Polyploid Origin Research Articles

Molecular cytogenetics of Bellevalia (Hyacinthaceae) species occurring in Greece

Eight taxa of the genus Bellevalia (Hyacinthaceae) occurring in Greece, all with basic chromosome number x = 4, were cytologically studied using classical and molecular-cytogenetic techniques [fluorochrome banding with chromomycin A3, fluorescence in situ hybridization (FISH) using probes of 18S–5.8S–26S and 5S ribosomal RNA genes]. Two of the examined taxa are endemic, i.e., B. brevipedicellata and B. sitiaca, both restricted to the Island of Kriti. B. hyacinthoides and B. edirnensis are Balkan endemics, and the four remaining taxa, i.e., B. dubia subsp. boissieri, B. trifoliata, B. romana, and B. ciliata, are more widely distributed Mediterranean elements. Genome size, estimated by flow cytometry, ranged from 18.59 to 53.38 pg. The results of fluorochrome banding and FISH are reported for the first time for the genus Bellevalia. Despite the morphological similarity of the chromosome complement, which is in accordance with the general “basic” Bellevalia karyotype formula, the karyotypes of the studied species are clearly distinguished by the number and position of GC-rich bands and ribosomal DNA (rDNA) loci, revealing clear interspecific differentiation among the taxa. Additionally, examination of the polyploid species B. sitiaca and B. edirnensis and populations of B. hyacinthoides and B. ciliata with different ploidy levels permits discussion about the origin of polyploids and the taxonomic relationships among the taxa.

Syncytes during male meiosis resulting in 2n pollen grain formation in Lindelofia longiflora var. falconeri

Abstract Lindelofia longiflora (Royle ex Benth.) Baill. var. falconeri (Cl.) Brand (Family: Boraginaceae) is investigated cytologically (n= 12) for the first time from the cold deserts of Pangi Valley, Chamba District (Himachal Pradesh) in India. We report the formation of syncytes and 2n pollen grains in the species. During meiosis, the majority of the pollen mother cells (PMCs) exhibited 12 bivalents, equal segregation of chromosomes during anaphases, regular tetrads, and normal-sized pollen grain formation. Occasionally, two proximate PMCs fused during the early stages of prophase-I and resulted in the formation of syncytes. The frequency of syncytes in the accession is rather low, at 25 out of 1866 (1.33%). Such syncyte PMCs are detectable during meiosis due to their larger size compared to typical PMCs. The syncytes or polyploid cells showed normal 24 bivalents and depicted perfectly regular meiotic course. But the products of such PMCs yield 2n or larger sized pollen grains that are almost double the size of typical normal or n pollen grains. The origin of syncytes as a consequence of the fusion of meiocytes during the early stages of meiosis-I could be attributed to low temperature stress conditions prevailing in the Pangi Valley, where temperature during May and June dip to below freezing, the time the plants enters the reproductive/flowering bud stage. It is possible that such apparently fertile 2n pollen grains originating from syncytes might play a role in the origin of intraspecific polyploids in the species.

Allozyme Variation in Diploid, Polyploid and Mixed-Ploidy Populations of the Pilosella alpicola Group (Asteraceae): Relation to Morphology, Origin of Polyploids and Breeding System

The Pilosella alpicola group includes four species (P. alpicola s.str., P. ullepitschii, P. rhodopea and P. serbica) with allopatric distributions (Alps, Balkans, Carpathians) and contrasting cytotype patterns (diploid, diploid-polyploid and polyploid species). Whereas diploid taxa (P. ullepitschii and P. serbica) reproduce sexually, the mode of reproduction of polyploid cytotypes reflects their origin: autopolyploids of P. rhodopea reproduce sexually, while allopolyploid cytotypes of P. alpicola s.str. apomictically. We used allozymes to elucidate overall genetic variation within the group and to test their utility for taxon discrimination, assessment of polyploid origin and possible correlations with breeding systems. Variation of five allozyme systems encoded by eight polymorphic loci and 29 alleles was studied in 20 populations and 298 plants representing all taxa. Allozymes were proved to be only of limited usefulness for the taxonomic classification within the P. alpicola group. The Western Carpathian populations of P. ullepitschii formed the only genetically well-differentiated group. The same allele suite shared by all cytotypes of P. rhodopea and presence of both balanced and unbalanced heterozygotes in tetraploids was consistent with autopolyploid origins of polyploids and provided further evidence for a primary contact zone. An isolated relic population of P. rhodopea from the Southern Carpathians exhibited lowered values of genetic diversity when compared to the core area. Pronounced fixed heterozygosity was found in P. alpicola s.str., supporting its allopolyploid origin. In accordance with assumptions, genotypic variability was significantly higher in sexually reproducing diploid and diploid-polyploid taxa than in apomictic P. alpicola s.str.

Polyploid formation pathways have an impact on genetic rearrangements in resynthesized Brassica napus

• Polyploids can be produced by the union of unreduced gametes or through somatic doubling of F(1) interspecific hybrids. The first route is suspected to produce allopolyploid species under natural conditions, whereas experimental data have only been thoroughly gathered for the latter. • We analyzed the meiotic behavior of an F(1) interspecific hybrid (by crossing Brassica oleracea and B.rapa, progenitors of B.napus) and the extent to which recombined homoeologous chromosomes were transmitted to its progeny. These results were then compared with results obtained for a plant generated by somatic doubling of this F₁ hybrid (CD.S₀) and an amphidiploid (UG.S₀) formed via a pathway involving unreduced gametes; we studied the impact of this method of polyploid formation on subsequent generations. • This study revealed that meiosis of the F₁ interspecific hybrid generated more gametes with recombined chromosomes than did meiosis of the plant produced by somatic doubling, although the size of these translocations was smaller. In the progeny of the UG.S₀ plant, there was an unexpected increase in the frequency at which the C1 chromosome was replaced by the A1 chromosome. • We conclude that polyploid formation pathways differ in their genetic outcome. Our study opens up perspectives for the understanding of polyploid origins.

Microsatellite marker development for Galax urceolata (Diapensiaceae)

Microsatellite markers were developed for Galax urceolata to investigate genetic diversity, population structure, and polyploid origins (auto- vs. allopolyploid), and to estimate the minimum number of independent cytotype origins. Ten primer sets have been developed, and preliminary study indicates that all loci are appropriate for population-level genetic investigations. All loci are polymorphic with 6 to 46 alleles per locus. Expected heterozygosity ranges from 0.1007 to 0.6085. The microsatellite markers presented will facilitate analyses of polyploid origins, genetic diversity, geographic structure, and gene flow.

Amplified Fragment Length Polymorphism Data Reveal a History of Auto- and Allopolyploidy in New Zealand Endemic Species of Plantago (Plantaginaceae): New Perspectives on a Taxonomically Challenging Group

Amplified fragment length polymorphism (AFLP) data were generated for most of the ∼12 species and subspecies of endemic New Zealand Plantago (Plantaginaceae) to test species boundaries and interpret polyploid origins. Phylogenetic, network, principal-coordinates, structure, and quantitative analyses of the AFLP data were generally congruent and complementary regarding the main lineages, and the incongruences allowed some inferences of alloploidy and hybridization. Within the largely diploid group III, P. novae-zelandiae, P. lanigera, P. obconica, and P. aucklandica were genetically distinct. Within group II, there was little genetic differentiation between octoploids P. masoniae and P. triandra, and multiple alloploid origins were inferred for decaploid and dodecaploid cytotypes of P. unibracteata. Within group I, the 16-ploid P. sp. “Sylvester” is suggested to be an autoploid of octoploid P. raoulii, octoploid P. picta is clearly a genetically distinct entity, and species boundaries of octoploids P. raoulii, P. spathulata, and P. aff. spathulata are unclear. Additional genetic and molecular cytogenetic studies on New Zealand species and their close relatives from Australia and southern South America are needed to further investigate polyploid origins. AFLP is a useful tool for elucidating the origins and evolutionary history of closely related polyploid species in taxonomically challenging groups such as endemic New Zealand Plantago.

Bridging global and microregional scales: ploidy distribution in Pilosella echioides (Asteraceae) in central Europe.

A detailed knowledge of cytotype distribution can provide important insights into the evolutionary history of polyploid systems. This study aims to explore the spatial distribution of different cytotypes in Pilosella echioides at various spatial scales (from the whole distributional range to the population level) and to outline possible evolutionary scenarios for the observed geographic pattern. DNA-ploidy levels were estimated using DAPI flow cytometry in 4410 individuals of P. echioides from 46 populations spread over the entire distribution range in central Europe. Special attention was paid to the cytotype structure in the most ploidy-diverse population in south-west Moravia. Five different cytotypes (2x, 3x, 4x, 5x and 6x) were found, the last being recorded for the first time. Although ploidy-uniform (di- or tetraploid) sites clearly prevailed, nearly one-quarter of the populations investigated harboured more (up to all five) cytotypes. Whereas penta- and hexaploids constituted only a minority of the samples, a striking predominance of the triploid cytotype was observed in several populations. The representative sampling confirmed previous data on cytotype distribution, i.e. the spatial aggregation of mixed-ploidy populations in south-west Moravia and Lower Austria and the predominance of ploidy-uniform populations in other parts of the area investigated. Recurrent origin of polyploids from diploid progenitors via unreduced gametes and their successful establishment are considered the key factors promoting intrapopulational ploidy mixture ('primary hybrid zones'). As an alternative to the generally accepted theory of cytotype co-existence based on the development of different means of inter-ploidy reproductive isolation, it is suggested that a long-term ploidy mixture can also be maintained in free-mating populations provided that the polyploids originate with a sufficient frequency. In addition, the prevalence (or subdominance) of the triploid cytotype in several mixed-ploidy populations represents the first evidence of such a phenomenon in plant systems with exclusively sexual reproduction.

Comparative morphology and phylogeny of Nicotiana section Suaveolentes (Solanaceae) in Australia and the South Pacific

Molecular phylogenies alone have failed to resolve evolutionary relationships of Nicotiana L. section Suaveolentes Goodsp. (Solanaceae), a section largely comprising Australian endemics. Comparative morphology of Suaveolentes is illustrated and characters, together with the chromosome number, coded for phylogenetic analysis. Morphological characters included discrete characters of seeds and trichomes studied by scanning electron and light microscopy, and gap-coded quantitative measurements of flowers and vegetative organs. These data were analysed using both maximum parsimony and splits network, and compared to and combined with molecular analyses based on published nuclear and chloroplast-DNA sequences. Among morphological characters, there was a high level of homoplasy, possibly attributable to convergent evolution, hybridisation and introgression, and the underlying polyploid origin of the group. There was some conflict between the morphological and molecular datasets; however, overall there was a level of concordance that identified a phylogenetic sequence of taxa that reflected a reduction in the chromosome number. With N. africana as the functional outgroup, analyses showed N. fragrans (New Caledonia and Tongatapu) and N. fatuhivensis (Marquesas islands, formerly N. fragrans var. fatuhivensis) to be a basal lineage in the Australian and South Pacific clade of Suaveolentes. N. forsteri (eastern mainland Australia, Lord Howe Island and New Caledonia) is the sister taxon to all other Australian species of Suaveolentes, which form a well supported monophyletic group. Within this Australian endemic group, species with a chromosome count of n = 24 or 23 are early lineages. Two clades with a reduced chromosome number are the ‘N. simulans clade’ (all taxa with n = 20) and the ‘N. suaveolens clade’ (n = 19, 18, 16, or 15). The position of N. cavicola (n = 20 or 23) is equivocal, and it and several other species require further study. Some widespread taxa require greater sampling of populations to test for variation in morphology, DNA sequences and chromosome number for further elucidation of the evolutionary history of Suaveolentes.

GENETIC ARCHITECTURE OF SEXUAL DIMORPHISM IN A SUBDIOECIOUS PLANT WITH A PROTO-SEX CHROMOSOME

The rise of sexual dimorphism is thought to coincide with the evolution of sex chromosomes. Yet because sex chromosomes in many species are ancient, we lack empirical evidence of the earliest stages of this transition. We use QTL analysis to examine the genetic architecture of sexual dimorphism in subdioecious octoploid Fragaria virginiana. We demonstrate that the region housing the male-function locus controls the majority of quantitative variation in proportion fruit set, confirming the existence of a proto-sex chromosome, and houses major QTL for eight additional sexually dimorphic traits, consistent with theory and data from animals and plants with more advanced sex chromosomes. We also detected autosomal QTL, demonstrating contributions to phenotypic variation in sexually dimorphic traits outside the sex-determining region. Moreover, for proportion seed set we found significant epistatic interactions between autosomal QTL and the male-function locus, indicating sex-limited QTL. We identified linked QTL reflecting trade-offs between male and female traits expected from theory and positive integration of male traits. These findings indicate the potential for the evolution of greater sexual dimorphism. Involvement of linkage groups homeologous to the proto-sex chromosome in these correlations reflects the polyploid origin of F. virginiana and raises the possibility that chromosomes in this homeologous group were predisposed to become the sex chromosome.

Genome composition and origin of the polyploid Aegean grass Avenula agropyroides (Poaceae)

Aim The Aegean is a hotspot of plant biodiversity, with its island biota harbouring a large number of endemic taxa. To investigate the relationship between biogeography, polyploid speciation and genomics in the Aegean we used the biogeographically isolated highly polyploid eastern Mediterranean grass species Avenula agropyroides (2n = 70) as an example of complicated polyploid origin. Location Mediterranean, Aegean. Methods To clarify the origin of A. agropyroides, we conducted chromosome studies using repetitive DNAs as hybridization probes in fluorescent in situ hybridization experiments, chromosome banding methods and DNA sequence analyses of plasmid-cloned nuclear ribosomal (nr) ITS1–5.8S–ITS2 DNA. Results Decaploid A. agropyroides had near-autopolyploid karyotype structure and contained characteristic sequence motifs of nrDNA repeats not encountered in any of the diploids studied. Special repeat types found in one of its accessions (Crete) showed that A. agropyroides originated from a diploid species with a hybrid background. One of the genomes involved was close to both that of extant species (Avenula aetolica, Avenula compressa, Avenula hookeri, Avenula schelliana, Avenula versicolor) distributed mostly in the eastern Mediterranean to Asia and North America and also to the west Mediterranean (Avenula bromoides). The other resembled that of exclusively western Mediterranean species (Avenula albinervis, Avenula levis, Avenula marginata, Avenula sulcata). Main conclusions Avenula agropyroides represents a remarkable polyploid in the eastern Mediterranean, conserving the genome structure of a diploid species that no longer exists. This highlights how the Aegean has been less affected than other Eurasian regions by the repeated shifts of climatic zones and vegetation belts since the Late Tertiary.

Wx intron variations support an allohexaploid origin of the sweetpotato [Ipomoea batatas (L.) Lam

To clarify the polyploid origin of the sweetpotato, we analyzed retentions of three distinctive types of Waxy intron 2 (Wx-In2) variants among 27 sweetpotato lines and 24 selected relatives and their phylogenetic relationships with Wx-In2 from 11 closest relatives. The three types of Wx-In2 effectively distinguish three diploid constituent genomes of very close homeology in the sweetpotato: Type I is characteristic of some loci in Genome I and III, and Types II and III are specific to loci in Genome II and III. The Type I Wx-In2 variation was found to be retained in 19 sweetpotato lines, Ipomoea littoralis Blume (4×), I. tabascana (4×), and I. tenuissima (2×); Type II to be retained in all 27 sweetpotato lines, I. littoralis Blume, and two I. trifida accessions; Type III to be retained in 13 sweetpotato lines, I. tenuissima, and four distantly related species. Because of the nature of independent random divergence of orthologous intronic sequences, these highly selective retentions of genome-specific or characteristic sweetpotato Wx-In2 variations among four diploid or tetraploid sweetpotato relatives are consistent only with separate lineages of diploid genomes of the sweetpotato. Such an allohexaploid origin of the sweetpotato probably occurred via hybridization between I.tenuissima and I.littoralis Blume, derived earlier from I. trifida and an unidentified species sibling to I. tenuissima. However, neither the involvement of I. tabascana nor a multiple origin of the sweetpotato can be ruled out. The inference is supported by maximal likelihood relationships between the three types of Wx-In2 from the sweetpotato and Wx-In2 from its 11 closest relatives.

Origins and introgression of polyploid species in Mentzelia section Trachyphytum (Loasaceae)

Polyploid speciation has been important in plant evolution. However, the conditions that favor the origination and persistence of polyploids are still not well understood. Here, we examine origins of 16 polyploid species in Mentzelia section Trachyphytum. • We used phylogeny reconstructions based on DNA sequences from plastid regions and the nuclear gene isocitrate dehydrogenase (idh) to construct hypotheses of introgression and polyploidization. • Molecular data suggest that homoploid hybridization has been surprisingly common in Trachyphytum. Diploid species had unequal involvement in polyploid origins, but most polyploid taxa had allopolyploid origins from extant progenitors. A few polyploids with extreme phenotypes did not appear to have extant progenitors. We infer that the progenitors of these species were derived from extinct diploid lineages or ancestral lineages of multiple extant diploids. In agreement with other recent studies, we recovered molecular evidence of multiple phylogenetically distinct origins for several polyploid taxa, including the widespread octoploid M. albicaulis. • Evidence of high levels of introgression and allopolyploidy suggests that hybridization has played an important role in the evolution of Trachyphytum. Although idh sequences exhibited complicated evolution, including gene duplication, deletion, and recombination, they provided a higher percentage of informative characters for phylogeny reconstruction than the most variable plastid regions, allowing tests of hypotheses regarding polyploid origins. Given the necessity for rapidly evolving low-copy nuclear genes, researchers studying hybridization and polyploidy may increasingly turn to complex sequence data.

Hybridization and polyploidy of an aquatic plant,Ruppia(Ruppiaceae), inferred from plastid and nuclear DNA phylogenies

The monogeneric family Ruppiaceae is found primarily in brackish water and is widely distributed on all continents, many islands, and from subartic to tropical zones. Ruppia taxonomy has been confusing because of its simplified morphology yet high phenotypic plasticity and the existence of polyploidy and putative hybrids. This study addresses the current classification of species in the genus, the origin of putative hybrids and polyploids, and the distribution of Ruppia species. • Separate molecular phylogenetic analyses using plastid DNA and nuclear-encoded PHYB data sets were performed after chromosome observations. • The resultant trees were largely congruent between genomes, but were incongruent in two respects: the first incongruence may be caused by long outgroup branches and their effect on ingroup rooting, and the second is caused by the existence of heterogeneous PHYB sequences for several accessions that may reflect several independent hybridization events. Several morphological species recognized in previous taxonomic revisions appear paraphyletic in plastid DNA and PHYB trees. • Given the molecular phylogenies, and considering chromosome number and morphology, three species and one species complex comprising six lineages were discerned. A putative allotriploid, an allotetraploid, and a lineage of hybrid origin were identified within the species complex, and a hybrid was found outside the species complex, and their respective putative parental taxa were inferred. With respect to biogeography, a remarkably discontinuous distribution was identified in two cases, for which bird-mediated seed dispersal may be a reasonable explanation.

Dating the origins of polyploidy events

is a widespread speciation mechanism, particularly in plants. Estimating the time of origin of polyploid species is important for understanding issues such as gene loss and changes in regulation and expression among homoeologous copies that coexist in a single genome owing to polyploidy. Polyploid species can originate in various ways; the effects of mode of origin, genetic system, and sampling on estimates of the age of polyploid origin using distances between alleles of polyploids and their diploid progenitors, or between homoeologous loci in a polyploid genome, are explored. Even in the simplest cases, simulations confirm that different loci are expected to give very different estimates of the date of origin. The time of polyploid origin is at least as old as the time estimated from comparison of an allele sampled from the polyploid with the most closely related allele in the diploid progenitor. The polyploidy literature often does not make clear the longstanding observation that the divergence of homoeologous copies in an allopolyploid tracks the divergence of diploid species, not the origin of the polyploid. Estimating the date of origin of a polyploid is difficult, and in some circumstances impossible. Skepticism about dates of polyploid origins is clearly warranted.

Cytotype Distribution and Colonization History of the Steppe PlantIris aphylla

The polyploid origin and colonization history of Iris aphylla in central and southeastern Europe were investigated by means of karyological and AFLP analyses. The study indicated two chromosome races within I. aphylla based on chromosomal numbers 2n = 24 and 2n = 48. The overall chromosome counts and determined ploidy suggest tetraploidy as the most common ploidy level for this species in Europe. Only one diploid I. aphylla population was found, with a restricted distribution area in the Slovak Karst. The analyses showed that tetraploids arose independently at least three or more times in the past. The karyological results were in agreement with the AFLP data and strongly suggested that migration between the two Carpathian Mts. sites occurred recently via the Dukielska Pass. The low number of AFLP fragments unique to each population supports the hypothesis of recent range expansion and colonization of different environments in central Europe by I. aphylla. All populations were genetically depauperated and had 0–3 unique bands, indicating that not enough time elapsed for many unique fragments to form through mutation. We also observed relatively high and significant differentiation between the one diploid and all investigated tetraploid I. aphylla populations (ΦST = 0.470, p < 0.001). The high and significant values of genetic differentiation can be explained by several factors such as anthropopression and natural succession, which have fragmented the open calcareous habitats in central Europe.

Evidence of natural hybridization in Brachiaria humidicola (Rendle) Schweick. (Poaceae: Panicoideae: Paniceae)

Polyploidy and aneuploidy are conspicuous features of chromosomal evolution in the Poaceae; at least 80% of grass species are of polyploid origin (Stebbins 1985). The original basic chromosome number for the Poaceae is not known and has been the subject of much speculation. In the large tribe Paniceae, which includes the genus Brachiaria, the most common basic chromosome number is x = 9 (De Wet 1986). The predominance of x = 9 in Brachiaria genus had been reported in several studies (Basappa et al. 1987; Honfi et al. 1990; Bernini and Marin-Morales 2001; Mendes-Bonato et al. 2002, 2006; Utsunomiya et al. 2005). More recently, a new chromosome number, x = 6, was determined for B. dictyoneura (Risso-Pascotto et al. 2006). There is a large predominance of polyploid forms in the genus Brachiaria and tetraploids prevail. The origin of polyploidy, however, has never been pursued in this genus. Cytological evidence from analysis of meiotic behaviour, including chromosome pairing at diakinesis, led to the suggestion that autotetraploidy and segmental allopolyploidy could have contributed to evolution in some species (Mendes-Bonato et al. 2002, 2006; Utsunomiya et al. 2005). On the other hand, in two pentaploid accessions of B. brizantha (2n = 5x = 45) there was clear evidence of allopolyploidy (Mendes et al. 2006). This paper reports the unusual cytological behaviour during meiosis in one accession of Brachiaria humidicola from the Embrapa Beef Cattle germplasm collection.

Evolution and polyploid origins in North American ArcticPuccinellia(Poaceae) based on nuclear ribosomal spacer and chloroplast DNA sequences

The proportion of polyploid plant species increases at higher latitudes, and it has been suggested that original postglacial Arctic immigrants of some large groups, including grasses, were polyploid. We analyzed noncoding nuclear and chloroplast DNA of all North American diploid Puccinellia (Poaceae) and a subset of arctic polyploids to hypothesize evolutionary relationships among diploids and to evaluate the parentage of polyploids. Diploids formed three lineages: one uniting arctic species P. arctica and P. banksiensis; a second comprising arctic species P. tenella, P. alaskana, P. vahliana, and P. wrightii; and a third uniting the two temperate species P. lemmonii and P. parishii. The arctic species P. angustata (hexaploid) and P. andersonii (primarily octoploid) apparently derive from the P. arctica-P. banksiensis lineage based on ITS and chloroplast sequences, and share an ancestor with arctic triploid/tetraploid P. phryganodes based on nrDNA sequences. Sequence comparisons also suggest tetraploid P. bruggemannii evolved from two arctic lineages: P. vahliana-P. wrightii and P. arctica-P. banksiensis. These patterns and the predominance of arctic rather than temperate diploid species support the idea that diploid Puccinellia recolonized the Arctic from northern glacial refugia like Beringia, and also formed stabilized polyploid hybrids during these refugial events or subsequently during postglacial colonization.

Impact of Chromatin Transfer and Spindle Abnormalities on Pollen Fertility and Pollen Size in Plantago lanceolata L.

The phenomenon of cytomixis involving chromatin transfer and spindle abnormalities were recorded for the first time in the diploid (n=6) accession of Plantago lanceolata scored from Parvati Valley, Kullu District, Himachal Pradesh (India). Cytomixis with chromatin transfer involved 2–3 PMCs (pollen mother cells) at different stages of meiosis from early prophase-I to T-II, but the maximum frequency of PMCs involved in cytomixis was recorded during the early prophase stages of meiosis-I (11.03%). The transfer of chromatin material among meiocytes was observed to be both partial and complete resulting into PMCs with hypo- and hyperploid chromosome numbers (2n=14, 15). During cytomixis, nucleolus also transmigrated to neighbouring PMCs and the PMCs with supernumerary nucleoli also resulted. Other meiotic abnormalities associated with cytomixis included chromatin stickiness, out of plate bivalents, laggards, bridges, unoriented chromosomes during A-I/T-I and A-II/T-II, and micronuclei. The effect of chromatin transfer, spindle abnormalities and associated meiotic anomalies on meiotic behaviour, pollen fertility and pollen size has been discussed. The cytomixis in P. lanceolata seems to be a natural phenomenon under genetic control and certainly have played a role in the origin of aneuploids and polyploids.

Dating and functional characterization of duplicated genes in the apple (Malus domestica Borkh.) by analyzing EST data

BackgroundGene duplication is central to genome evolution. In plants, genes can be duplicated through small-scale events and large-scale duplications often involving polyploidy. The apple belongs to the subtribe Pyrinae (Rosaceae), a diverse lineage that originated via allopolyploidization. Both small-scale duplications and polyploidy may have been important mechanisms shaping the genome of this species.ResultsThis study evaluates the gene duplication and polyploidy history of the apple by characterizing duplicated genes in this species using EST data. Overall, 68% of the apple genes were clustered into families with a mean copy-number of 4.6. Analysis of the age distribution of gene duplications supported a continuous mode of small-scale duplications, plus two episodes of large-scale duplicates of vastly different ages. The youngest was consistent with the polyploid origin of the Pyrinae 37-48 MYBP, whereas the older may be related to γ-triplication; an ancient hexapolyploidization previously characterized in the four sequenced eurosid genomes and basal to the eurosid-asterid divergence. Duplicated genes were studied for functional diversification with an emphasis on young paralogs; those originated during or after the formation of the Pyrinae lineage. Unequal assignment of single-copy genes and gene families to Gene Ontology categories suggested functional bias in the pattern of gene retention of paralogs. Young paralogs related to signal transduction, metabolism, and energy pathways have been preferentially retained. Non-random retention of duplicated genes seems to have mediated the expansion of gene families, some of which may have substantially increased their members after the origin of the Pyrinae. The joint analysis of over-duplicated functional categories and phylogenies, allowed evaluation of the role of both polyploidy and small-scale duplications during this process. Finally, gene expression analysis indicated that 82% of duplicated genes, including 80% of young paralogs, showed uncorrelated expression profiles, suggesting extensive subfunctionalization and a role of gene duplication in the acquisition of novel patterns of gene expression.ConclusionsThis study reports a genome-wide analysis of the mode of gene duplication in the apple, and provides evidence for its role in genome functional diversification by characterising three major processes: selective retention of paralogs, amplification of gene families, and changes in gene expression.

Genetic variation in the allotetraploid Dryopteris corleyi (Dryopteridaceae) and its diploid parental species in the Iberian Peninsula

Studies on genetic diversity help us to unveil the evolutionary processes of species and populations and can explain several traits of diploid-polyploid complexes such as their distributions, their breeding systems, and the origin of polyploids. We examined the allozyme variation of Dryopteris aemula and D. oreades, diploid ferns with highly fragmented habitats, and the allotetraploid D. corleyi to (1) analyze the putative relationship between both diploids and the tetraploid, (2) compare the levels of genetic variation among species and determine their causes, and (3) assess the breeding system of these taxa. The allozymic pattern of D. corleyi confirms that it derived from D. aemula and D. oreades. The lack of genetic diversity in D. aemula, a species of lowland habitats, may be due to genetic drift associated with the contraction of populations in the last glaciation. By contrast, the alpine D. oreades had moderate intrapopulation genetic variation, which may derive from the expansion of populations during the last glaciation. In the latter species, low interpopulational variation suggested effective gene flow (spore exchange), and genotype frequencies in Hardy-Weinberg equilibrium indicated cross-fertilization of gametophytes. Evolutionary history appears to be an essential element in the interpretation of genetic variation of highly fragmented populations.