clones demonstrated a significant yield increase of the hybrid progeny from 2n = 2x gametes from the diploids over the hybrid progeny from n = 2x gametes from the chromosome doubled tetraploid counterparts. The yield gain ranged from a 12% increase to a 32% increase. Theoretical comparisons indicated the 2n = 2x gametes from diploids would have 12.5 to 50% more heterozygous loci, on average, than the n = 2x gametes derived from somatic doubling. These results confirm the importance of heterozygosity on alfalfa yield, and the results demonstrate that 2n gametes formed by first division restitution offer a unique method for producing highly heterotic alfalfa hybrids.     

Experimental evidence on the affinities ofPapaver somniferum (Papaveraceae)

Results obtained from crossing experiments betweenP. somniferum subsp.somniferum (2n = 22) and subsp.setigerum (2n = 44),P. glaucum (2n = 14) andP. gracile (2n = 14) and from the observation of meiotic chromosome pairing in the various hybrids obtained do not provide straightforward evidence for the hypothesis thatP. somniferum originated as a triploid hybrid between taxa similar toP. glaucum andP. gracile (Kadereit 1986a, b).—On the one hand, the pattern of crossability found reflects the closer similarity of subsp.somniferum toP. glaucum and of subsp.setigerum toP. gracile, which was interpreted as segregation of parental characters, and the high frequency of 2n = 28 chromosomes among F₂-progeny from the hybrid subsp.somniferum × subsp.setigerum (2n = 33) might reveal n = 7 as the base number also ofP. somniferum. On the other hand, however, the general difficulty of obtaining hybrids, and the low incidence of bivalent formation in their meiosis, probably indicating a lack of chromosome homology between the different species, do not fit the above hypothesis.—These results are in marked contrast to the morphological similarity between the three species involved.