Genetic relationships among Hylocereus and Selenicereus vine cacti (Cactaceae): evidence from hybridization and cytological studies

BACKGROUND AND AIMS: Hylocereus and Selenicereus are native to tropical and sub-tropical America. Based on its taxonomic status and crossability relations it was postulated that H. megalanthus (syn. S. megalanthus) is an allotetraploid (2n = 4x = 44) derived from natural hybridization between two closely related diploid taxa. The present work aimed at elucidating the genetic relationships between species of the two genera. METHODS: Crosses were performed and the putative hybrids were analysed by chromosome counts and morphological traits. The ploidy level of hybrids was confirmed by fluorescent in situ hybridization (FISH) of rDNA sites. Genomic in situ hybridization (GISH) was used in an attempt to identify the putative diploid genome donors of H. megalanthus and an artificial interploid hybrid. KEY RESULTS: Reciprocal crosses among four diploid Hylocereus species (H. costaricensis, H. monacanthus (syn. H. polyrhizus), H. undatus and Hylocereus sp.) yielded viable diploid hybrids, with regular chromosome pairing. Reciprocal crosses between these Hylocereus spp. and H. megalanthus yielded viable triploid, pentaploid, hexaploid and aneuploid hybrids. Morphological and phenological traits confirm the hybrid origin. In situ detection of rDNA sites was in accord with the ploidy status of the species and hybrid studied. GISH results indicated that overall sequence composition of H. megalanthus is similar to that of H. ocamponis and S. grandiflorus. High sequence similarity was also found between the parental genomes of H. monacanthus and H. megalanthus in one triploid hybrid. CONCLUSIONS: The ease of obtaining partially fertile F1 hybrids and the relative sequence similarity (in GISH study) suggest close genetic relationships among the taxa analysed.     

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.     

Mitotic and meiotic irregularities in somatic hybrids of Lycopersicon esculentum and Solanum tuberosum.

Chromosome numbers were determined in metaphase complements of root-tip meristems of 107 tomato (+) potato somatic hybrids, obtained from five different combinations of parental genotypes. Of these hybrids 79% were aneuploid, lacking one or two chromosomes in most cases. All four hybrids that were studied at mitotic anaphase of root tips showed laggards and bridges, the three aneuploids in a higher frequency than the single euploid. Hybrid K2H2-1C, which showed the highest percentage of aberrant anaphases, possessed 46 chromosomes. Fluorescence in situ hybridization with total genomic DNA showed that this hybrid contained 23 tomato, 22 potato, and 1 recombinant chromosome consisting of a tomato chromosome arm and a potato chromosome arm. The potato parent of K2H2-1C was aneusomatic in its root tips with a high frequency of monosomic and trisomic cells and a relatively high frequency of cells with one fragment or telosome. Meiotic analyses of three tomato (+) potato somatic hybrids revealed laggards, which occurred most frequently in the triploid hybrids, and bridges, which were frequently present in pollen mother cells (PMCs) at anaphase I of hypotetraploid K2H2-1C. We observed putative trivalents in PMCs at diakinesis and metaphase I of eutriploid A7-82A and quadrivalents in part of the PMCs of hypotetraploid K2H2-1C, suggesting that homoeologous recombination between tomato and potato chromosomes occurred in these hybrids. All three hybrids showed a high percentage of first division restitution, giving rise to unreduced gametes. However, shortly after the tetrad stage all microspores completely degenerated, resulting in exclusively sterile pollen.     

Cytology and mating systems in the climbing cacti Hylocereus and Selenicereus

Chromosome numbers and meiotic behavior are reported for the climbing cacti species Hylocereus undatus, Hylocereus polyrhizus, and Selenicereus megalanthus. The Hylocereus spp. are diploid (2n = 22), while S. megalanthus is a tetraploid (2n = 44). Irregular chromosome disjunction at anaphase I in pollen mother cells of S. megalanthus is probably the major cause of its reduced pollen viability and may contribute to low seed set, low number of viable seeds and, consequently, low fruit mass. A pollination study confirmed self-incompatibility in H. polyrhizus and a weakened incompatibility reaction in H. undatus and S. megalanthus. Major crossability barriers do not exist between the Hylocereus spp. investigated. Reciprocal intergeneric crosses were successful between Hylocereus spp. and S. megalanthus, suggesting that an Hylocereus sp. might be one of the diploid progenitors of the tetraploid S. megalanthus. The implications of the results on cacti nomenclature and systematics are briefly discussed.     

Chromosome pairing in triploid intergeneric hybrids ofFestuca pratensis withLolium multiflorum, revealed by GISH

Genomic in situ hybridisation (GISH) was used to reveal chromosome pairing in two partly fertile, triploid (2n = 3x = 21) hybrids obtained by crossing the diploid (2n = 2x = 14) Festuca pratensis Huds. (designated FpFp), used as a female parent, with the autotetraploid (2n = 4x = 28) Lolium multiflorum Lam. (designated LmLmLmLm), used as a male parent. The pattern of chromosome pairing calculated on the basis of the mean values of chromosome configurations identified in all 100 PMCs analysed, was: 0.71I Lm + 2.24I Fp + 2.18II Lm/Lm + 0.54II Lm/Fp + 4.18III Lm/Lm/Fp. A relatively high number of Lm/Lm bivalents and Fp univalents, and a low number of Lm/Fp bivalents and Lm univalents indicated that the pairing was preferential between L. multiflorum chromosomes. Other observations regarding chromosome pairing within the Lm/Lm/Fp trivalents also confirmed this preferential pairing in the analysed triploids, as the Fp chromosome was not randomly located in the chain- and frying-pan-shaped trivalents. The similarities and differences in chromosome pairing at metaphase I and the level of preferential pairing between Lolium chromosomes in the different triploid Lolium-Festuca hybrids are discussed.     

TRANSMISSION AND SIGNIFICANCE OF B CHROMOSOMES IN ANTHURIUM WAROCQUEANUM

Somatic and meiotic chromosomes of one plant of Anthurium warocqueanum J. Moore and its selfed offspring were analyzed. The parent showed 2n = 30 + 3B in both somatic cells and pollen mother cells. The B chromosomes divided normally in somatic cells, but meiotic associations of Bs varied. Three configurations of three B chromosomes were observed at metaphase I of parent meiosis: one trivalent, one bivalent and one univalent, or three univalents. The number of B chromosomes in offspring ranged from 0 to 6, indicating their transmission from both male and female gametes. Offspring with two B chromosomes appeared in greatest frequency. It was hypothesized that both male and female gametes of the 3 B parent frequently contained one B chromosome through the normal distribution of the bivalent Bs at meiosis and the elimination of the univalent B chromosome due to lagging. Examination of pollen mother cells of offspring also revealed irregular behavior of B chromosomes. With a high number of B chromosomes, normal A chromosome bivalent formation seemed to be reduced. No phenotypic effects of B chromosomes were observed.     

Metaxenia in the vine cacti Hylocereus polyrhizus and Selenicereus spp

BACKGROUND AND AIMS: Flowers of the vine cacti of the genera Hylocereus and Selenicereus grown in Israel must be hand pollinated due to self-incompatibility and lack of efficient pollinators. In controlled pollination experiments, it was found that the time elapsed between pollination and ripening depends on the source of the pollen. Therefore a study was made of some effects of the pollen source on fruit development. METHODS: Flowers of Hylocereus polyrhizus were pollinated on the same day with different pollen sources and the stigmas were covered. Fruits were collected 4 d after reaching full colour. RESULTS: Pollinating flowers of Hylocereus polyrhizus with Selenicereus grandiflorus and S. megalanthus pollen delayed ripening by 1 and 3 weeks, respectively, as compared with ripening of fruits pollinated with Hylocereus undatus pollen. Other fruit characteristics affected by the pollen source were fruit size, pulp dry weight and number of seeds per fruit, all being significantly reduced, while peel dry weight was significantly increased by S. megalanthus pollen. Total soluble sugar content was reduced in H. polyrhizus fruits pollinated with S. grandiflorus pollen. No other major traits were affected. CONCLUSIONS: The results are evidence for the existence of metaxenia, i.e. an effect of pollen on maternal tissues, in cacti fruits. This pollen effect on the fruit-ripening time may be used for extending the marketing period of H. polyrhizus fruits.     

Organization and variation analysis of 5S rDNA in different ploidy-level hybrids of red crucian carp × topmouth culter

Through distant crossing, diploid, triploid and tetraploid hybrids of red crucian carp (Carassius auratus red var., RCC♀, Cyprininae, 2n = 100) × topmouth culter (Erythroculter ilishaeformis Bleeker, TC♂, Cultrinae, 2n = 48) were successfully produced. Diploid hybrids possessed 74 chromosomes with one set from RCC and one set from TC; triploid hybrids harbored 124 chromosomes with two sets from RCC and one set from TC; tetraploid hybrids had 148 chromosomes with two sets from RCC and two sets from TC. The 5S rDNA of the three different ploidy-level hybrids and their parents were sequenced and analyzed. There were three monomeric 5S rDNA classes (designated class I: 203 bp; class II: 340 bp; and class III: 477 bp) in RCC and two monomeric 5S rDNA classes (designated class IV: 188 bp, and class V: 286 bp) in TC. In the hybrid offspring, diploid hybrids inherited three 5S rDNA classes from their female parent (RCC) and only class IV from their male parent (TC). Triploid hybrids inherited class II and class III from their female parent (RCC) and class IV from their male parent (TC). Tetraploid hybrids gained class II and class III from their female parent (RCC), and generated a new 5S rDNA sequence (designated class I-N). The specific paternal 5S rDNA sequence of class V was not found in the hybrid offspring. Sequence analysis of 5S rDNA revealed the influence of hybridization and polyploidization on the organization and variation of 5S rDNA in fish. This is the first report on the coexistence in vertebrates of viable diploid, triploid and tetraploid hybrids produced by crossing parents with different chromosome numbers, and these new hybrids are novel specimens for studying the genomic variation in the first generation of interspecific hybrids, which has significance for evolution and fish genetics.     

Extensive citrus triploid hybrid production by 2x?×?4x sexual hybridizations and parent-effect on the length of the juvenile phase

The citrus fresh market demands the production of seedless citrus fruits, as seedy fruits are not accepted by consumers. The recovery of triploid plants has proven to be the most promising approach to achieve this goal, since triploids have very low fertility, are generally seedless and do not induce seeds in other cultivars by cross pollination. Triploid plants can be recovered by 2x?×?4x sexual hybridization. In this work, we present an effective methodology to recover triploid plants from 2x?×?4x hybridizations based on in vitro embryo rescue, ploidy level analysis by flow cytometry and genetic origin of triploid plants. The pollen viability of diploid and tetraploid citrus genotypes was analyzed by comparing the pollen germination rate in vitro. The pollen viability of tetraploid (doubled-diploid) genotypes is generally reduced but sufficient for successful pollination. Triploid embryos were identified in normal and undeveloped seeds that did not germinate under greenhouse conditions. The influence of parents and environmental conditions on obtaining triploid plants was analyzed and a strong interaction was noted between the parents and environmental conditions. The parental effect on the length of the juvenile phase was also demonstrated through observations of a large number of progeny over the last 15?years. The juvenile phase length of the triploid hybrids obtained with 'Fortune' mandarin as female parent and tetraploid 'Orlando' tangelo as male parent was shorter than the juvenile phase obtained with a clementine as female parent and tetraploids of 'Nova', 'W. Leaf' and 'Pineapple' male parents. Key message Effective methodology to recover citrus triploid plants from 2x?×?4x sexual hybridizations and the parental effect on the length of the juvenile phase.     

Development of monosomic alien addition lines and introgression of genes from Oryza australiensis Domin. to cultivated rice O. sativa L.

Oryza australiensis, a diploid wild relative of cultivated rice, is an important source of resistance to brown planthopper (BPH) and bacterial blight (BB). Interspecific hybrids between three breeding lines of O. sativa (2n=24, AA) and four accessions of O. australiensis (2n=24, EE) were obtained through embryo rescue. The crossability ranged from 0.25% to 0.90%. The mean frequency of bivalents at diakinesis/metaphase I in F₁ hybrids (AE) was 2.29 to 4.85 with a range of 0–8 bivalents. F₁ hybrids were completely male sterile. We did not obtain any BC₁ progenies even after pollinating 20,234 spikelets of AE hybrids with O. sativa pollen. We crossed the artificially induced autotetraploid of an elite breeding line (IR31917-45-3-2) with O. australiensis (Acc. 100882) and, following embryo rescue, produced six F₁ hybrid plants (AAE). These triploid hybrids were backcrossed to O. sativa. The chromosome number of 16 BC₁ plants varied from 28 to 31, and all were male sterile. BC₂ plants had 24–28 chromosomes. Eight monosomic alien addition lines (MAALs) having a 2n chromosome complement of O. sativa and one chromosome of O. australiensis were selected from the BC₂ F₂ progenies. The MAALs resembled the primary trisomies of O. sativa in morphology, and on the basis of this morphological similarity the MAALs were designated as MAAL-1, -4, -5, -7, -9, -10, -11, and -12. The identity of the alien chromosome was verified at the pachytene stage of meiosis. The alien chromosomes paired with the homoeologous pairs to form trivalents at a frequency of 13.2% to 24.0% at diakinesis and 7.5% to 18.5% at metaphase I. The female transmission rates of alien chromosomes varied from 4.2% to 37.2%, whereas three of the eight MAALs transmitted the alien chromosome through the male gametes. BC₂ progenies consisting of disomic and aneuploid plants were examined for the presence of O. australiensis traits. Alien introgression was detected for morphological traits, such as long awns, earliness, and Amp-3 and Est-2 allozymes. Of the 600 BC₂ F₄ progenies 4 were resistant to BPH and 1 to race 6 of BB. F₃ segregation data suggest that earliness is a recessive trait and that BPH resistance is monogenic recessive in two of the four lines but controlled by a dominant gene in the other two lines.     

Production and viability of unreduced gametes in triploid interspecific blueberry hybrids

Summary Three triploid (2n=3x=36) blueberry hybrids were obtained by hand-pollinating approximately 7,000 flowers of tetraploid highbush blueberry cultivars (based on Vaccinium corymbosum L.) with pollen from the diploid species V. elliottii Chapm. Meiotic analysis of these triploids revealed trivalents, bivalents and univalents in all metaphase I cells, with lagging chromosomes evident at anaphase I. Pollen of the three triploids was mostly aborted and did not stain with acetocarmine. However, the three triploids did produce from 0.9%–1.3% giant pollen grains that stained with acetocarmine and were present as monads, dyads or triads, rather than the normal tetrads. Pollination of 10,853 flowers of hexaploid V. ashei Reade cultivars with pollen from the triploids produced 266 berries, which averaged fewer than two fully-developed seeds per berry. One triploid clone showed partial female fertility when crossed to hexaploids, self-pollinated, or intercrossed with other triploids. Ploidy levels of the resulting hybrids were determined.Florida Agricultural Experiment Station Journal Series No. 8672     

Pairing and recombination at meiosis of Brassica rapa (AA) × Brassica napus (AACC) hybrids

Interspecific crosses contribute significantly to plant evolution enabling gene exchanges between species. The efficiency of interspecific crosses depends on the similarity between the implicated genomes as high levels of genome similarity are required to ensure appropriate chromosome pairing and genetic recombination. Brassica napus (AACC) is an allopolyploid, resulting from natural hybridization between Brassica rapa (AA) and Brassica oleracea (CC), both being diploid species derived from a common ancestor. To study the relationships between genomes of these Brassica species, we have determined simultaneously the pairing and recombination pattern of A and C chromosomes during meiosis of AAC triploid hybrids, which result from the interspecific cross between natural B. napus and B. rapa. Different AAC triploid hybrids and their progenies have been analysed using cytogenetic, BAC-FISH, and molecular techniques. In 71% of the pollen mother cells, homologous A chromosomes paired regularly, and usually one chromosome of each pair was transmitted to the progeny. C chromosomes remained mainly univalent, but were involved in homoeologous pairing in 21.5% of the cells, and 13% of the transmitted C chromosomes were either recombined or broken. The rate of transmission of C chromosomes depended on the identity of the particular chromosome and on the way the hybrid was crossed, as the male or as the female parent, to B. napus or to B. rapa. Gene transfers in triploid hybrids are favoured between A genomes of B. rapa and B. napus, but also occur between A and C genomes though at lower rates.     

百合‘Siberia’בManissa’种间杂交后代的染色体核型分析

以东方百合杂种系(Oriental hybrids group)品种‘Siberia’为母本,OT杂种系(interspecific hybrids betweenOriental and Longiflorum/OT group)品种‘Manissa’为父本杂交获得种间杂交F1代,对亲本及F1代株系的染色体数目和形态特征进行了分析。结果显示,母本东方百合‘Siberia’为二倍体即24条染色体,而父本‘Manissa’是高度杂合后代,为三倍体即36条染色体。杂交F1代的8个株系中有6个株系为二倍体即24条染色体,有2个株系为非整倍体,染色体条数分别为25和26条。母本‘Siberia’的核型为4m(1SAT)+10st(1SAT)+10t,父本‘Manis-sa’的核型为3m+18st(1SAT)+15t(1SAT),均属3B型。杂种F1代核型出现了多种类型,其中株系a、b、h为3B型,株系c、d、e、f为3A型,株系g为4B型。与亲本染色体形态相比,F1代株系出现了随体及端部着丝点染色体较多等染色体形态结构特征,而亲本没有这些特征。从染色体的形态、随体来看,子代为真杂种,在遗传上均偏向于母本。     

POLLEN COMPETITION AND RELATEDNESS IN HAPLOPAPPUS SECTION ISOPAPPUS (COMPOSITAE). II.

Pollen competition in mixed pollinations involving a time handicap was studied among the five aneuploid members of section Isopappus of Haplopappus. Foreign pollen was applied to the stigmas of a plant followed after a delay of 0–180 min by domestic pollen. Domestic pollen was prepotent over foreign pollen in all cases, but with increasing delay an increasing percentage of hybrids appeared in the progeny. The time interval required to produce 50% of hybrids in the progeny (at which time the two pollen types were competitively matched in potency) was determined for each cross. The interval is correlated with relatedness, with chromosome number, and with length of the style of the pollen parent. It can be used as an estimate of the potential frequency of hybridization in nature, with all other factors being equal.     

Comparison of Nicotiana tabacum and Nicotiana nesophila hybrids produced by ovule culture and protoplast fusion

Summary Somatic hybrids were produced between Nicotiana tabacum and N. nesophila, two species incapable of conventional sexual hybridization. Sexual hybrids, though, could be produced between these two species by using ovule culture only when N. nesophila was female. Clones of somatic hybrids were compared with sexual hybrids. Statistically significant variation was observed between clones, but not between sexual hybrids, for pollen viability, flower morphology, leaf morphology, and trichome density. As all clones of somatic hybrids have 96 chromosomes, the variability could not be explained by interclonal variation in chromosome number. Variation between somatic hybrids could be the result of cytoplasmic segregation or recombination, mitotic recombination or small chromosomal rearrangements prior to plant regeneration. Variation between clones could be exploited as these interspecies hybrids are now being used to incorporate disease resistance into cultivated tobacco.     

甘蔗-滇蔗茅杂交F_1花粉母细胞减数分裂过程GISH分析

研究甘蔗属与滇蔗茅属间远缘杂交F1染色体遗传行为具有重要科学意义,可为发掘利用滇蔗茅野生优异基因资源提供细胞学依据。本研究采用常规压片技术对可育父本及不育杂交F1花粉母细胞减数分裂过程进行比较观察,结果显示可育父本云南95-19减数分裂正常,而不育杂交F1分裂异常;进一步对F1花粉母细胞减数分裂过程进行基因组荧光原位杂交(GISH,genome in situ hybridization)分析,结果表明:滇蔗茅与甘蔗属热带种的亲缘关系较远,双亲染色体在F1细胞中不能进行同源配对,终变期,15条滇蔗茅染色体以单价体形式存在,花粉母细胞减数分裂细胞中存在滞后染色体、染色体丢失和不均衡分离现象。甘蔗-滇蔗茅属间远缘杂交F1花粉母细胞减数分裂异常因而杂交F1花粉完全败育。     

The formation of the polyploid hybrids from different subfamily fish crossings and its evolutionary significance

This study provides genetic evidences at the chromosome, DNA content, DNA fragment and sequence, and morphological levels to support the successful establishment of the polyploid hybrids of red crucian carp x blunt snout bream, which belonged to a different subfamily of fish (Cyprininae subfamily and Cultrinae subfamily) in the catalog. We successfully obtained the sterile triploid hybrids and bisexual fertile tetraploid hybrids of red crucian carp (RCC) (female symbol) x blunt snout bream (BSB) (male symbol) as well as their pentaploid hybrids. The triploid hybrids possessed 124 chromosomes with two sets from RCC and one set from BSB; the tetraploid hybrids had 148 chromosomes with two sets from RCC and two sets from BSB. The females of tetraploid hybrids produced unreduced tetraploid eggs that were fertilized with the haploid sperm of BSB to generate pentaploid hybrids with 172 chromosomes with three sets from BSB and two sets from RCC. The ploidy levels of triploid, tetraploid, and pentaploid hybrids were confirmed by counting chromosomal number, forming chromosomal karyotype, and measuring DNA content and erythrocyte nuclear volume. The similar and different DNA fragments were PCR amplified and sequenced in triploid, tetraploid hybrids, and their parents, indicating their molecular genetic relationship and genetic markers. In addition, this study also presents results about the phenotypes and feeding habits of polyploid hybrids and discusses the formation mechanism of the polyploid hybrids. It is the first report on the formation of the triploid, tetraploid, and pentaploid hybrids by crossing parents with a different chromosome number in vertebrates. The formation of the polyploid hybrids is potentially interesting in both evolution and fish genetic breeding.     

Pathways to polyploidy: indications of a female triploid bridge in the alpine species <Emphasis Type="Italic">Ranunculus kuepferi</Emphasis> (Ranunculaceae)

Polyploidy is one of the most important evolutionary processes in plants. In natural populations, polyploids usually emerge from unreduced gametes which either fuse with reduced ones, resulting in triploid offspring (triploid bridge), or with other unreduced gametes, resulting in tetraploid embryos. The frequencies of these two pathways, and male versus female gamete contributions, however, are largely unexplored. Ranunculus kuepferi occurs with diploid, triploid and autotetraploid cytotypes in the Alps, whereby diploids are mostly sexual, while tetraploids are facultative apomicts. To test for the occurrence of polyploidization events by triploid bridge, we investigated 551 plants of natural populations via flow cytometric seed screening. We assessed ploidy shifts in the embryo to reconstruct female versus male gamete contributions to polyploid embryo and/or endosperm formation. Seed formation via unreduced egg cells (B_III hybrids) occurred in all three cytotypes, while only in one case both gametes were unreduced. Polyploids further formed seeds with reduced, unfertilized egg cells (polyhaploids and aneuploids). Pollen was highly variable in diameter, but only pollen >27 μm was viable, whereby diploids produced higher proportions of well-developed pollen. Pollen size was not informative for the formation of unreduced pollen. These results suggest that a female triploid bridge via unreduced egg cells is the major pathway toward polyploidization in R. kuepferi, maybe as a consequence of constraints of endosperm development. Triploids resulting from unreduced male gametes were not observed, which explains the lack of obligate sexual tetraploid individuals and populations. Unreduced egg cell formation in diploids represents the first step toward apomixis.     

SYNTHETIC HYBRIDS OF NEW WORLD AND OLD WORLD AGROPYRONS. I. TETRAPLOID AGROPYRON SPICATUM × DIPLOID AGROPYRON CRISTATUM

Four interspecific hybrids of tetraploid A. spicatum × diploid A. cristatum ‘Fairway’ were obtained by controlled pollinations of emasculated and unemasculated spikes of A. spicatum. Most vegetative and spike characteristics of the hybrids were intermediate between those of the parent species. Tetraploid A. spicatum behaved cytologically as an autotetraploid, with mean chromosome associations of 0.09 I, 7.95 II, 0.03 III, and 2.98 IV being observed in 103 cells interpreted. The diploid A. cristatum was cytologically regular and formed 7 bivalents in 160 of 163 cells examined. Meiosis in the triploid hybrids was highly irregular, and these plants were completely sterile. Chromosomes of A. spicatum and A. cristatum differed sufficiently in size so that they could be distinguished in the hybrids. Seven bivalents and 7 univalents were formed in 90.5% of 262 cells interpreted at metaphase I. Mean chromosome associations of 6.87 I, 6.98 II, and 0.05 III were observed in the hybrids. Most chromosome pairing was due to autosyndesis of A. spicatum chromosomes, but A. cristatum chromosomes occasionally paired among themselves and with A. spicatum chromosomes. Tetraploid A. spicatum was considered to be an autotetraploid. On the basis of cytological evidence, A. cristatum, A. spicatum, and their interspecific hybrids were represented by genome formulae of AA, BBBB, and ABB, respectively.     

Chromosome numbers, meiotic behavior, and pollen viability of species of Vriesea and Aechmea genera (Bromeliaceae) native to Rio Grande do Sul, Brazil

Chromosome number, meiotic behavior, and pollen viability were analyzed in 15 species of two genera, Vriesea and Aechmea, native to Rio Grande do Sul, Brazil. This study is the first cytogenetic analysis of these taxa. The chromosome numbers are all n = 25, consistent with the proposed base number of x = 25 for Bromeliaceae. All examined taxa displayed regular bivalent pairing and chromosome segregation at meiosis. Observed meiotic abnormalities include univalents in metaphase I; missing or extra chromosomes and precocious division of centromeres in metaphase II; laggards in telophase I and anaphase II/telophase II. The high pollen viability (>88%) reflects a regular meiosis.