Doubling Of Chromosome Number Research Articles

Dramatic difference in rate of chromosome number evolution among sundew (Drosera L., Droseraceae) lineages.

Chromosome number change is a driver of speciation in eukaryotic organisms. Carnivorous sundews in the plant genus Drosera L. exhibit single chromosome number variation both among and within species, especially in the Australian Drosera subg. Ergaleium D.C., potentially linked to atypical centromeres that span much of the length of the chromosomes. We critically reviewed literature on chromosome counts in Drosera, verified the taxonomy and quality of the original counts, and reconstructed dated phylogenies. We used the BiChrom model to test whether rates of single chromosome number increase, decrease, and chromosome number doubling differed between D. subg. Ergaleium and the other subgenera and between self-compatible and self-incompatible lineages. The best model for chromosome evolution among subgenera had equal rates of chromosome number doubling but higher rates of single chromosome number change in D. subg. Ergaleium than the other subgenera. Contrary to expectation, self-incompatible lineages had a significantly higher rate of single chromosome loss than self-compatible lineages. We found no evidence for an association between differences in single chromosome number changes and diploidization after polyploidy or centromere type. This study presents an exemplar for critically examining published cytological data and rigorously testing factors that may impact the rates of chromosome number evolution.

Estimation of Nuclear Genome Size of Three Species of Camponotus (Mayr, 1861) (Hymenoptera: Formicidae: Formicinae) and Their Cytogenetic Relationship

The chromosome variability among ant species is remarkable, and the processes generating such variation are still under discussion since polyploidy has been observed in some distinct taxa. The chromosome number of species belonging to the Camponotus, subgenera Myrmothrix and Myrmobrachys, are highly different, whereas, the first subgenus has double the number of chromosomes of the second. In order to test the hypothesis of chromosome number doubling through polyploidy, the genome sizes of Camponotus (Myrmothrix) rufipes, Camponotus (Myrmothrix) renggeri and Camponotus (Myrmobrachys) crassus were estimated by flow cytometry. The chromosome number of specimens from the nests studied was also defined. No significant variation was noted in the genome size among them. The mean haploid genome size value (1C) of workers for the three species was 286.16 Mpb (0.29 pg). The polyploidy hypothesis can be ruled out as an evolutionary step linking the karyotype variations among the three studied species since the genome size of C. crassus with 2n = 20 chromosomes was the same as that of C. rufipes and C. renggeri with 2n = 40. The lack of variation in the amount of DNA between the related species C. rufipes and C. renggeri also demonstrate that flow cytometry is not an adequate approach to distinguish them. Our results highlight the importance of combining distinct methods, DNA quantification, and cytogenetics from the same colony. Understanding the path of chromosome evolution of three species with distinct degrees of relatedness should provide further information in enriching our knowledge about the Minimum Interaction Theory.

Agmatoploidy and symploidy: a critical review.

Agmatoploidy is a type of chromosome rearrangement that involves the fragmentation of an entire chromosome complement, generating a diploid with double its original chromosome number. Agmatoploidy and other related karyotype changes, such as symploidy (the opposite change, promoted by chromosome fusion), partial agmatoploidy, polyagmatoploidy, etc., are restricted to species with holokinetic chromosomes and are assumed to play an important role in their karyotype evolution. However, a critical review of the literature shows that examples of chromosome number doubling by agmatoploidy are rare and not clearly demonstrated, while partial agmatoploidy and partial symploidy seem to be the same as ascending and descending disploidy, respectively. It is therefore proposed here that these terms should be avoided or even abolished.

Cytological and Molecular Characterization of Homoeologous Group-1 Chromosomes in Hybrid Derivatives of a Durum Disomic Alien Addition Line

Earlier, we produced a stable Fusarium head blight (FHB)-tolerant durum wheat (Triticum turgidum L.) alien disomic addition line, DGE-1 (released in 2008), incorporating chromosome 1E of diploid wheatgrass [Lophopyrum elongatum (Host) A. Love; 2n = 2x = 14; EE]. For stable FHB resistance, the ultimate goal would be to transfer resistance from 1E into the durum chromosome 1A or 1B. Such transfer is more likely to occur when these target chromosomes are in a single dose. Therefore, we crossed DGE-1 with ‘Langdon’ substitution lines 1D(1A) and 1D(1B) and produced hybrid derivatives with chromosomes 1A, 1B, 1D, and 1E in a single dose. As an important fi rst step, we standardized a method to readily identify these group-1 chromosomes. Multicolor fl uorescent genomic in situ hybridization that we standardized earlier is useful for such identifi cation but is tedious and expensive. To identify these chromosomes more expeditiously and economically in a large plant population, we also used chromosome-specifi c molecular markers and identifi ed each of the group-1 chromosomes. We used marker Xwmc333 to identify chromosome 1A, Xwgm18 for 1B, Xwmc147 for 1D, and Xedm17 for 1E. Xedm17, which profi les chromosome 1E, also generated two characteristic bands for chromosome 1D and may therefore be useful in identifying both chromosomes simultaneously. DURUM WHEAT (Triticum turgidum L.; 2n = 4x = 28; AABB) is an important cereal used for human consumption in the United States, Canada, and most European countries, including Italy, Turkey, France, Romania, and Ukraine. On the evolutionary ladder, durum wheat is a forerunner of bread wheat (Triticum aestivum L.; 2n = 6x = 42; AABBDD) and evolved in nature long before the latter (see Jauhar, 2006). Durum wheat is a natural hybrid, having resulted from hybridization between related wild species in conjunction with doubling of chromosome number. Th us, it enjoys the benefi ts of both polyploidy and hybridity. It has a unique cytogenetic architecture and its chromosomes can be divided into seven homoeologous groups of two chromosomes each (Fig. 1). Th e chromosomes within a homoeologous group are genetically and evolutionarily related. Durum wheat can tolerate addition of certain chromosomes, of related species, syntenic to those within a homoeologous group. A related species, diploid wheatgrass [Lophopyrum elongatum (Host) A. Love; 2n = 2x = 14; EE genome] is an excellent source of Fusarium head blight (FHB) resistance (Jauhar and Peterson, 1998). In our earlier work on durum germplasm enhancement (DGE), we produced a stable alien disomic addition line, DGE-1, incorporating chromosome 1E of L. elongatum (Jauhar and Peterson, 2008; Jauhar et al., 2009). DGE-1 is the fi rst alien addition line of its kind with a novel source of FHB resistance from chromosome 1E. Th e ultimate goal would be to transfer FHB resistance from the added alien chromosome 1E into the durum chromosome complement. Homoeologous group-1 chromosomes 1A and/or 1B of durum wheat would naturally be the logical Published in The Plant Genome 4:102–109. Published 21 June 2011. doi: 10.3835/plantgenome2011.01.0002 © Crop Science Society of America 5585 Guilford Rd., Madison, WI 53711 USA An open-access publication All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. USDA-ARS, Northern Crop Science Lab., Fargo, ND 58102. Mention of trade names or commercial products in this publication is solely to provide specifi c information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. Received 26 Aug. 2010. *Corresponding author (prem.

Production of viable male unreduced gametes in Brassica interspecific hybrids is genotype specific and stimulated by cold temperatures.

BackgroundUnreduced gametes (gametes with the somatic chromosome number) may provide a pathway for evolutionary speciation via allopolyploid formation. We evaluated the effect of genotype and temperature on male unreduced gamete formation in Brassica allotetraploids and their interspecific hybrids. The frequency of unreduced gametes post-meiosis was estimated in sporads from the frequency of dyads or giant tetrads, and in pollen from the frequency of viable giant pollen compared with viable normal pollen. Giant tetrads were twice the volume of normal tetrads, and presumably resulted from pre-meiotic doubling of chromosome number. Giant pollen was defined as pollen with more than 1.5 × normal diameter, under the assumption that the doubling of DNA content in unreduced gametes would approximately double the pollen cell volume. The effect of genotype was assessed in five B. napus, two B. carinata and one B. juncea parents and in 13 interspecific hybrid combinations. The effect of temperature was assessed in a subset of genotypes in hot (day/night 30°C/20°C), warm (25°C/15°C), cool (18°C/13°C) and cold (10°C/5°C) treatments.ResultsBased on estimates at the sporad stage, some interspecific hybrid genotypes produced unreduced gametes (range 0.06 to 3.29%) at more than an order of magnitude higher frequency than in the parents (range 0.00% to 0.11%). In nine hybrids that produced viable mature pollen, the frequency of viable giant pollen (range 0.2% to 33.5%) was much greater than in the parents (range 0.0% to 0.4%). Giant pollen, most likely formed from unreduced gametes, was more viable than normal pollen in hybrids. Two B. napus × B. carinata hybrids produced 9% and 23% unreduced gametes based on post-meiotic sporad observations in the cold temperature treatment, which was more than two orders of magnitude higher than in the parents.ConclusionsThese results demonstrate that sources of unreduced gametes, required for the triploid bridge hypothesis of allopolyploid evolution, are readily available in some Brassica interspecific hybrid genotypes, especially at cold temperatures.

New Promising Citrus Triploid Hybrids Selected from Crosses between Monoembryonic Diploid Female and Tetraploid Male Parents

The breeding of citrus triploid hybrids started at Istituto Sperimentale per l'Agrumicoltura spell out in Acireale, Italy, in 1978 (Starrantino and Reforgiato, 1981). The strategy used has been to cross a monoembryonic 2x female parent with a 4x male parent. The imbalance between the embryo and endosperm ploidy (3:4) makes seeds from such crossings incapable of germinating in vivo. However, in vitro cultivation has been used to rescue the embryos. In this paper we report the main characteristics of triploid hybrids from 22 different parents, including sweet orange [Citrus sinensis (L.) Osbeck], mandarin (C. reticulata Blanco), clementine (C. reticulata Blanco), grapefruit (C. paradisi Macf.), pummelo [C.maxima], tangor (C. reticulata × C. sinensis), lemon [C. limon (L.) Burm. f.], citron (C. medica L.), and Fortunella hindsii (Champ.) Swing. Tetraploid parents are duplex because they originate from the doubling of chromosome number of the nucella or of other somatic tissues. The segregation and recombination process results in triploid hybrids with characteristics that are nearer to the 4x parent than the 2x one. This strategy is important in obtaining seedless hybrids similar to a parent after generations of backcrossing.

Revealing unknown or extinct lineages within Isoëtes (Isoëtaceae) using DNA sequencesfrom hybrids

Isoëtes, a heterosporous lycopod with a fossil record dating back to the Paleozoic, has numerous putative allopolyploids (resulting from hybridization events coupled with doubling of chromosome number). By using the highly variable nucleotide sequences from the second intron of a LFY homologue in Isoëtes, species could be delimited and hybrid origins determined. The data suggest that reticulate evolution is both common and complex within a more derived species complex of Isoëtes. Sequences of identifiable parentage and sequences that are unlike any diploid species known were recovered, leading to the conclusion that one or both of the putative parents have not yet been discovered or are extinct. A range of observations concerning allopolyploid speciation were categorized as follows: (1) verification of previous hypotheses regarding parentage (e.g., I. riparia, I. appalachiana), (2) determination that two morphologically distinct allotetraploid species can share the same parentage (I. azorica and I. acadiensis), (3) recognition of a cryptic allotetraploid species, indicated by the presence of different parental genomes (I. "appalachiana" from Florida), and (4) identification of allotetraploid species with one or two unknown parents (e.g., I. tuckermanii, I. acadiensis, I. azorica, and I. hyemalis). Some sequences from diploid species are remarkably uniform among populations (e.g., I. echinospora from various locations in North America, Iceland, and Wales), while others are variable at the subspecies level (e.g., northern and southern populations within I. engelmannii).

The effect of colchicine on triticale anther-derived plants: Microspore pre-treatment and haploid-plant treatment using a hydroponic recovery system

In cereals, chromosome doubling of microspore-derived haploid plants is a critical step in producing doubled haploid plants. This investigation was undertaken to study the effect of incorporation of colchicine in the induction medium for anther culture, and the effect of colchicine on anther culture-derived plants of triticale grown under controlled greenhouse conditions. In the latter case, chromosome doubling of adult sterile plants derived from anther culture of fourteen triticale populations was attempted, where androgenetic plants with non-dehiscent anthers were cloned and subjected to the colchicine treatment, and then grown with the aid of hydroponics. The hydroponic system provided optimal conditions for recovery of the affected haploids from the toxic effects of colchicine treatment and all colchicine-treated plants survived. A topcross-F1 (TC1F1) population with timopheevii cytoplasm produced the highest percentage of plants with seed-set either due to chromosome doubling by colchicine (98%) or spontaneous doubling of chromosome number (15%). Colchicine-treated anthers performed inferior than control in both induction and regeneration phases. One of the key observation of this study was the reversal from reproductive stage back to the vegetative stage which in turn enabled further cloning of haploid plants under hydroponic conditions once they were identified as sterile. The one hundred percent survival rate of in vitro-derived plants, 100% survival rate of colchicine treated haploid plants and the high chromosome doubling success rate (X = 82.3) observed in this study imply that a temperature-controlled greenhouse with an hydroponic system provides an efficient environment for inducing chromosome doubling of haploid plants in cereals.

Somatic segregation in rice

somatic (diplontic) selection would sufficiently rapidly elim inate heterozygous cell lineages after tissue differentiation has started. An alternative explanation is elimination of one of the genomes, followed by chromosome doubling of the remaining (single) genome, resulting in a homozygous diploid. There are numerous examples of genome separation in hybrids (Sybenga 1992), in which centromere behaviour is species dependent, and in which the centromeres of one species are activated later than those of the other species. The late centromeres fail to reach the metaphase plate simultaneously with the early centromeres, and the central space of the anaphase‐telophase group is occupied by chromosomes of the early species. Only the periphery is then available for the late chromosomes. This is reflected by the position of the chromosomes at interphase or mitosis. In extreme cases the late chromosomes do not reach telophase, and are eliminated. This occurs especially in young embryos where rapid divisions require well synchronized centromere activity. This phenomenon is used for producing haploids of one species from hybrids with another species. Spontaneous chromosome doubling is rare, but when the haploids are artificially doubled, homozygous diploids result. In a number of insects, imprinting of the chromosomes of one parent has a comparable effect: one set of chromosomes is eliminated. This mechanism of elimination of one specific genome can not have operated in the particular case considered here, as homozygosity for markers of both parents is recorded. Random loss of (initially) single chromosomes with equivalent centromeres from diploid hybrid nuclei, first re sulting in genetic instability and consequently in rapid loss of other chromosomes until a balanced haploid set remains, is common in some imperfect fungi (Pontecorvo 1958). The haploid state is the natural somatic state, and a series of un balanced mitoses does not present a threat to the very restricted differentiation. If this would occur in hybrid plants, chromosome number doubling would result in homozygous diploid sectors. However, in plants, the diploid state is the natural state and mitosis is normally very stable when the

Chromosomal location of rRNA genes in Diprion pini detected by in situ hybridization

Diprion pini belongs to a small family of sawflies with n = 7 as the modal chromosome number. This species has 14 acrocentric chromosomes and one carries a satellite. In situ hybridization to mitotic chromosomes of a Drosophila rDNA probe was carried out according to two protocols. The biotinylated probe was detected with peroxidase-conjugated extravidin and diaminobenzidine or with fluorescein-conjugated extravidin (FISH). These two techniques showed that only one chromosome per haploid complement responds to the probe used. The centric fission hypothesis is consequently more likely than polyploidization in order to explain the chromosome number doubling in D. pini. The sites of probe hybridization are located on the satellite and its carrier short arm, which are heterochromatin-rich. The propensity of rDNA and heterochromatin for self duplication and accretion most likely created the satellite and the short arms. This process is also known to be one of the mechanisms by which extra segments may arise.

Somatic chromosome number doubling of selected potato genotypes using callus culture or the colchicine treatment of shoot nodes in vitro

SummaryExperiments were carried out to double the somatic cell chromosome numbers of a monoploid and dihaploid of Solanum tuberosum and a genotype of S. circaeifolium subsp. quimense. Colchicine was used in vitro on shoot nodes from which the axillary meristems had been removed. Plants with doubled chromosome numbers were obtained from shoots grown from the tertiary, sub‐axillary meristems of all three genotypes. The callus culture of stem and leaf explants was found to produce more shoots with doubled chromosome numbers than the colchicine treatment in the case of the dihaploid and quimense genotypes but no shoots were obtained from callus culture of the monoploid.Fifty‐two % of the shoots from the dihaploid and 63% from the quimense clone were ploidy doubled in the case of the best callus culture system. Using a sub‐lethal dose of colchicine, the dihaploid yielded 37% ploidy‐doubled shoots whereas all the shoots produced from the monoploid were doubled and the quimense clone produced 27% doubled plants.Callus culture was highly dependent upon the type of growth medium and other, unknown, factors. The colchicine treatment, although yielding fewer products, was more reliable for achieving ploidy doubling in selected clones if the number of plants produced is not important.

The chromosomes of Diprion pini and D. similis (Hymenoptera: Diprionidae): implications for karyotype evolution.

Earlier cytological investigations characterize the family Diprionidae with a modal chromosome number of seven. This study shows that Diprion pini and D. similis have 14 acrocentric chromosomes (n = 14 for haploid males and 2n = 28 for diploid females). In D. pini, rRNA genes are located on the satellite and the round short arm of the chromosome carrying this segment. We studied six populations with no evidence of chromosomal polymorphism at the species level. Our results disagree with those published previously and reopen the question of karyotype evolution. Chromosome morphology, ISH and C-banding results support the hypothesis of chromosome number doubling by centric fission and not by polyploidization, followed by the growth of short arms by means of pericentromeric DNA amplification.

Intermeiocyte Connections and Cytomixis in Intergeneric Hybrid of Roegneria ciliaris (Trin.) Nevski with Psathyrostachys huashanica Keng.

The F1 intergeneric hybrids of Psathyrostachys huashanica Keng with Roegneria ciliaris (Trin.) Nevski was made and cytologically studied. Chromatin transfer through conjugation tube or conjugation opening before, during and after meiosis was observed in these hybrids. Consequently, unusual nuclear behaviors frequently observed include coenocytism, cell size variation and abnomal chromosome or nuclei number, non-synchronous or multipolar division, and delayed chromatin condensation. In addition, cytomixis was observed in early stages of micro-sporogenesis. These events could lead to spontaneous chromosome number doubling and, in turn, the origin of alloautoploid species.

Allelism of Endosperm Balance Number (EBN) in Mexican tuber-bearing Solanum species

Endosperm Balance Number (EBN) is a genetic, dose-dependent crossability system functioning in tuber-bearing Solanum species. Each species has been assigned 1EBN, 2EBN, or 4EBN. Species thus designated cross only within their EBN group. Doubling of chromosome number also doubles the EBN. The ploidy: EBN ratio is not consistent among Solanum species. Some diploids are 2EBN while others are 1EBN. Some tetraploids are 4EBN while others are 2EBN. Species from Mexico typically have EBNs that are one-half of their ploidy [e.g. 2x(1EBN), 4x(2EBN)]. Hybrids of Mexican species and a South American species, 2x(1EBN) S. Commersonii, and its 4x(2EBN) colchicine derivative were made and crossed to 1, 2, and 4EBN standard testers to determine the relationship of the genetic organization of EBN among and within these species. Diploid hybrids crossed only to 1EBN standard testers. Hybrids of 4x(2EBN) S. commersonii and 4x(2EBN) Mexican species crossed almost exclusively to 2EBN standard testers. Complex tetraploid hybrids involving S. commersonii, S. stenophyllidium (a Mexican diploid), and Mexican tetraploids of series Longipedicellata also crossed only to 2EBN testers. The apparent lack of recombination and segregation for EBN in these hybrids indicates that the genomes of the Mexican diploid and tetraploid species carry EBN in a way genetically similar to that of the South American species S. Commersonii.

Karyotypic evolution of a human undifferentiated large cell carcinoma of the lung in tissue culture : Burholt DR, Shackney SE, Ketterer DM et al. Cancer Cell Biology and Genetics Laboratory, Allegheny-Singer Research Institute, Pittsburgh, PA 15212. Cancer Res 1989;49:3355-61

Serial cytogenetic studies were performed on a cell line derived from a pleural effusion from a patient with undifferentiated large cell carcinoma of the lung. The initial sample had a broad range of chromosome numbers per cell, with a hypodiploid/pseudodiploid stem line and a hypotetraploid sideline. A sequence consisting of a doubling of chromosome number per cell followed by chromosome loss was observed repeatedly during 40 culture passages. The presence of metaphase spreads showing evidence of endoreduplication suggested this as a likely mechanism for the doubling of chromosome number per cell. Eleven marker chromosomes were observed in the cells of the primary sample; these markers persisted through all subsequent passages. Chromosomes 1, 2, 6, 7, 8, 11, and 16 were consistently overrepresented; each of these chromosomes was involved in marker formation. Chromosomes 4, 5, 9, 10, 19, 21, and 22 were consistently underrepresented. Every chromosome, either in its normal form and/or as part of a marker, was represented on the average by at least one copy per diploid cell. Eighteen new marker chromosomes were observed during the course of cell cultivation; one of these evolved into a clonal marker over the course of six cell passages. Of the new marker chromosomes that were formed during the observation period, the majority were found in hypotetraploid cells.

Chromosome number doubling of 2x potato lines with diverse genetic background through tissue culture

Twelve 2x potato clones of diverse genetic origin were tested for doubling of their chromosome number by placing stem and petiole segments in the regeneration medium (Hermsen et al., 1981). Eleven of the 12 clones yielded doubled plants (2n=4x=48). The percentages of regenerated tetraploid plants varied from 0 to 77% among the genotypes tested. The high rates of successful doubling from many genetically different clones, demonstrates the applicability of this method as a means to convert valuable 2x potato germplasm into the 4x level.

An unusual strain of human keratinocytes which do not stratify or undergo terminal differentiation in culture.

We have characterized an unusual cell phenotype in third passage cultures of a human keratinocyte strain derived from newborn foreskin epidermis. The cells had the same DNA fingerprint pattern as the second passage, morphologically normal, keratinocytes; they formed desmosomes and expressed the keratin profile characteristic of normal keratinocytes in culture. However, unlike normal keratinocytes, the cells did not grow as compact colonies and did not stratify or undergo terminal differentiation, even after TPA treatment or suspension culture. For these reasons we named them ndk for "nondifferentiating keratinocytes." The ndk cells also differed from normal keratinocytes in that they did not require a feeder layer and were not stimulated by cholera toxin to proliferate. The ndk cells had an absolute requirement for hydrocortisone and their growth rate was increased when epidermal growth factor was added to the medium. Although ndk failed to undergo terminal differentiation in culture, they were not transformed, since they were still sensitive to contact inhibition of growth, did not proliferate in soft agar, and had a limited lifespan in culture. The appearance of the ndk phenotype was correlated with a doubling of chromosome number and the presence of a lp marker chromosome. We suggest that these cells are a useful experimental adjunct to cultures of normal keratinocytes, in which proliferation and terminal differentiation are tightly coordinated, because in ndk cells there appears to be a block in terminal differentiation.

Fixation of genes and breakup of linkage blocks in selfed generations.

Occurrence of meiotic crossover and transmission of recombined chromosomes in selfed generations after a cross of two pure lines were simulated by applying the Monte Carlo method and the changes in the magnitude of heterozygosity and in the length of parental linkage blocks with the generations were examined. The number of generations required for the fixation of all the genes on a chromosome was proportional to the logarithm of the number of crossovers per chromosome (NCR). Increase in NCR from 1 to 10 resulted in the delay of fixation generation from 4.25 to 7.32. A cumulative distribution curve with NCR=z for the frequency of chromosomes homozygous for the whole length was roughly equivalent to the curve for independent factors with the number of factors equal to z multiplied by 3.3 to 4.0. The number of generations needed for the fixation of the whole genome depended upon the number of pairs of homologous chromo-somes involved in the genome as well as NCR. When NCR=2, for instance, the generation when 95% or more of the individuals come to complete fixation is Fg for a single pair of chromosomes, while F12 and F18 for a set of 7 and 12 pairs, respectively. Delay of fixation generation with increase of NCR was similar irrespective of the number of chromosomes. The average number of breakups of the linkage blocks increased from 0.95 to 7.31 in proportion to the increase in NCR from 1 to 10 and the maximum number ranged from 5 to 17. For a particular value of NCR, there was a significant positive correlation between the number of generations needed for fixation and the number of breakups. When 1% of the individuals with the highest number of breakups was selected, the average number of breakups was 2 (NCR=1) to 5 (NCR=10) times that of the original, while the number of generations needed for fixation changed into F9 or so irrespective of NCR. Increase in NCR only in a single generation was effective in raising the number of breakups when the increase occurred in early generations. It was found moreover by simulation that the fixation of the whole genome in as early as F2 by applying the technique of anther culture and doubling of chromosome number resulted in a narrower distribution of the number of breakups and highly recombined genotypes could not be obtained.

Isolation of tetraploid clones with high efficiency from diploid 3Y1 rat fibroblasts treated with sodium butyrate.

Sodium butyrate causes proliferation arrest with a G2 (4C) DNA content and induces formation of tetraploid cells upon removal of the inhibitor, in rat 3Y1 diploid fibroblasts. We isolated tetraploid clones from the butyrate-treated 3Y1 cells with high efficiency; among 21 clones randomly isolated, 5 were pure diploid, 7 were mainly tetraploid with a small contaminating diploid population, and 7 were pure tetraploid. Among the pure tetraploid clones, two showed doubled chromosome numbers with slightly broader distributions than that seen in parental 3Y1 cells. Butyrate further induced polyploid formation in the tetraploid cells thus produced, but octaploid cells that resulted could not be maintained for prolonged cultivation. We found no difference between the tetraploid and the (parental and parallel isolated) diploid clones in terms of colony-forming ability, proliferation rate, and sensitivity to density-dependent inhibition of proliferation. These results suggest that doubling of chromosome number by itself does not cause a change in proliferation property. The tetraploid clones had lower average saturation densities possibly due to enlargement of cell size represented by higher cellular protein content.

Calyx forms in dihaploids in relation to the origin ofSolanum tuberosum

Dihaploids ofSolanum tuberosum may have a regular form of calyx with relatively short sepals similar to that found in the diploid, wild species,S. sparsipilum, or an irregular form with relatively long sepals similar to that found in the cultivated, diploid species,S. stenotomum. The regular form is due to a dominant gene which may be strongly linked to theS genes controlling self-incompatibility. The results support the hypothesis thatS. tuberosum is a segmental allotetraploid produced by the doubling of chromosome number of a hybrid betweenS. stenotomum and a species with a regular calyx.