Occurrence of unreduced pollen in diploid blueberry species,Vaccinium sect. Cyanococcus

Summary A total of 1475 individuals belonging to 43 natural populations of seven diploid (2x) blueberry species (Vaccinium section Cyanococcus) and two natural interspecific 2x hybrid populations were evaluated for unreduced pollen production. Significant differences were found in the frequency of unreduced pollen producers between species and within and between populations of the same species. Individuals with 1% or more unreduced pollen were considered unreduced pollen producers. The average frequency of unreduced pollen producers in these diploid species was 13.5%, ranging from 7.4% (V. corymbosum) to 18.4% (V. darrowii). The frequency of unreduced pollen grains in individual clones varied from <-1% to 28.6%. The production of unreduced pollen was not associated with male fertility. The widespread occurrence of unreduced pollen in the diploid species should allow the introgression of this germ plasm to the tetraploid level via unilateral sexual polyploidization.New Jersey Agricultural Experiment Station, Publication No. D-12163-18-91, supported by State and U.S. Federal funds, USDA-ARS Specific Cooperative Agreement No. 58-3615-9-068     

Unreduced gametes and neopolyploids in natural populations of Achillea borealis (Asteraceae)

Polyploidy is a major mechanism of speciation and adaptation, yet little is known about the origins of polyploids in natural species. I investigated gametic nonreduction and neopolyploid formation in natural tetraploid populations of Achillea borealis (Asteraceae), an autopolyploid complex consisting of tetraploid and hexaploid cytotypes. Cytological analyses of tetraploid populations revealed the occurrence of reduced (n=2x) as well as unreduced 'big' (2n=4x) and 'jumbo' (4n=8x) pollen grains, which were clearly distinguished by size. Production of unreduced pollen was monitored in two tetraploid populations in 1997 and 1998. Mean population-level frequencies of unreduced pollen ranged from 0.030 to 0.538%, with as few as one-third and as many as one-half of sampled plants producing unreduced grains. Eight individuals were found to produce >1% unreduced pollen, with highest observed frequencies of 7.0, 13.2 and 15.8%. Experimental crosses using high unreduced pollen producers as male parents generated viable seeds. However, the frequency of neohexaploids in the progeny of experimental crosses (0.388%) was similar to that observed in progeny of randomly selected, open-pollinated control parents (0.465%). These results suggest that unreduced eggs are the most likely source of new polyploids. In spite of the inefficiency of unreduced pollen in unilateral sexual polyploidization, the overall rate of neohexaploid formation (one in 233) was several orders of magnitude greater than estimates of genic mutation rates.     

Variation patterns of parthenogenetic plants derived from “unreduced” embryo-sacs of Solanum tuberosum subspecies andigena (Juz. et Buk.) Hawkes

Summary Parthenogenetic seed induction was performed on one clone of Solanum tuberosum subspecies andigena (2n=4x=48) using S. phureja (2n=2x=24) marker inducer clones. The parthenogenetic population when grown was found to contain both diploid and tetraploid individuals presumably arising from reduced and unreduced gametes, respectively. Variation patterns in the diploid and tetraploid sub-populations, as well as a population obtained by selfing the parental clone, were compared to try and elucidate the origin of the tetraploid parthenotes. From the results of this one generation it appeared that the tetraploid parthenogenetic plants had been produced by a mechanism equivalent to second division restitution (SDR).     

Genetic relationships between diploid and allotetraploid cherry species (Prunus avium, Prunus x gondouinii and Prunus cerasus)

Prunus avium L. (diploid, AA, 2n=2x=16), Prunus cerasus L. (allotetraploid, AAFF, 2n=4x=32) species, and their hybrid Prunus x gondouinii Rehd., constitute the most widely cultivated cherry tree species. P. cerasus is supposed to be an hybrid species produced by the union of unreduced P. avium gametes and normal P. fruticosa gametes. A continuum of morphological traits between these three species makes their assignation difficult. The aim of this paper is to study the genetic relationships between tetraploid and diploid cherry species. In all, 114 genotypes belonging to these species were analyzed using 75 AFLP markers. The coordinates of these genotypes on the first axis of a correspondence analysis allowed us to clearly distinguish each species, to identify misclassifications and to assign unknown genotypes to one species. We showed that there are specific alleles in P. cerasus, which are not present in the A genome of P. avium and which probably come from the F genome of P. cerasus. The frequencies of each marker in the A and the F genomes were estimated in order to identify A and F specific markers. We discuss the utility of these specific markers for finding the origin of the A and F genomes in the allopolyploid species.     

Karyotype analysis and polyploidy in Palaua and a comparison with its sister group Fuertesimalva(Malvaceae)

Palaua(Malveae,Malvaceae)comprises 15 species endemic to the hyperarid coastal desert of Chile and Peru.So far,chromosome counts have been known for two diploid species(2n=2x=10)only.Here we report new chromosome numbers for 12 species of Palaua and four of its sister group Fuertesimalva.Karyotypes including 4,6-diamidino-2-phenylindole dihydrochloride(DAPI)/chromomycin(CMA3)fluorescent banding are presented for selected species representative of each of the main clades of Palaua.An important finding is the discovery of polyploids in one exclusively tetraploid species(P.trisepala)and four species with mixed diploid and tetraploid cytotypes(P.dissecta,P.mollendoensis,P.moschata,and P.tomentosa).The diploid and tetraploid karyotypes are all unimodal,symmetrical and show one or two pairs of satellite chromosomes with their associated CMA+/DAPI- band depending on the cytotype.For some of the tetraploids an autopolyploid origin is suggested.     

2n+n Hybridization of Apomictic Paspalum dilatatum with Diploid Paspalum Species

Common dallisgrass (Paspalum dilatatum) is an apomictic pentaploid (2n=5x=50) of hybrid origin with irregular meiosis and with the genome formula IIJJX. The I and J genomes are homologous to those of diploid P. intermedium and P. jurgensii, respectively, but the source of the X genome is unknown. Members of the X genome may have genes of special biological significance, including those controlling apomixis. Common dallisgrass was crossed with several diploid Paspalum species in an attempt to identify the source of the X genome. Since common dallisgrass is apomictic, all hybrids produced will be formed by fertilization of an unreduced egg (2n+n). Any hybrid showing 30 chromosome bivalents at meiosis would indicate that the male diploid parent has a chromosome set that is homologous to the X genome of dallisgrass. Over 36,000 spikelets of dallisgrass were emasculated and dusted with pollen of 15 different diploid species (diploid species bearing I or J genomes were excluded). Only five (P. chaseanum, P. equitans, P. fasciculatum, P. notatum, and P. simplex) produced 2n+n hybrids with P. dilatatum. Meiotic chromosome behavior was similar in all hexaploid hybrids showing ca. 20 bivalents and 20 univalents. Results indicated a very low rate of 2n+n hybridization; none of the five diploid species possessed the X genome. Because several diploid species failed to hybridize with 5x dallisgrass, other methods should be attempted. Molecular markers specific for the X genome may help solve the question.     

Karyotype analysis and polyploidy in Palaua and a comparison with its sister group Fuertesimalva (Malvaceae)

Palaua (Malveae, Malvaceae) comprises 15 species endemic to the hyperarid coastal desert of Chile and Peru. So far, chromosome counts have been known for two diploid species (2n = 2x = 10) only. Here we report new chromosome numbers for 12 species of Palaua and four of its sister group Fuertesimalva. Karyotypes including 4',6'-diamidino-2-phenylindole dihydrochloride (DAPI)/chromomycin (CMA3) fluorescent banding are presented for selected species representative of each of the main clades of Palaua. An important finding is the discovery of polyploids in one exclusively tetraploid species (P. Trisepald) and four species with mixed diploid and tetraploid cytorypes (P. Dissecta, P. Mollendoensis, P. Moschata, and P. Tomentosa). The diploid and tetraploid karyotypes are all unimodal, symmetrical and show one or two pairs of satellite chromosomes with their associated CMA+/DAPI-band depending on the cytotype. For some of the tetraploids an autopolyploid origin is suggested.     

The Biology of Allium monanthum (Liliaceae) I. Polyploid Complex and Variations in Karyotype

Abstract Plants of Allium monanthum Maxim., whose gender expression are usually dioecious, but rarely hermaphrodite or gynomonoecious, proved to constitute a polyploid complex, consisting of diploid, triploid, and tetraploid individuals. The basic chromosome complement of this species consists of seven metacentric or submetacentric chromosomes and one acrocentric, the latter possessing a satellite on the short arm. Thus, the karyotype formula is expressed as 7V+11 (x=8). The diploid plants (2n = 16) were confined to central Honshu, Japan. Typical female plants possessed the standard karyotype, whereas male plants were heterozygous for two kinds of translocations. The 3x plants (2n=24) are somewhat widely distributed in the areas from the Kanto to Hokuriku district in Honshu. All female triploid plants possessed the standard karyotype. The geographical distribution of 4x plants (2n=32) which express mostly a female phenotype occurred nearly throughout the whole areas investigated; they are geographically isolated from the 2x plants. A majority of 4x plants had the standard karyotype. The remaining tetraploids were of the aberrant type, 4x/51, which has five acrocentric chromosomes, and two aneuploids 4x+1 and 4x-1. Both 3x and 4x forms seem to be of autopolyploid origin. Three kinds of aberrant nucleolar chromosomes with an extra satellite or an inseried secondary constriction were found in the heterozygotes for translocations of 2x plants and also in some plants of the 3x form. These aberrant plants usually form their own homogeneous populations, but were somewhat scattered throughout the range in their distribution. Thus, these individuals are considered to have perpetuated these types of chromosome aberrations which originated in the remote past.     

The genome composition of hexaploid Psammopyrum athericum and octoploid Psammopyrum pungens (Poaceae: Triticeae).

The genomic constitution of two species in the genus Psammopyrum, i.e., Ps. athericum (2n = 6x = 42) and Ps. pungens (2n = 8x = 56), was studied by genomic in situ hybridization (GISH). In Ps. athericum, one diploid chromosome set hybridized to a genomic probe from Pseudoroegneria ferganensis (St genome), one diploid set to a probe from Agropyron cristatum (P genome), and one diploid set to a probe from Thinopyrum junceiforme (EbEe genomes) or Th. bessarabicum (Eb genome). Substituting the St-genome probe with an L-genome probe from Festucopsis serpentinii resulted in exactly the same hybridization pattern, suggesting a genomic constitution of EStP or ELP for Ps. athericum. The same probes used on Ps. pungens showed two diploid sets of chromosomes hybridizing to the St-genome probe, one diploid set hybridizing to the P-genome probe, and one diploid set hybridizing to the EbEe-genome probe. The L-genome probe hybridized to approximately 14 of the chromosomes that were labeled by the St-genome probe. Hence the genomic constitution for Ps. pungens is proposed to be EStStP or EStLP.     

High transmission of paternal plastid DNA in alfalfa plants demonstrated by restriction fragment polymorphic analysis

Summary A high frequency of paternal plastid transmission occurred in progeny from crosses among normal green alfalfa plants. Plastid transmission was analyzed by hybridization of radiolabeled alfalfa plastid DNA (cpDNA) probes to Southern blots of restriction digests of the progeny DNA. Each probe revealed a specific polymorphism differentiating the parental plastid genomes. Of 212 progeny, 34 were heteroplastidic, with their cpDNAs ranging from predominantly paternal to predominantly maternal. Regrowth of shoots from heteroplasmic plants following removal of top growth revealed the persistence of mixed plastids in a given plant. However, different shoots within a green heteroplasmic plant exhibited paternal, maternal, or mixed cpDNAs. Evidence of maternal nuclear genomic influence on the frequency of paternal plastid transmission was observed in some reciprocal crosses. A few tetraploid F₁ progeny were obtained from tetraploid (2n=4x=32) Medicago sativa ssp. sativa x diploid (2n=2x=16) M. sativa ssp. falcata crosses, and resulted from unreduced gametes. Here more than the maternal genome alone apparently functioned in controlling plastid transmission. Considering all crosses, only 5 of 212 progeny cpDNAs lacked evidence of a definitive paternal plastid fragment.Contribution No. 89-524-J from the Kansas Agricultural Experiment Station, Kansas State University, Manhattan     

浙江大罗山七倍体薤白的发现及其成因分析

该试验对浙江大罗山一个薤白种群的13个个体进行了染色体计数和核型分析,并对探讨七倍体薤白的可能形成机制进行了讨论。结果表明:(1)大罗山薤白种群为混倍种群,其中3个个体为七倍体,染色体组型是2n=7x=46m(2SAT)+10sm(2SAT),核型为2B型;10个个体为四倍体,染色体组型是2n=4x=26m(1SAT)+6sm(1SAT),核型为2B型。薤白种群的混倍性和七倍体均为首次报道。(2)对七倍体薤白的成因分析认为,七倍体是通过三倍体和四倍体未减数配子结合产生;随体染色体数目并不与植株的倍性相对应,而且并不都是出现于同源染色体上;薤白种内倍性增大与其物种进化的趋势一致,即倍性越大,种群越进化。     

AFLP-based estimation of 2n gametophytic heterozygosity in two parthenogenetically derived dihaploids of Rosa hybrida L

Two dihaploid Rosa hybrida L. genotypes, derived through parthenogenesis by using irradiated pollen, were crossed with clonally propagated plants of the diploid species Rosa rugosa Thunb. and Rosa wichuraiana Crép., respectively. Three progeny groups were obtained which contained numerous polyploids, as determined by flow cytometry. Production of fertile 2n female gametes is apparently very common in one of these R. hybrida dihaploid derivatives, whereas the other one is able to produce fertile 2n pollen. Hence, an amplified fragment length polymorphism (AFLP) study was performed on the parental plants and the resulting hybrid offspring in order to estimate (1) the respective genomic parental contributions, and (2) the level of heterozygosity transmitted by the 2n unreduced gametes. Comparison of the levels of transmitted parental heterozygosity revealed that two types of 2n gametes were produced simultaneously, presumably resulting from restitution at the first and at the second meiotic division, respectively. Received: 15 February 2001 / Accepted: 22 May 2001     

Triploidy in Equisetum subgenus Hippochaete (Equisetaceae, Pteridophyta)

BACKGROUND AND AIMS: The genus Equisetum is cytologically uniform, having a base chromosome number of x = 108. All previously known species and hybrids that have been counted represent diploids with a sporophytic chromosome number of 2n = 216. Biosystematic studies on Equisetum subgenus Hippochaete revealed evidence that triploids occur in nature. The objective of this study was to confirm that triploid plants exist in the natural environment. METHODS: Flow cytometry was used to establish nuclear DNA values and cytological investigations of meiosis were carried out to obtain information on chromosome number and pairing behaviour. KEY RESULTS: Triploidy exists in three morphologically different hybrid taxa. Two of these are morphologically intermediate between a primary diploid hybrid and a parent, while the third apparently combines genomes from all three Central European Hippochaete species. Nuclear 1C DNA values for the four European Hippochaete species range from 21.4-31.6 pg. For the hybrids, the 1C DNA values not only occupy the same range as the species, but their total DNA amounts agree closely with values predicted by adding the 1C DNA values of each parental genome. Chromosome counts confirm diploidy in the species E. hyemale and E. variegatum and in the hybrid E. xtrachyodon (= E. hyemale x E. variegatum). For the triploids (2n approximately 324), cytological information is presented for the first time. CONCLUSIONS: Triploid taxa may have originated by backcrossing or by crossing of a diploid hybrid with an unrelated diploid species. As tetraploid plants are unknown, these crossings probably involve diploid gametophytes that developed from unreduced diplospores. By repeated crossing events or backcrossing, reticulate evolution patterns arise that are similar to those known for a number of ferns and fern allies.     

Cytogenetic analysis of Lycopersicon esculentum (+) Solanum etuberosum somatic hybrids and their androgenetic regenerants

The aim of the study was to characterize genomic relationships among cultivated tomato (Lycopersicon esculentum Mill.) (2n=2x=24) and diploid (2n=2x=24) non-tuberous wild Solanum species (S. etuberosum Lindl.). Using genomic in situ hybridization (GISH) of mitotic and meiotic chromosomes, we analyzed intergeneric somatic hybrids between tomato and S. etuberosum. Of the five somatic hybrids, two plants were amphidiploids (2n=4x=48) mostly forming intragenomic bivalents in their microsporocytes, with a very low frequency of multivalents involving the chromosomes of tomato and S. etuberosum (less than 0.2 per meiocyte). Tomato chromosomes showed preferential elimination during subsequent meiotic divisions of the amphidiploids. Transmission of the parental chromosomes into microspores was also evaluated by GISH analysis of androgenic plants produced by direct embryogenesis from the amphidiploid somatic hybrids. Of the four androgenic regenerants, three were diploids (2n=2x=24 or 2n=2x+1=25) derived from reduced male gametes of the somatic hybrids, and one plant was a hypertetraploid (2n=4x+4=52). GISH revealed that each anther-derived plant had a unique chromosome composition. The prospects for introgression of desirable traits from S. etuberosum into the gene pool of cultivated tomato are discussed. Received: 2 August 2000 / Accepted: 4 December 2000     

Identification of mitotic chromosomes of tuberous and non-tuberous solanum species (Solanum tuberosum and Solanum brevidens) by GISH in their interspecific hybrids.

GISH (genomic in situ hybridization) was applied for the analysis of mitotic chromosome constitutions of somatic hybrids and their derivatives between dihaploid clones of cultivated potato (Solanum tuberosum L.) (2n = 2x = 24, AA genome) and the diploid, non-tuberous, wild species Solanum brevidens Phil. (2n = 2x = 24, EE genome). Of the primary somatic hybrids, both tetraploid (2n = 4x) and hexaploid (2n = 6x) plants were found with the genomic constitutions of AAEE and AAEEEE, respectively. Androgenic haploids (somatohaploids) derived from the tetraploid somatic hybrids had the genomic constitutions of AE (2n = 2x = 24) and haploids originating from the hexaploid hybrids were triploid AEE (2n = 3x = 33 and 2n = 3x = 36). As a result of subsequent somatic hybridization from a fusion between dihaploid S. tuberosum (2n = 2x = 24, genome AA) and a triploid somatohaploid (2n = 3x = 33, genome AEE), second-generation somatic hybrids were obtained. These somatic hybrids were pentaploids (2n = 5x, genome AAAEE), but had variable chromosome numbers. GISH analysis revealed that both primary and second-generation somatic hybrids had lost more chromosomes of S. brevidens than of S. tuberosum.     

Segmental allopolyploidy and paleopolyploidy in species of Leucaena benth: evidence from meiotic behaviour analysis

In this paper we report meiotic behaviour in 28 accessions of the tetraploid (2n = 4x = 104 or 112) Leucaena confertiflora, L. diversifolia, L. involucrata, L. leucocephala, L. x spontanea and the diploid (2n = 2x = 52 or 56) L. shannonii and L. macrophylla. We compare and discuss our data with that on literature about polyploidy in the genus. Despite the general predominance of bivalent formation, quadrivalents and other associations were found in all the taxa analysed. In the diploid species. multiple associations were found in up to 62% of the cells in L. shannonii and 97.6% in L. macrophylla. In the tetraploid taxa irregularities such as univalents, trivalents and other multivalents were observed in varying proportions, up to 55% in L. involucrata. Mean meiotic indexes per accession in the diploids and tetraploids were over or near 90%, but pollen fertility varied from 54.2% to 87.3%. The rather frequent presence of quadrivalents in the diploid species supports a paleopolyploid origin. For the tetraploid taxa, the presence of quadrivalents reflect chromosomal homology due to polyploid origin. Even if an autotetraploid origin cannot be ruled out by cytological data alone, considering other existing evidence, it is probable that L. confertiflora, L. pallida, L. leucocephala and L. involucrata are segmental allopolyploids. However, an autopolyploid origin for L. diversifolia cannot be ruled out nor cytologically nor by other existing data.     

Nature of 2n gamete formation and mode of inheritance in interspecific hybrids of diploid Vaccinium darrowi and tetraploid V. corymbosum

RAPD markers were used to determine the level of heterozygosity transmitted via 2n gametes from V. darrowi selection Florida 4B (Fla 4B) to inter-specific hybrids with tetraploid V. corymbosum cv Bluecrop. The tetraploid hybrid US 75 was found to contain about 70% of Fla 4B's heterozygosity, a value attributed to a first-division restitution (FDR) mode of 2n gamete production. Crossovers during 2n gamete formation were evidenced by the absence of 16 dominant alleles of Fla 4B in US 75, and direct tests of segregation in a diploid population involving Fla 4B. RAPD markers that were present in both Fla 4B and US 75 were used to determine the mode of inheritance in a segregating population of US 75xV. corymbosum cv Bluetta. Thirty-one duplex loci were identified which segregated in a 51 ratio, indicating that US 75 undergoes tetrasomic inheritance.     

Mendelian segregation for two-factor apomixis in Erigeron annuus (Asteraceae)

The inheritance of asexual seed development (apomixis) in Erigeron annuus (Asteraceae) was evaluated in a triploid (2n=3x=27) population resulting from a cross between an apomictic tetraploid (2n=4x=36) pollen parent and a sexual diploid (2n=2x=18) seed parent. Diplospory (unreduced female gametophyte formation) and autonomous development (embryo and endosperm together) segregated independently in the population yielding four distinct phenotype classes: (1) apomictic plants combining diplospory and autonomous development, (2) diplosporous plants lacking autonomous development, (3) meiotic plants with autonomous (though abortive) development and (4) meiotic plants lacking autonomous development. Each class was represented by approximately one-quarter of the population (n=117), thus corresponding to a two-factor genetic model with no linkage (chi(2)=2.59, P=0.11). Observations demonstrate that autonomous embryo and endosperm development (jointly) may occur in either reduced or unreduced egg cells. The cosegregation of the traits is attributed to tight linkage or pleiotropy. The data are consistent with the hypothesis that autonomous development in E. annuus is regulated by a single fertilization factor, F, which initiates development of both the embryo and the endosperm in the absence of fertilization.     

Formation of unreduced megaspores (diplospory) in apomictic dandelions (Taraxacum officinale, s.l.) is controlled by a sex-specific dominant locus

In apomictic dandelions, Taraxacum officinale, unreduced megaspores are formed via a modified meiotic division (diplospory). The genetic basis of diplospory was investigated in a triploid (3x = 24) mapping population of 61 individuals that segregated approximately 1:1 for diplospory and meiotic reduction. This population was created by crossing a sexual diploid (2x = 16) with a tetraploid diplosporous pollen donor (4x = 32) that was derived from a triploid apomict. Six different inheritance models for diplospory were tested. The segregation ratio and the tight association with specific alleles at the microsatellite loci MSTA53 and MSTA78 strongly suggest that diplospory is controlled by a dominant allele D on a locus, which we have named DIPLOSPOROUS (DIP). Diplosporous plants have a simplex genotype, Ddd or Dddd. MSTA53 and MSTA78 were weakly linked to the 18S-25S rDNA locus. The D-linked allele of MSTA78 was absent in a hypotriploid (2n = 3x - 1) that also lacked one of the satellite chromosomes. Together these results suggest that DIP is located on the satellite chromosome. DIP is female specific, as unreduced gametes are not formed during male meiosis. Furthermore, DIP does not affect parthenogenesis, implying that several independently segregating genes control apomixis in dandelions.